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The International Electromagnetic Field (EMF) Dosimetry Project was initiated at a NATO Advanced Research Workshop on Radio Frequency Radiation Dosimetry in Slovenia in 1998. The mission of the project is to promote and develop high quality EMF dosimetry for the assessment of human exposure and for in vitro and in vivo experimental systems. The intention is to create an internationally accepted Dosimetry Handbook which will be a living and substantially on-line document with integrated software tools and guides for dosimetry measurements and calculations. The primary benefactor of this project would be the public health, via assurance of the quality and transportability of human and experimental data; the ability to acquire robust scientific information is dependent upon accurate and precise dosimetry. The previous versions of the handbook, dealing mainly with CW signals, have provided the dosimetric bases of human and experimental RF studies as well as current exposure guidelines. The use of telecommunications technologies, particularly those incorporating pulse-modulated (and in the future ultrawideband) signals, has become more common since the last version of the handbook was published (in 1986). Dosimetry specific to such systems will be a major topic for the International EMF Dosimetry Handbook. The major improvements in computational dosimetry in the last 15 years also indicate that a new Handbook is due. MCL hosts the Handbook's website and co-ordinates the work of the leading international EMF/RF dosimetry experts who will contribute to the writing of the Handbook. Many of these experts attended the Slovenia meeting and have already agreed to participate in the Project if support can be found for its management and administration. Such support has been provided by the UK DTI.

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Page 1: International Electromagnetic Field (EMF) Dosimetry Project

The International Electromagnetic Field (EMF) Dosimetry Project was initiated at a NATO Advanced Research Workshop on Radio Frequency Radiation Dosimetry in Slovenia in 1998. The mission of the project is to promote and develop high quality EMF dosimetry for the assessment of human exposure and for in vitro and in vivo experimental systems. The intention is to create an internationally accepted Dosimetry Handbook which will be a living and substantially on-line document with integrated software tools and guides for dosimetry measurements and calculations. The primary benefactor of this project would be the public health, via assurance of the quality and transportability of human and experimental data; the ability to acquire robust scientific information is dependent upon accurate and precise dosimetry.

The previous versions of the handbook, dealing mainly with CW signals, have provided the dosimetric bases of human and experimental RF studies as well as current exposure guidelines. The use of telecommunications technologies, particularly those incorporating pulse-modulated (and in the future ultrawideband) signals, has become more common since the last version of the handbook was published (in 1986). Dosimetry specific to such systems will be a major topic for the International EMF Dosimetry Handbook. The major improvements in computational dosimetry in the last 15 years also indicate that a new Handbook is due.

MCL hosts the Handbook's website and co-ordinates the work of the leading international EMF/RF dosimetry experts who will contribute to the writing of the Handbook. Many of these experts attended the Slovenia meeting and have already agreed to participate in the Project if support can be found for its management and administration. Such support has been provided by the UK DTI.

Page 2: International Electromagnetic Field (EMF) Dosimetry Project

Project aims and objectives

1. Develop a plan to update and expand 4th edition of the Radiofrequency Radiation Dosimetry Handbook: The last edition of the Radiofrequency Radiation Handbook was published in 1986. Advances in technologies have enhanced the capability to measure and predict energy absorption during exposures to RF electromagnetic fields. Also, the global reach of the Internet make it possible for the next version of the document to be widely available in an enhanced electronic form in addition to a traditional paper version.

2. Recruit international scientists and engineers to serve as project leaders and contributors to the new handbook: Understanding energy absorption during exposure to electromagnetic fields requires a team approach. Physicists, engineers, biologists, physiologists, and health physicists will be recruited to write the chapters essential for an integrated Dosimetry Handbook. As an open document, contributions from many sources are welcomed, but its structure and validity will be ensured by the project leaders.

3. Establish an Internet presence for International EMF Dosimetry project: By employing an open international forum, the Internet, the project should proceed more rapidly, at less cost, with the results accepted internationally.

4. Develop an open copyright statement that will allow authors of text and software to retain the copyright to their contributions: this approach will encourage the development of a new handbook that could continually evolve with developing technologies. The ability to retain an open source copyright would encourage software contributions.

5. Develop working relationships with potential dosimetry user societies, including: Health Physics Society, International Labour Organisation, WHO and ICNIRP as well as UK national bodies: The new dosimetry handbook will address the professional needs of individuals responsible for protecting public and occupational health. The new handbook will be user-friendly from an introductory level up to an advanced research level.

Keywords: EMF, dosimetry handbook, online, international, software

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Background

During the past 20 to 30 years, society has become more dependent upon the use of technologies incorporating electromagnetic fields, and particularly telecommunications. These new and useful technologies have been embraced by the public but it has become increasingly important to be able to measure the emitted fields accurately and predict the amount of energy absorbed by biological tissue and cell cultures. Such issues were the impetus behind the development of the original Radiofrequency Radiation Dosimetry Handbooks (1976, 1978, 1980, 1986).

The original handbooks were written at a level that was easy to comprehend by the novice researcher and their contents have withstood the test-of-time. Specific absorption rate (SAR) values predicted by more recent computational approaches are in many instances quite comparable to those presented in the most recent dosimetry handbook. More than 2000 hardcopies of the Radiofrequency Radiation Dosimetry Handbook have been distributed and it is one of the cornerstone publications in bioelectromagnetics research.

Over the past few years, the explosive increased use in wireless communication devices has resulted in a flurry of meetings to develop or revise exposure standards. The ability to measure or predict energy absorption during exposure to radiofrequency EMFs is the basis of these standards. Recent advances in technologies utilised in dosimetry measurements permit better mapping of EMF absorption, and assessment of the biological responses to EMF exposure. Increased computer power permits the prediction of localised SAR values at higher resolutions. Reports produced by respected committees, such as the Independent Expert Group on Mobile Phones, highlight the importance of scientific replication of experimental results. Such replications are possible only when accurate dosimetry has been conducted in the original report. One of the primary goals of the Dosimetry Handbook has been to provide instruction in such methodology. Accurate experimental dosimetry in conjunction with well-replicated experiments will provide the best possible scientific evidence on which to base public health policy.

The new applications of EMF technologies and the marked improvements in computational dosimetry since 1986 will require substantial additions to the Dosimetry Handbook. To be useful from an introductory level up to the advanced research level, the new dosimetry handbook must be user friendly and maintain the high quality of scientific data reported in the four previous versions. The content will be expanded to the entire frequency range used for communications and elated applications and will reflect the tremendous advances in computational and analytical dosimetry since 1986. The new handbook will include data for researchers conducting in vivo or in vitro experiments, epidemiologists, electrical engineering professionals, medical professionals, and those involved in compliance testing and establishing exposure standards.

Development of the handbook will employ the Internet as an open international forum. Technologies such as hypertext markup language (HTML), World Wide Web (WWW), virtual reality markup language (VRML), and JAVA TM will be incorporated to permit sharing of ideas and results on an international basis. This aspect of the new Handbook would allow it to be used as an on-line resource by researchers: standard, verified dosimetric models of animals or computational model of exposure facilities could be accessed remotely, removing the need for dedicated dosimetric effort whilst ensuring good quality dosimetry. As an international group,

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contributors to International EMF dosimetry project could also facilitate efforts to harmonise the EMF exposure standards that are currently unique to each country .

The need for a new version of the dosimetry handbook is evident. Questions will continue to be raised concerning the way in which the emitted energy from new and emerging devices interacts with biological systems. Furthermore, the evaluation of a system's effectiveness and operational safety will continue to be a challenge to those developing and operating this new technology The new version of the Handbook must not only be up to date, it must be able to evolve at a rate that matches the development of EMF technology and devices.

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References

Durney, C.H., Iskander, M.F., Massoudi, H., Allen, S.J., and Mitchell, J.C. Radiofrequency Radiation Dosimetry Handbook, 3rd Edition, USAFSAM-TR-80-32, 1980.

Durney, C.H., Johnson, C.C., Barber, P.W., Massoudi, H., Iskander, M.F., Lords, J.L., Ryser, D.K., Allen, S.J., and Mitchell, J.C. Radiofrequency Radiation Dosimetry Handbook, 2nd Edition, USAFSAM-TR-78-22, 1978.

Durney, C.H., Massoudi, H., and Iskander, M.F. Radiofrequency Radiation Dosimetry Handbook, 4th Edition, USAFSAM-TR-85-73, 1986.

Johnson, C.C., Durney, C.H., Barber, P.W., Massoudi, H., Allen, S.J., and Mitchell, J.C. Radiofrequency Radiation Dosimetry Handbook, 1st Edition, USAFSAM-TR-76-35, 1976.

Klauenberg, B.J and Miklavcic D. (eds). Radio Frequency Radiation Dosimetry and its Relationship to the Biological Effects of Electromagnetic Fields. Proceedings of the NATO Advanced Research Workshop , Gozd Martuljek, Slovenia, October 1998. NATO Science Series 3. High Technology - Vol. 82. Kluwer Academic, Dordrecht, 2000. Mason, P.A., Klauenberg, B.J., Chadwick, P., Gajsek, P., Walters, T.J., Hurt, W.D. and Ziriax J.M. International EMF Dosimetry Project. In Biological Effects, Health Consequences and Standards for Pulsed Radiofrequency Fields: Proceedings of ICNIRP/WHO International Seminar, Erice. Sicily, November 1999.

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EMF Dosimetry Handbook Copyright

The Handbook itself will not "own" any of the contributions in a formal sense.

Contributors must agree to their work being published by the Handbook. They will retain the rights to use all or portions of their contributions in future publications.

Reproduction or quotation of the published materials by others will be encouraged, with the proviso that the quotation or copy be accompanied by a complete citation, including the author of the material being copied or quoted.

Any material that contributors supply must be either non-copyright or copy-righted to the contributor with permission for free dissemination.

Where a chapter contains material originating with a third party, permission to publish under these terms must be attained by the Chapter contributor prior to the chapter being delivered to the Handbook project.

We wish to encourage the use of on-line computational/analytical models. As far as possible we would like these to be published under the same copyright agreement as the rest of the chapter. However, it is recognised that for some organisations and individuals such models represent intellectual property and a commercial resource. Where models cannot be published without constraint, two possibilities exist: models could be run "on-line" without the ability of the user to download the model, or links could be provided to third party sites. We would not allow such links to be used as advertisements or solely to generate business for the model-holder.

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Mechanisms of electrostimulation: Application to electromagnetic field exposure standards at frequencies below 100 kHz.

J. Patrick Reilly, Metatec Associates, 12516 Davan Drive, Silver Spring, MD, 20904, USA and The John Hopkins University Applied Physics Laboratory Laurel, MD, USA

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1. Introduction The term “dose” as applied to electromagnetic safety parallels its use in pharmacology; it refers to the quantity of an agent (in this case electrical energy) that can potentially result in a biological effect. Past editions of this Dosimetry Handbook have focused on methods for measuring or calculating the absorption of electromagnetic energy by the human body, that is, the administered “dose.” In this work, I intend to go beyond an exposition of the absorption and distribution of electrical energy in the body.

It would be a laudable achievement if I could specify a safe “dose” of electrical energy and explain how to measure it. But the subject is far too complex to be reduced to a number, or even a single table of numbers. Instead, we are faced with a myriad of physical and biological relationships that account for human reactions to electrical energy – relationships that involve spatial and temporal characteristics of the electrical forces within the body, factors related to the human subject, the method of application of the energy, and the environment. To further complicate the problem, we are confronted with variations in the outcomes of biological experiments that defy causal explanations, and for which we are consequently reduced to formulations based on probability to describe adverse biological reaction thresholds. Even the definition of “adverse” is not obvious and requires careful delineation.

Considering these complications, I believe the subject of electrical dosimetry is best approached through an exposition of the biophysical forces and mechanisms that account for human reactions, whether adverse, beneficial, or benign. I have attempted to take this approach in this chapter for the portion of the electromagnetic spectrum below 100 kHz. The investigator who develops standards for electrical exposure needs an understanding of underlying biophysical mechanisms to help interpret experimental data, extrapolate from particular experimental conditions to more general conditions that may require regulation, and devise methods whereby the important quantities can be measured or calculated.

Numerous mechanisms have been advanced to account for human reactions to electrical energy. Among these, it is important to distinguish between established and proposed mechanisms. An established mechanism is defined here as one having the following properties: (a) it can be used to predict biological effects in humans; (b) it can be explicitly modelled using equations or parametric relationships; (c) it has been verified in the intact human; (d) it is supported by strong evidence; (e) it is widely accepted among experts in the scientific community. Mechanisms not having these characteristics are classified as proposed. I have identified established and proposed mechanisms based on these criteria in my recent book [Reilly, 1998a], and in other

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publications; [Reilly, 2000a, 2000b, 2002]. I will draw from these and other publications in this chapter.

Of the established mechanisms, the one that has the most impact on standards at the relatively low frequencies treated in this chapter is an excitable tissue effect, referred to here as electrostimulation; another established electrical mechanism for biological reactions is a thermal one. At frequencies below 100 kHz, electrostimulation reaction thresholds will typically be lower than thermal reaction thresholds. Above 100 kHz, thermal effects typically exhibit lower thresholds of reaction than do electrostimulation effects. However, with pulsed waveforms of low duty factor, the frequencies at which electrostimulation thresholds are lower than thermal thresholds can extend into the megahertz region.

Electrostimulation produces short-term effects, that is, it results in acute reactions that are manifested within seconds, (usually a fraction of a second) after the exposure begins. It dominates over thermal thresholds at frequencies below 100 kHz and as low as 1 Hz. Below that, magnetohydrodynamic mechanisms can be the most sensitive ones responsible for the human reactions.

Although there are many questions remaining about electrostimulation effects, we largely understand the underlying mechanisms, we can verify theoretical mechanisms in humans and animals, the experimental results are robust, and we can define biological end points in the intact human. It is therefore valuable to define limits to human exposure based on our understanding of acute excitable tissue effects.

Other mechanisms of interaction that fit into the proposed category relate to long-term or chronic exposure effects [Olden, 1999; Reilly, 1998a]. These mechanisms are typically mentioned in connection with hypotheses concerning adverse health effects, including cancer, reproductive effects, and nervous system disorders from chronic exposure to low-level electric and magnetic fields. Standard-setting and advisory groups, while not dismissing long-term exposure mechanisms as irrelevant, have concluded that the evidence and body of knowledge concerning them is presently insufficient to derive a human exposure limit [ICNIRP, 1998; IEEE, 2002]. Progress in research on proposed mechanisms should nevertheless be monitored and evaluated as to whether any one can be included in the list of established mechanisms.

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1. Principles of nerve and muscle excitation Every biological cell maintains a potential difference between its interior and exterior; usually the interior is negative with respect to the exterior. Nerve and muscle cells respond to electrical stimuli by becoming “excited,” a state in which the neural membrane undergoes a marked change of conductivity that leads to a large change in the cellular potential. The excited state is triggered when the potential difference across the cellular membrane is sufficiently reduced from its normal resting state. This potential change, called an action potential, propagates along the nerve’s axon. In an afferent neuron (e.g., a nerve conveying information from a sensory receptor to the brain) the action potential normally travels to the spinal cord and thence to the central nervous system (CNS). In an efferent nerve cell (e.g., one conveying information from the brain to muscle cells), the action potential is initiated in the CNS, from whence it propagates to muscle connections called motor end plates. Communication from one nerve cell to another or at the motor end plate takes place across junctions called synapses, most typically by means of chemical agents called neurotransmitters.

These normal processes can be activated or modified by electrical forces introduced into the body through applied current or electromagnetic induction for medical diagnosis or therapy. If uncontrolled, the same forces can be detrimental.

Excitable tissue effects are typically observed shortly after the application of the stimulus, often within milliseconds to seconds. These "acute" effects stand in contrast to responses to chronic electromagnetic exposure effects that many investigators have studied at much lower exposure levels for possible implications on human health.

1.1. Cellular polarisation

Biological cells normally maintain a potential, Vr, in which the interior of the cell is negative with respect to its exterior. Typical values of Vr for nerve and muscle cells are -65 and -90 mV respectively. Considering the membrane potential (≈ 0.1 V) and thickness (≈ 10-8 m), the electric field developed across the resting membrane is around 107 V/m. The conductivity of the excitable membrane is controlled by the this enormous electric field. Disturbances from the resting condition can lead to profound changes in the membrane's electrical properties, and ultimately initiate the functional responses of nerve and muscle.

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Figure 1. Representation of current flow around elongated cell placed in medium having a uniform electric field (uniform current density). The membrane is assumed to be semi-permeable to current flow.

Figure 1 illustrates the distribution of current flow around an elongated cell within a medium having a uniform electric field (i.e., uniform current density). The cell is presumed to be oriented with its long axis parallel to the undisturbed field. The flux lines suggest that the current through the membrane, and hence the disturbance of membrane polarisation, is greatest at the ends of the fibre. The anode-facing end of the cell will be hyperpolarised, and the cathode-facing end will be depolarised. In an alternating field, the sites of hyperpolarisation and depolarisation alternate every one-half cycle of the field oscillation.

We can analyse the potential disturbances of the elongated cell using the theory of electrical coaxial cables. Consider a cable of length 2L in a longitudinal static field of strength E. The steady-state solution for membrane voltage is given by [Sten-Knudsen, 1960]:

(1)

where X = x/λ, x is the longitudinal distance from the centre of the cell, λ is the space constant, and 2L is the length of the cell. X = 0 is taken as the centre of the cell, and the ends are at ± L. The space constant λ, also known as the electrotonic distance of the membrane, defines the distance along the membrane that a steady-state voltage disturbance due to point current

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injection will decay to e-1 of the value at the disturbed location. Space constants for invertebrate nerve are in the range 0.23 - 0.65 cm [Rall, 1977].

Figure 2. Normalised membrane voltage of a finite cable immersed in a static field of strength E. Cable length = 2L. Voltage has odd-valued symmetry about X = 0.

Figure 2 illustrates Eq. (1) for several cable lengths. Since Vm has odd-valued symmetry about X = 0, only one quadrant of the function needs to be illustrated. The maximum membrane voltage occurs at the ends of the fibre, and has the value

(2)

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For very long cells L → ∞, and the voltage at the cell's terminus is Eλ, a value which is closely approached even for fibres of modest length. For instance, with L/λ = 2 (total length = 4λ), the membrane voltage at the ends is ±0.964Eλ.

As evident in Eqs. (1) and (2), the fundamental force for membrane polarisation is the in-situ electric field, E, rather than current density, J. Although it is also possible to describe electrostimulation effects in terms of current density, as has been a common practice in the past [Bernhardt, 1988; ICNIRP, 1998; IEEE, 1999], the in situ electric field is a more fundamental descriptor. Of course, we can relate the two by J = Eσ, where σ is the conductivity of the medium. However, the conversion introduces an additional parameter (σ) about which there may be some additional uncertainty in an applied situation. The calculation of the in situ electric field is less sensitive to assumptions of tissue conductivities compared to internal current density. Consequently, it is preferable to express membrane polarisation effects, including nerve and muscle excitation, in terms of the in-situ E-field rather than current density. To my knowledge, the IEEE low-frequency standard [IEEE, 2002] is the first to specify basic restrictions for the general public in terms of the in-situ electric field.

1.2. Polarisation of nerve cells within an electric field

A nerve cell is an extremely elongated cell: the length of a sensory nerve innervating the fingertip or toe has a length of about one metre. Figure 3 illustrates modes of stimulation of a nerve cell, designated as end, bend, and spatial gradient modes [Reilly, 1998a; Reilly and Diamant, 2003]. The illustration shows a myelinated nerve, which, due to its significantly lower threshold as compared with an unmyelinated nerve, is a good choice for electrical stimulation models.

An action potential is initiated by depolarisation of the cellular membrane from its resting potential. Depolarisation occurs at points along the membrane experiencing current efflux. As illustrated in the figure, current efflux could occur at a site where the nerve is terminated, such as with a sensory receptor or motor end plate, where the nerve undergoes a sharp bend, or where a spatial gradient of the electric field exists. In practice, all three of these modes can be take place at one time. The site where excitation first occurs will be the one in which the depolarisation is maximal, and this site determines the threshold of excitation.

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Figure 1 Modes of neural stimulation. Excitation is initiated at points of maximal current efflux across neural membrane. Potential excitation sites consist of fibre terminals, sharp bends, and maximal gradient of E-field. Arrows indicate current flow. The wrappings along the long process (the nerve axon) consist of insulating myelin; the uninsulated portions are called “nodes of Ranvier.”

We model stimulation of myelinated nerve using an equivalent circuit model (Figure 4) which contains circuit elements for the electrical conductivity at each node of Ranvier [McNeal, 1976].

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Figure 2 Equivalent circuit models for excitable membranes. The response near the excitation threshold requires that the membrane conductance be described by a set of nonlinear differential equations. [After McNeal, 1976].

The terms Ve,n are potentials at the exterior of each node of the myelinated axon with respect to a distant electrode in the medium, Vi,n are the interior potentials, Cm is membrane capacitance at the node, and Rm is its resistance. The membrane potential of this circuit model can be expressed as

(3)

which is a discrete form of the cable equation, where Vn is the membrane potential at node n. The term , which is the driving function for membrane polarization change, is a second difference (approximately the second derivative) of the spatial potential measured along the long axis of the nerve fibre, or, equivalently, the first derivative of the

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longitudinal electric field. The term Ii,n representing the ionic current flowing across the membrane (i.e., through the element Rm in Fig. 4) is governed by a set of nonlinear differential equations applying to the myelinated nerve membrane [Frankenhaeuser and Huxley, 1964].

Equation (3) has been developed as a computer model consisting of an arbitrary number nodes, and with a threshold criterion based on propagation of an action potential; it is described as the Spatially Extended Nonlinear Node (SENN) model [Reilly et al., 1985, 1998a], which is an extension of a previously developed model [McNeal, 1976]. The SENN model alternates the magnitude of extra-nodal potentials (Ve,n) between threshold and no-threshold conditions to determine the threshold of excitation within 1%. It accommodates arbitrary spatial and temporal distributions of the extra-nodal potentials, Ve,n.

Because equation (3) requires a finite gradient of the longitudinal E-field to cause a change in Vn, one might suppose that excitation of a nerve is impossible in an E-field which lacks a spatial gradient. If we were to restrict our attention to a mathematically ideal nerve fibre of infinite length, such a conclusion would be correct. However, a nerve that terminates or bends (Figure 3) will experience a second derivative of the external potential function, even if the field itself lacks a significant spatial gradient. The end and bend sites are typically where excitation is initiated when current is introduced into the biological medium through cutaneous electrodes or through magnetic induction.

Within a uniform field, thresholds of excitation are inversely proportional to fibre diameter. This occurs because the nodal separation is proportional to fibre diameter such that d = 100 D, where d is node separation, and D is fibre diameter [McNeal, 1976]. Consequently, the voltage difference from node to node is directly proportional to fibre diameter. For example, a 10-µm fibre would have an internode spacing of 1 mm. The membrane potential change at the terminal node is approximately equal to Vm = Ed. Compare this result with the cable relationship mentioned above, for which Vm = Eλ.

The distribution of myelinated fibre diameters in peripheral nerve effectively covers the range 2 - 20 µm. Since the lowest thresholds correspond to the largest fibres, we use a 20 µm fibre to model minimum response thresholds for peripheral nerve stimulation.

1.3. Strength-duration law of excitation

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Figure 3 Strength-duration relationships derived from the myelinated nerve model: current thresholds and charge thresholds for single-pulse monophasic and for single-cycle biphasic stimuli with initial cathodic phase, point electrode 2 mm radially distant from the centre of a 20-µm fibre. Threshold current refers to the peak of the stimulus waveform. Charge refers to a single phase for biphasic stimuli. [From Reilly et al., 1985].

Figure 5 shows thresholds of excitation based on the SENN model of a point electrode 2 mm radially distant from a 20-µm nerve fibre and with waveforms consisting of a monophasic square wave, a biphasic square wave, and a single cycle of a sine wave. Thresholds are shown in terms of charge units (right vertical axis), and peak current (left vertical axis). Threshold charge is determined by QT = IT tp, where IT and QT are peak current and charge thresholds, respectively, and tp is the phase duration (the duration between zero crossings of the stimulus waveform). For the monophasic stimulus, peak current thresholds fall to a minimum plateau called rheobase

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as the pulse width is increased above 1 ms. At short durations (<10 µs), thresholds converge to a minimum charge value. Stimulus energy is not a pertinent descriptor of excitation thresholds, as some have erroneously supposed.

Strength duration functions

When thresholds are plotted against phase duration, the result is termed a strength-duration (S-D) function. For a monophasic stimulus, the S-D function conforms to the empirical relationships

(4)

(5)

where Io and Qo are minimum (rheobase) thresholds of current and charge, and τe is a parameter known as a strength-duration time constant. Equations (4) and (5) are redundant in that one can be derived from the other. Note that two parameters completely describe the S-D law of the excitable tissue: the rheobase and the strength-duration time constant. The relationships of Eqs. (4) and (5) can also be derived from theoretical models involving polarisation of a segment of an idealised linear membrane [Reilly, 1998a].

Strength-duration parameters

As noted above, an S-D curve is defined by two parameters: rheobase, and an S-D time constant, τe. Rheobase can be defined in terms current applied to an electrode contacting the biological medium (I), the current density within the medium (J), or the electric field within the medium (E). No matter which of these metrics we choose to use, the form of the S-D curve (Eq. 4) is the same when expressing the ratio of the threshold quantity to the rheobase (the left-hand term in Eq. 4). In cases of direct electrode contact, we may know the conducted current, but not its density and distribution within the body. In such cases, it is preferable to express the threshold and rheobase metrics as current values, IT and Io. With magnetic field induction, the induced

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electric field is the metric of choice, in which case the threshold and rheobase quantities are designated ET and Eo.

Although the S-D relationships discussed here are universal properties of excitable tissue, both τe and Eo vary with the type of tissue being stimulated and the method of stimulation. The smallest values of τe apply to nerve excitation; values for direct muscle excitation are approximately 10 times greater, and those for synaptic processes are about 100 greater. The parameter τe is not just a property of the tissue being stimulated, but also of the locality of the stimulation. The smallest values apply to the most focal application of current, such as with a point electrode adjacent to the nerve fibre. As the stimulation current is applied more gradually across the spatial dimensions of the neuron, the time constant increases.

Terminated axon models

When a small active electrode is placed near a nerve fibre, the spatial derivative of the electric field along the fibre appears as a driving force in Eq. (3). However, in chance electrical exposure, including exposure to environmental electric and magnetic fields, nerve excitation typically occurs where the derivative of the in situ electric field is insufficient to account for the reaction. Such cases can be modelled as excitation of a terminated or bent nerve fibre within a locally constant field, as illustrated in Fig. 3.

Table 1. Excitation requirements for end and bend modes of stimulation.

Bend Angle (deg)

Bend Node (#)

Eo (V/m) τe (µs)

0 1 6.15 120.7 90 2 8.55 115.7 90 4 9.84 105.1 90 6 9.96 103.9 90 8 9.96 103.9 180 2 6.50 93.4 180 4 5.45 96.3 180 6 5.10 101.5 180 8 4.98 103.9

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Thresholds apply to a 20-µm nerve fibre within a constant E-field that is oriented parallel to the nerve beyond the bend point (21-node fibre modelled). Source: Reilly and Diamant, 2003.

Table 1 [Reilly and Diamant, 2003] lists rheobase thresholds and S-D time constants from the SENN model of a 20-µm fibre that is either sharply bent, or terminated at a node; thresholds are given in terms of the electric field external to the affected fibre. Although the lowest tabulated threshold applies to a fibre with a sharp 180° bend at a location distant from the terminus, such a condition would not be realistic for practical fibre trajectories. For the remaining cases, the straight, terminated fibre provides the lowest practical threshold. Note that gradual bends would necessitate higher excitation thresholds, since the second derivative of voltage would necessarily be lower as compared with a sharp bend. In light of these observations, the terminated fibre is a good model for determining the minimum thresholds of nerve excitation in chance electrical exposures. Accordingly, the SENN model suggests Eo = 6.15 V/m, and τe = 121 µs for a 20 µm myelinated nerve.

For the straight, terminated fibre in a constant electric field, rheobase thresholds are inversely proportional to fibre diameter, and the strength-duration time constant is independent of fibre diameter. Since a 20 µm fibre diameter is at the outer limits of fibre diameters found within the human peripheral nervous system, the thresholds for the fibre modelled in Table 1 correspond to minimum thresholds of excitation within the peripheral nervous system.

The SENN model values correspond well to experimental data. Median experimental values of τe with magnetic stimulation are reported in the range 146 - 152 µs [Barker et al., 1991; Mansfield and Harvey, 1993; Bourland et al., 1991a], although larger values have also been reported [Havel, 1997; Nyenhuis et al., 1990; Bourland et al., 1999]. Values of τe with contact current stimulation encompass a fairly wide range that includes the values observed with magnetic stimulation. The threshold electric field with a coil encircling the forearm was found to be 5.9 V/m (Havel et al., 1997), which is quite close to the SENN model value of 6.15 V/m. In addition, an underlying neural excitation assumption of 6.15 V/m correctly reproduces the distribution of let-go current thresholds in adults [Sweeney, 1993]. Furthermore, thresholds of excitation with pulsed magnetic stimulation calculated with Eo = 6.15 V/m are reasonably consistent with experimentally determined thresholds, as explained in Sect. 7.1.

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It is often convenient to express S-D functions as asymptotic boundaries rather than a complex function, as in Eq. (4). For instance, the asymptotic approximation to excitation thresholds can be expressed as

ET = Eo for tp ≥ τe (6a)

ET = Eo(τe/tp) for tp ≤ τe (6b)

where ET is the threshold of the in situ electric field, Eo is the rheobase, tp is the duration of the excitation pulse (also called phase duration).

Table 2 lists various parameters that will be used in this chapter’s development of electrical dosimetry. For the present, I will comment only on the S-D parameters that have been discussed up to now. The other parameters listed in the table will be discussed presently.

Table 2. Models for established thresholds of reaction: median in situ E-field thresholds.

Reaction Eo peak

(V/m)

τe

(ms)

fe

(Hz) Synapse activity alteration, brain 0.075 25.0 20

10-µm nerve excitation, brain 12.30 0.149 3350

20-µm nerve excitation, body 6.15 0.149 3350

Cardiac excitation 12.0 3.0 167

(a) Interpretation of Table as follows:. Ei = E0 for ; for

.

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Also, for ; for .

(b) (V/m-pk) refers to the temporal peak of the electric field.

(c) Adapted from Reilly [1998a]

The 3rd row of Table 2 lists S-D parameters for peripheral nerve excitation. To determine basic restrictions in electromagnetic field standards, it is conservative to assume a small value of τe, rather than a large one. Consequently, Table 2 adopts a value of τe = 149 µs as suggested by an average of the lower experimental values mentioned above. The theoretical rheobase of Eo = 6.15 V/m for a 20-µm fibre is considered a median within a distribution of minimum thresholds in healthy adults.

1.4. Sinusoidal and other biphasic stimuli

A biphasic wave is generally less effective for excitation than a monophasic wave of the same magnitude and duration. This occurs because the biphasic phase reversal tends to reverse a developing action potential initiated on the initial phase. The result is higher thresholds of excitation for biphasic stimuli, as is evident by comparing the monophasic and biphasic square wave thresholds in Fig. 5.

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Figure 4 Thresholds of excitation vs. phase duration for three waveforms: monophasic square; continuous sinewave; single cycle sinewave. SENN model results for terminated 20-µm diameter fibre in constant electric field. [From Reilly and Diamant, 2002].

To further illustrate biphasic effects, Fig. 6 also shows SENN model thresholds for three functions. The horizontal axis indicates the phase duration, tp, defined as the duration of the monophasic wave, or the half-cycle time of an oscillating square wave or a sine wave function. The mode of excitation modelled is a 20 µm diameter myelinated axon terminated within a locally constant electric field oriented parallel to the fibre – conditions that are associated with minimum excitation thresholds.

Strength-frequency functions

Sinusoidal thresholds can be depicted versus frequency through the relationship f = 1/(2tp). The resulting curve appears as a U-shaped strength-frequency (S-F) function with a minimum plateau that is approximately equal to the rheobase of a monophasic squarewave stimulus, and with rising thresholds at both low and high frequencies. The low-frequency upturn typically occurs at

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about 10 Hz for a variety of excitable tissue types; it is a consequence of the slow rate of rise of a low-frequency sinusoidal wave, and is known as accommodation. The high-frequency upturn is designated fe in Table 2. A U-shaped threshold curve can be derived from the SENN model when it is exercised with sinusoidal waveforms; it is also evident in data from a variety of laboratory experiments [Reilly, 1998a].

In Fig. 6, as tp decreases (increasing frequency), thresholds eventually converge to a function proportional to tp-1 (i.e., are proportional to frequency).

If the stimulus waveform consisted of an oscillating square wave rather than a sinusoid, thresholds would not have the low-frequency upturn, although high frequency thresholds would still rise in proportion to frequency as with sinusoidal waveforms (Fig. 5). The low frequency upturn would also not occur if the sinewave stimulus were initiated at a peak rather than a zero crossing because the waveform would appear similar to the initial phase of a square wave.

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Figure 7 Excitation thresholds as a function of number of cycles of sinusoidal stimulation. Stimulus duration stepped in half-cycle increments out to four cycles, and full cycle increments beyond that. Dashed lines indicate duration of stimulus. Point electrode, 2 mm distant from the centre of a 20-µm fibre. [From Reilly, 1998a].

Figure 7 illustrates SENN model results that show the variation of thresholds with number of cycles of sinusoidal stimulation, where the waveforms starts at a zero crossing [Reilly, 1998a].

The threshold at one-half cycle of stimulation is relatively low because the waveshape is monophasic. At one cycle of stimulation, however, the waveshape is a charge-balanced biphasic wave, which results in a relatively high threshold. Thresholds alternate with half-cycle increments in a sawtooth pattern, gradually falling until reaching a minimum plateau when the total duration of stimulation is about one millisecond, which is effectively equivalent to continuous stimulation. Similar threshold relationships with the number of cycles of sinusoidal

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stimulation have been demonstrated experimentally in both human [Budinger et al., 1991] and bovine [Reinemann et al., 1999] subjects.

The minimum threshold for continuous sinusoidal stimuli converges to a value approximately equal to the threshold of a monophasic square wave having a duration equal to the phase duration (half-cycle time) of the sinusoidal wave (Fig 6). This fact allows us to approximately designate the same peak threshold for both a monophasic square wave and a continuous sine wave of the same phase duration, as in Table 2.

Asymptotic expressions for the S-F function can be expressed as

ET = Eo for f ≤ fe (7a)

ET = Eo(f/fe) for f ≥ fe (7b)

fe = 1/(2τe) (7c)

The phase duration and frequency relationship expressed in Eq. (7c) has been determined using a theoretical model of myelinated nerve [Reilly and Diamant, 2002]. Because of the nonlinear electrodynamics of excitable tissue, Eq. (7c) differs from linear systems for which a relationship τ = 1/(2πf) would be anticipated.

Table 2 lists S-F parameters in the 4th column. The rheobase values for the

S-F function correspond to those for the S-D function. The parameter fe in the S-F function is derived from the S-D time constant as in Eq. (7c).

Quasi sinusoidal stimulation

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Standards for electrical exposure often refer to sinusoidal frequency as a parameter. Strictly speaking, one can never test a physical system with a pure sine wave, which is a mathematical construst that exists in time from minus to plus infinity. In practice, any stimulus must have a beginning and an end, whether it is from a computer model or a physical test. The fact that the applied waveform is actually gated on and off may be important. The mathematical abstraction of a "sine-wave" can only be approximated.

In the SENN model results of Fig. 6, sinusoidal stimuli start with zero phase at t = 0, i.e., at a zero crossing. Were the sine wave to start at a different phase, the threshold results might be quite different, particularly at frequencies below 10 Hz. For example, the low frequency threshold upturn would not exist if the sine wave were initiated at 90°. In that case the stimulus would appear as a suddenly switched pulse of long duration, and would lead to a rheobase threshold, rather than an elevated one. Consequently, one can not validly test a so-called "dc" response by switching on a constant current.

Thermal versus electrical thresholds with sinusoidal stimulation

As noted above, thresholds for continuous sinusoids beyond a critical frequency increase in proportion to frequency. An example of this is seen in studies of human electrocutaneous perception [Dalziel and Mansfield, 1950; Chatterjee et al., 1986]. These studies show that above 100 kHz, thresholds achieve a maximum plateau as a result of thermal perception due to tissue heating. One should not assume, however, that 100 kHz always marks the divide between mechanisms dominated by thermal and electrostimulation effects. For pulsed stimuli, the frequency above which thermal perception dominates electrical thresholds depends greatly on the fraction of on-time (duty factor). This occurs because the heating capacity of electrical current (i.e., its rms value) is proportional to the square root of the duty factor. Consequently, one can extend the frequency where thermal perception dominates electrical perception by using pulsed sinusoidal stimuli of low duty factor [Reilly, 1998b].

This point is illustrated in Fig. 8, which is an adaptation of the experimental data of Chatterjee and colleagues [1986].

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Figure 8 Median finger touch perception threshold of sinusoidal current with duty factor, df, as a parameter. Perception curve for 100% duty factor (df = 1) determined from experimental data on adult males. [Data for df = 1 after Chatterjee et al., 1986].

The curve labelled df = 1 represents the average experimental threshold of perception for finger contact; the threshold is given in Fig. 8 as peak current rather than rms current, as originally presented in the cited paper; the horizontal axis represents the frequency. Since the thermal threshold depends on the rms value of the applied current, it follows that the peak thermal current is related to duty factor as

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( 8)

where IT,d is the thermal threshold with an arbitrary value of df, and IT,1 is the threshold with df = 1. Equation (8) ensures that IT,d has a constant rms value, regardless of the duty factor (with minor perturbations if the pulse does not contain an integral number of half cycles). The remaining curves in Fig. (8) apply to values of df ranging from 0.1 to 0.001. Note that the frequency separating the thermal and electrostimulation perception is advanced in inverse proportion to df1/2.

How high in frequency do frequency-proportional thresholds apply? Experiments with sinusoidal stimulation of rats show reasonable correspondence of frequency-proportional thresholds up to 1 MHz, the highest frequency tested [LaCourse et al., 1985]. The strength-duration law has also been theoretically demonstrated down to 2 ns using the SENN model [Reilly and Diamant, 2002a], thereby suggesting that the strength-frequency curve may be extended to several hundred megahertz. Other experimental data verify the strength-duration law for human sensory thresholds with monophasic pulsed stimuli as short as 0.1 µs, the shortest duration tested in humans [Reilly, 1998a], and to 1 ns in frog neuromuscular stimulation experiments [Rogers et al., 2002].

1.5. Electrical stimulation of muscle and cardiac tissue

The most sensitive means of exciting skeletal muscle is via electrostimulation of the motor neurons that innervate it. Consequently, thresholds for muscle stimulation normally follow those for nerve excitation – with transcutaneous stimulation of normal muscle tissue, S-D time constants of approximately 0.1 - 0.2 ms are observed. However, muscle tissue can be directly stimulated if the innervating nerves are desensitised, such as with the application of curare [Mortimer, 1981], or other means of denervation [Sunderland, 1978]. In such cases, time constants for motor reaction of several milliseconds are observed, although rheobase remains approximately unaffected.

The heart is a special type of muscle tissue. The heart is most sensitive to electrical excitation (a premature beat) during the diastolic (relaxed) period. It is most sensitive to fibrillation during the partial recovery period after the systolic (contractile) phase. Since excitation thresholds are much lower than fibrillation thresholds, the former is appropriate for electrical safety considerations.

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Experimental data from several sources suggest that rheobase for the heart is about 12.0 V/m at the median of a statistical distribution among healthy canines [Reilly, 1989a]. At the one-percentile rank, the rheobase is approximately 6.0 V/m – a factor of two smaller.

Excitation thresholds of the heart exhibit an S-D law, much like that for nerve excitation, except that the time constants are shifted to much larger values, similar to those observed with skeletal muscle stimulation. Like nerve tissue, time constants for cardiac excitation increase as the stimulus current is applied less focally to the stimulated tissue [Reilly, 1989a, Fig. 6.2]. Typical values of τe for cardiac tissue with large area stimulation is about 3 ms, which is the value listed in Table 2 – a factor of ten or more greater than τe for nerve stimulation.

1.6. Nonsinusoidal waveforms∗

Customarily, electromagnetic field standards specify maximum permissible exposure (MPE) limits to sinusoidal fields, and these limits are specified as a function of frequency. In applied situations, however, the exposure waveform of interest may not be strictly sinusoidal, such as with pulsed sinusoidal waveforms, mixed frequency waveforms, those having harmonic distortion, and a great variety of transient functions that may or may not be repeated. Examples of devices producing nonsinusoidal electromagnetic waveforms to which workers or the general public may be exposed include metal detectors, pulse modulators of radars, magnetic resonance imaging devices, video displays, anti-theft devices, and the charging circuits of cellular telephones [Jokela, 2000].

To develop electrostimulation standards, the analyst needs to identify the threshold of excitation as a first step. How is one to proceed with such a task when confronted with the myriad of possible exposure waveforms? One approach is to apply the temporal variation as a driving function to a nerve model that includes the nonlinear electrodynamics of the neural membrane, such as embodied in the SENN model. Unfortunately, this may beyond the resources of most individuals who are interested in the application of standards. It would be desirable to define a relatively simple method that could be used for evaluation or that could be incorporated into an electronic sensor. This problem is not as intractable as it may seem, as noted below.

In some cases, the tests described here may be overly conservative. Such cases may occur when the waveform appears as a low frequency wave on which is superimposed a short duration impulse. The degree of conservatism would increase as the impulse becomes shorter in duration, and greater in amplitude. A more precise test would require evaluation of the threshold of a specific waveform with a neural excitation model, such as the SENN model [Reilly and Diamant, 2002b].

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Evaluation method using waveform peak and phase duration

The principles discussed in Sect. 2.4 suggest one means for estimating the excitation potential of a variety of pulsed waveforms. Recall that peak thresholds for a monophasic squarewave and a continued sinewave are approximately the same (Fig. 6). This observation leads to a simple, conservative algorithm for evaluating many other complex waveforms, particularly those displaying occasional prominent peaks: (1) Determine the peak value of the test wave; (2) determine the phase duration, tp, associated with the peak (i.e., the duration between zero crossings on either side of the peak); (3) determine the peak value of the threshold for a continuous sine wave with equivalent tp; (4) equate that value to the peak threshold of the test waveform. While this method is not exact, it provides a relatively simple, and usually conservative test of whether a test waveform is below the excitation threshold [Reilly and Diamant, 2002].

Figure 5 Synthesised threshold waveform using Fourier components with relative amplitudes of the threshold values of the individual sinewaves and phased for maximum peak (odd and even harmonics, i = 1 – 20), 60 Hz fundamental

Figure 9 can be used to illustrate this method. The illustrated waveform was synthesised by summing 20 individual sinewaves (Fourier Series components); the relative magnitude of each component was proportional to the sinusoidal threshold determined from the SENN model (Fig. 6), beginning with a fundamental frequency of 60 Hz, and continuing to the 20th harmonic (1200 Hz). The resulting waveform was then used as a time series to exercise the SENN model. The magnitude of this waveform, which is plotted in Fig. 9, was adjusted so that it was just at the SENN model threshold. In this case, the peak of the threshold waveform is 7.85 V/m.

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The phase duration of the waveform peak in Fig. 9 equals 0.72 ms. Using the relationship f ≈ 1/(2tp), one would ascribe an equivalent frequency of 1389 Hz. The SENN model threshold at that frequency is 7.80 V/m, which is within 2% of the threshold determined by exercising the SENN model with the waveform in Fig. 9. This procedure proves to be a conservative test for a wide variety of waveforms [Reilly and Diamant, 2002]. It is most conservative when the in situ waveform is biphasic and of short duration.

Evaluation based on frequency components

An alternative method for evaluating nonsinusoidal waveforms requires knowledge of the frequency components of the test waveform, either through Fourier decomposition, or perhaps through other a priori means. Formulae for applying single frequency exposure limits to multiple frequency waveforms have been expressed in standards in terms of the following inequality [ICNIRP, 1998; IEEE, 2002]:

(9)

where S1 is an evaluation metric, Ai is the amplitude of the ith frequency component of the exposure waveform, and MPEi is the single frequency maximum exposure limit, which is usually set at some fraction of the excitation threshold. According to the cited standards, if the inequality is satisfied, then the tested waveform is acceptable. The NRPB applies Eq. (9) to frequencies from 0 to 12 MHz; ICNIRP applies it from 1 Hz to 10 MHz, and the IEEE applies it from 0 to 5 MHz.

To appreciate the significance of Eq. (9), consider a case in which the Ai values are scaled such that the waveform from which they were synthesised is exactly at the threshold of excitation. In that case, S1 = 1 would indicate that the test is neither conservative nor permissive, S1 < 1 would indicate a permissive test (i.e., the result would inform the analyst that the test waveform is subthreshold, even though it is at threshold), and S1 > 1 would indicate a conservative test (would inform the analyst that the waveform is suprathreshold).

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In a recent study [Reilly and Diamant, 2002] we tested Eq. (9) with waveforms having vastly different characteristics. One such example is shown in Fig. 9. We treated the MPEi terms as the SENN model threshold for each frequency component present in the test waveform, and the Ai terms as the magnitude of each Fourier component. In this example, Eq. (9) yielded a value of 1.03, from which the analyst would conclude that the test waveform would have to be reduced by 3% to be below the excitation threshold. Since the test waveform was at the SENN model threshold, the test criterion would be conservative by 3% in this instance. In other waveforms examined in the cited reference, the procedure yields typically conservative results, although in some cases more so than in the above example.

Evaluation with linear signal processing methods

An investigation was made into linear signal processing algorithms for assessing compliance of test magnetic field waveforms with the ICNIRP standards [Jokela, 2000]. The proposed method could be implemented with a sensing coil that responded either to flux density, B, or to the time derivative of flux density, dB/dt, followed by a first-order RC filter. The filter could be either a high-pass or low-pass type, depending on whether the device was processing the B or dB/dt waveform. The peak value of the device’s output waveform is to be compared with an ICNIRP amplitude value for sinusoidal magnetic fields. Considering that it uses a single RC stage, and that the ICNIRP standard has more than one frequency corner below 100 kHz, a safety factor implicit in the method varies with the frequency or time scale of the test waveform.

The author concluded:

“On the basis of an electrophysiological model [Reilly, 1989; 1998a] ... a first-order RC-type filter response provides a reasonable worst-case model for the response of a myelinated nerve fibre to the magnetic field stimulation, even though the linear RC model is only a very crude substitute for the complex non-linear stimulation model. Comparisons of the calculated stimulation thresholds with the proposed peak limits suggest that the weighted exposure restriction approach is in good agreement with the basic exposure criteria of the ICNIRP. At the same time, the excessively strict restrictions arising from the application of the multiple-frequency rule (Eq. 9) are avoided.”

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The author illustrated the proposed peak restriction procedure with data measured from various sources of non-sinusoidal magnetic fields.

It should be noted that Jokela’s method is an ad hoc one, designed specifically for the ICNIRP magnetic field standards. Jokela’s proposed method has recently been endorsed by ICNIRP [2003]. It would be desirable to investigate the application of linear processing methods, such as the one proposed by Jokela, for more general application, and with multiple frequency corners in the linear filter.

2. Electrostimulation of the central nervous system 2.1. Excitation of CNS neurons

Electrostimulation of the central nervous system (CNS) has been long studied for medical purposes, including, pain control [Hosobuchi, 1985], electroconvulsive therapy [Malitz and Sackeim, 1986], functional control and prostheses [Agnew and McCreery, 1990; Ueno et al., 1990], auditory and visual prostheses [Brindley and Lewin, 1968; Umematsu et al., 1974] , and various forms of functional diagnosis [Amassian et al., 1987; Mykelbust et al., 1985; Yeomans, 1990]. Such applications include stimulation by direct electrode contact of the brain and spinal cord [Agnew and McCreery, 1990], transcutaneous methods [Geddes, 1987; Grandori and Rossini, 1988; Levy et al., 1984; Marsden et al, 1983], and focal magnetic induction via pulsed current into small coils [Amassian et al., 1989].

In most studies involving electrostimulation of the CNS, the mechanism of neural excitation occurs through stimulation of myelinated axons, as discussed in Sect. 2. The diameter distribution of neurons within the brain, however, is shifted to smaller values as compared with peripheral nerve. Consequently, a fibre diameter of 10 µm is more appropriate for analysis of sensitive neurons responding to brain stimulation, and the rheobase for excitation of a sensitive brain neuron is greater than that for a peripheral neuron. The SENN model predicts rheobase for a 10-µm myelinated nerve fibre within a uniform electric field to be 12.3 V/m – twice that of a 20-µm peripheral nerve fibre, as noted in Table 2.

Whereas fibre diameter significantly affects rheobase, the SENN model predicts that the S-D time constant (also noted in Table 2) is unrelated to the diameter of a myelinated nerve. To illustrate this point, the S-D curve developed with stimulation of the visual cortex in the cat

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[Ronner, 1990] has a time constant of about 140 µs, as determined by the ratio of the rheobase charge to rheobase current [Reilly, 1998a] – a value quite close to that determined with the SENN model, and one within the range of values observed in many studies of peripheral nerve excitation.

Stimulation of the motor cortex.

Since the brain contains a wide distribution of neuron diameters at various orientations relative to an induced electric field, it may require a sophisticated experiment to detect reactions at the threshold where some brain neurons begin to be excited. However, at sufficient suprathreshold levels, reactions may become obvious and can be clearly adverse.

The experiments of Hayes [1950] are enlightening in this regard. He applied 60 Hz electric current just above and ahead of the ears of anesthetised spider monkeys, while measuring the electric field within the brain via electrodes inserted through holes drilled in the skull. With current at 58 mA, Hayes reported strong jerking motions of the monkey with each application of the current. The attendant electric field measured within the monkey's brain ranged from 17 - 23 V/m, depending on the location of the measurement probe. Assuming that the reported currents are rms values, the peak fields would be in the range 24 - 32.5 V/m. These values are about 2 to 3 times the assumed rheobase values noted for excitation of a 10-µm nerve fibre (Table 2).

From the experiments of Hayes, we can derive scaling factors to determine the relationship between the electric field in the brain and the applied current for electrodes placed on the surface of the skin at the temples as: E/I = 293 V/m/A at the cortex, and 400 V/m/A at the midbrain. These conversion factors are in excellent agreement with those of Rush and Driscoll [1968; 1969] using a physical model of a human skull and brain in a saline bath, and an analytic model consisting of three concentric spheres having dimensions and conductivities specific to human extracranial tissue, cranium, and brain tissue.

Rossini and colleagues [1985] applied pulsed stimuli to various areas of the head and scalp of 23 human subjects in areas above the motor cortex. He reported current levels needed to cause motor stimulation equivalent to moderate voluntary contractions of the hand or leg. To properly interpret his results, one must take into account the different waveshapes and electrode configurations he used. One phase of his experiments used "unifocal" electrodes (an "active" electrode at the apex of the head, and a ring of "return" electrodes around the forehead) . The electric field in the brain under his active electrode is thought to mimic what one would expect with large electrode separations on the head, such as with temple-to-temple electrodes. For this

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case, Rossini used 100 - 150 µs pulses, and corresponding thresholds were in the range 60 - 106 mA.

The pulse widths used by Rossini are sufficiently short that his reported current thresholds would be above rheobase. If we assume for the S-D time constant τe = 128 µs, then from Eq. (1) we conclude that 100 and 150 µs pulses would be above rheobase by factors of 1.84 and 1.45, respectively. If we apply the larger factor to Rossini's reported thresholds, we calculate rheobase current in the range 32.6 - 57.6 mA. Using a scaling factor of 293 V/m/A (noted above from the experiments of Hayes), the corresponding field induced in the cortex of the brain would be in the range 9.55 - 16.9 V/m. If instead we had calculated rheobase by assuming that Rossini's pulse width was 150 µs, rheobase at the lower end of his threshold range would be calculated as 12.1 V/m. We thus conclude that his most sensitive subject exhibited motor responses that correspond to a rheobase field between 9.55 and 12.1 V/m within the motor cortex. Considering the number of subjects tested (23), the lowest measured threshold can be attributed to about a 4.3 percentile rank within a distribution of subject thresholds.

Other experiments reported by Rossini with much shorter pulse widths and closely spaced electrodes yielded much greater threshold values. For instance, with pulse widths in the range 27 - 48 µs and with electrode spacing of a few centimetres, excitation thresholds were in the range 440 - 940 mA.

Electrical induction of brain seizures.

Grand mal seizures can be induced with relatively high stimulus levels. Seizures induced during electroconvulsive therapy (ECT) are sufficiently intense that patients are routinely anesthetised and given muscle relaxants to avoid bone fractures from intense muscle contractions that might otherwise occur [Glenn and Weiner, 1985].

The waveforms used in ECT vary widely from one instrument to another, and single instruments sometimes provide the practitioner with a selection of waveshapes, including pulsed and sinusoidal functions [ECRI, 1990]. The frequency and pulse repetition rate also varies among devices. The amplitude and duration of the stimulus for both sinusoidal and pulsed waveforms are usually selectable. Although the ECT practitioner is provided with this vast array of choices, the dosimetry and optimisation of effective ECT parameters has been little studied, although a few systematic studies do exist.

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The distribution of seizure thresholds was determined by Weaver and Williams [1986] using repeated trains of pulses having widths of 1 ms and a train duration less than a few seconds. In 2003 treatments, the mean (and median) thresholds were 587 mA. The most sensitive 1% of patients exhibited thresholds of approximately 300 mA. Using the scaling factors noted above, the cited current values would be interpreted as an electric field in the brain between 172 and 235 V/m at the 50 percentile rank, and between 88 and 120 V/m at the one percentile rank. Through Eq. (4), we apply these rheobase values to a 300 µs pulse by multiplying the cited values by 1.11 (assuming τe = 128 µs).

The duration of the stimulus affects the seizure threshold. Stimulus duration, which is defined here as the total time during which the stimulus is applied, should not be confused with pulse or "phase" duration appearing in Eq. (4). The later refers to the duration of an individual pulse, or to the half cycle time of a sinusoidal stimulus. Practitioners of ECT typically use stimulus durations from a fraction of a second to a few seconds (usually less than 4). Weaver and Williams found that with trains of 1 ms pulses, a stimulus duration of 1.0 s resulted in many more seizures than durations in the range 0.5 - 0.75 s. It is opined by some that the relevant stimulus parameter for ECT is the product of the current amplitude and the stimulus duration [Sackheim, 1991]. It is not clear whether one can apply this rule beyond the few seconds that comprises the upper limit of stimulus duration available in most ECT instruments.

Factors that lower seizure thresholds are: young age; hydration; previous seizures within the last few minutes; sedative hypnotic withdrawal; irritative brain diseases; amphetamines and other stimulants; antidepressant drugs; phenothiazines and lithium; unilateral electrodes; the drugs pentylenetetrazol, pitressin, reserpine; sensory stimulation; and good stimulus electrode application. Factors that raise seizure thresholds are: old age; dehydration; previous seizures within last few days; sedative hynoptics and most anesthetics; poor stimulus electrode application; decreasing electrode distance; frontal electrode(s); and diffuse nonirritative brain disease in some patients [Glen and Weiner, 1985].

2.2. Synaptic mechanisms

Excitable cells communicate with one another across junctions called synapses. An action potential that has travelled along the presynaptic cell affects the post synaptic cell through the release of specialised chemicals called neuro transmitters in the case of chemical synapses, or electrically (ephaptic transmission) in the case of electrical synapses across a gap between the pre- and post-synaptic cell. Chemical synapses are the more sensitive and common of these two modes of intracellular communication in the CNS. The neurotransmitter that is released from the chemical synapse flows across the gap, where it binds to receptor sites in the postsynaptic cell. If the postsynaptic potential is sufficiently depolarised by the neurotransmitters and if other conditions are met, an action potential will be launched in the post synaptic cell.

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Both temporal and spatial integration contribute to the postsynaptic potential of neurons within the CNS [Dudel, 1989; Shepherd, 1998], and this integration takes place in an nonlinear fashion. With temporal integration, sequences of action potentials contribute to the post synaptic potential. With spatial integration, a particular neuron may combine the inputs from thousands of other neurons that form synapses on that one neuron. Typically, single neurons form branching structures, sometimes consisting of thousands of individual branches (called dendrites), each one of which communicates with other neurons across synaptic junctions. In the cortex of the human brain, the density of synapses can be as large as one billion per cubic millimetre [Shepherd and Koch, 1998].

Because of the complexity of synaptic circuits, it is not possible to devise an electrostimulation model based on a single neuron, such as we did in applications to peripheral nerves. Although the literature on synaptic mechanisms is enormous, we lack a model that can be used to determine the CNS effects of electrostimulation by externally applied electric fields. Consequently, we rely on experimental data, albeit sparse, on electrostimulation of synaptic processes.

Experimental data show that an important property of the synapse is that a relatively small change in presynaptic potential can have a much larger percentage change in postsynaptic potentials [Katz and Miledi, 1967]. If an electric field exists around a neuron, its cell body and dendrites will become polarised as explained by cable theory (Sect. 2.0), and these potentials may be amplified in the post synaptic cell through spatial and temporal integration. Thus, we might expect much lower thresholds of synaptic effects than observed with nerve excitation. These effects could be either excitatory or inhibitory, that is, could result in the excitation of a neuron that would otherwise not have been excited, or could inhibit excitation of a neuron that would otherwise have been excited.

Saunders and Jefferys [2002] found that published data from in vitro studies support the view that CNS effects resulting most probably from synaptic interactions occur at induced electric fields as low as 1 V/m – a factor of about ten below the postulated median threshold of excitation of sensitive brain neurons. They further argued on theoretical grounds that the CNS in vivo may be more sensitive to extremely low frequency electric fields and currents than in vitro preparations. Spatial and temporal integration of interacting groups of neurons were thought to be responsible for the greater sensitivity of in vivo exposures.

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Experimental in vivo data on electrostimulation of synapses from in situ electric fields come primarily from studies of electrostimulation of the retina, discussed below. These data indicate a much lower rheobase than that observed for nerve excitation. With respect to the S-D time constant of synapse electrostimulation, time constants of synaptic mechanisms are demonstrated from a small fraction of a millisecond in synaptic “microcircuits”, to hundreds of seconds in larger hierarchal structures [Shepherd and Koch, 1998]. Despite this enormous range, observed S-D time constants applicable to electrostimulation of synapses within the retina fall within a relatively narrow range, as discussed below.

2.3. Synapse electrostimulation within the retina

Whereas the nerve cell requires membrane depolarisation of approximately 15 to 20 mV to initiate an action potential, synaptic processes can be affected by altering the presynaptic membrane potential by less than one mV, and possibly as little as 60 µV, as with electrical stimulation of synapses in the retina [Knighton, 1975a,b] – a factor 250 times lower than minimum neural excitation thresholds. Consequently, the synapse is a potentially sensitive site for neural interaction with applied electrical stimuli.

An example of a synaptic polarisation effect is attributed to the phenomenon of electro- and magnetophosphenes, which are visual effects resulting from electric currents or magnetic fields applied to the head [Adrian, 1977, Barlow, 1947a, 1947b; Baumgart, 1951; Bergeron et al., 1995; Budinger et al., 1984; Carstensen, 1985; Clausen, 1955; Lövsund et al., 1980a,b; Silny, 1986]. Experimental evidence suggests that phosphenes result from modification of synaptic potentials in the receptors and neurons of the retina [Lövsund et al., 1980a, 1980b; Knighton, 1975a; 1975b], rather than excitation of the optic nerve or the visual cortex, although visual sensations with stimulation of the visual cortex have been demonstrated with much stronger stimuli [Brindley and Lewin, 1968; Brindley and Rushton, 1977; Ronner, 1990]. Phosphenes are most sensitive to current or an in situ electric field that is oriented in a radial direction with respect to the retina. This finding suggests that the electric field interacts with radially oriented neurons in accord with cable theory (see Sect. 2.2).

Using data from magnetophosphenes [Lövsund et al., 1980a,b] the corresponding induced E-field in the head at the most sensitive frequency tested (20 Hz) is 0.079 V/m-rms as calculated with an ellipsoidal model of the head (see Appendix A). At the retina, where the electrical interaction is thought to take place, the calculated field is 0.053 V/m-rms, which is consistent with the current density threshold of 0.008 A/m2 at the retina determined for electro-phosphenes [Lövsund et al., 1980b] assuming the conductivity of the brain is 0.15 S/m. The peak of 0.075 V/m is associated with an rms value of 0.053 V/m, and that peak value is noted as rheobase in Table 2. The internal E-field corresponding to phosphene perception at the optimum frequency is a factor of 100 or so below rheobase thresholds for neural stimulation.

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Experimental strength-duration data show that τe for phosphenes using electrodes on the temples is approximately 14 ms [Baumgart, 1951; Bergeron et al., 1995] and for electrically evoked potentials in the frog's eye, τe is in the range 14 - 36 ms [Knighton, 1975 a, 1975b]. These values are consistent with the phosphene data described above, but are about 100 times greater than corresponding values for peripheral nerves.

Relatively few data exist on synaptic polarisation effects by applied electric fields. Considering this dearth of data, I have made use of experimental data on synaptic effects from applied electric fields and I have assumed parallels with nerve excitation properties. One class of these properties concerns strength-duration and strength-frequency characteristics. For example, experimental data show a S-D time constant (τe) for phosphenes using electrodes on the temples is about 14 ms [Baumgart, 1951; Bergeron et al., 1995]. Knighton [1975a, 1975b] developed S-D curves for electrically evoked responses, and found τe in the range 14 to 36 ms (Fig. 10).

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Figure 10 Strength-duration curves for electrically evoked potentials in the retina of the frog’s eye. Curves represent various experimental procedures. [Adapted from Knighton, 1975].

An average strength-duration time constant for synapse effects is τe = 25 ms, which is listed in Table 2. Using the relationships noted for nerve excitation, a strength-frequency constant of fe = 20 Hz is expected above which in situ electric field thresholds should rise. This rise is indeed observed in the case of electrophosphene thresholds, although the rate of rise is greater than that observed with nerve excitation (Clausen, 1955; Adrian, 1977). Magnetophosphene strength-frequency curves reported by Lövsund and colleagues (1980a; 1980b) show a minimum at 20 Hz, and rising thresholds at lower frequencies, in accord with electrophosphene data. Thresholds above 20 Hz vary somewhat with the experimental parameters (background illumination and wavelength, subject visual acuity). Considering electro- and magneto-phosphene strength-frequency and strength-duration curves in total, it is reasonable to adopt a threshold curve similar to that found in electrostimulation of nerve and muscle, but with a much lower strength-frequency constant (or, equivalently, a larger strength-duration time constant), and with lower rheobase. Additional study of CNS synaptic interaction effects is needed to clarify these assumptions.

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Frequency sensitive thresholds for phosphenes have been experimentally tested only to a maximum frequency of about 75 Hz. I have made the conservative assumption that synaptic polarisation thresholds follow a frequency-proportional law above 20 Hz to a frequency of at least 760 Hz (above which peripheral nerve excitation limits dominate the magnetic field MPEs, as will be explained subsequently).

A contrary view was expressed in a recent standard published by the International Commission on Non-Ionizing Radiation Protection [ICNIRP, 1998]. The ICNIRP standard assumed that the critical frequency for frequency-proportional thresholds of electrophosphene-related effects is 1000 Hz, rather than 20 Hz as assumed in a recent IEEE standard [IEEE, 2002]. This assumption, together with the ICNIRP assumption that such reactions include the spinal cord (see 3.4 below), account for lower basic restrictions in the ICNIRP standards at frequencies above about 35 Hz as compared with the IEEE standard. Note that no information exists on frequency constants for synaptic effects, other than the retina studies noted above.

2.4. The spinal cord versus brain as the site of synapse electrostimulation

The spinal cord, which also contains synapses, performs many functions, such as control of posture and reflex activity. It is logical to inquire whether low electrical thresholds associated with synapse stimulation might be present in the spinal cord.

In pursuit of this question, note that tests have been conducted with human subjects whose torsos were subjected to the strong switched gradient fields of experimental MRI systems (See Sect. 7.1). Perception was sometimes preferentially reported in the small of the back at stimulus levels corresponding to nerve stimulation thresholds in accord with expectations from an elliptical induction model. These tests showed no observable effects below the neural threshold for perception. The lack of an observable effect below electrical perception thresholds suggests one of three possible explanations. One is that spinal synapse interactions did occur, but they were imperceptible to the subject. Another is that the induced field in the spinal column was below synapse interaction thresholds, even though the levels just outside of the spinal column were roughly two orders of magnitude above synapse thresholds observed in the retina. A third is that stimulation thresholds in the spinal cord were significantly greater than what has been assumed for synaptic effects in brain neurons (Table 2).

Considering the lack of data to suggest observable effects from stimulation of the spinal cord at the levels attributed to synapse thresholds, I have concluded that protection standards should focus on synaptic effects in the brain, rather than the spinal cord [Reilly, 2002]; this view has been adopted in the recent low-frequency exposure standards of the IEEE [IEEE, 2002].

Not all scientists agree with this view. Indeed, standards published by the International Commission on Non-Ionizing Radiation Protection [ICNIRP, 1998a], include the spinal cord as well as the rest of the torso in basic restriction protection levels which were derived from

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electrophosphene reactions. One consequence of this decision is that the ICNIRP basic restrictions for exposure of the torso are lower than those of the IEEE in some portions of the frequency spectrum.

Subsequent to the publication of their standard, ICNIRP published responses to questions [ICNIRP, 1998b]. One question and answer were as follows:

Q. Is the basic restriction of 10 mA/m2 based only on the threshold for acute effects in the central nervous system, or does it apply to other tissues in the trunk of the body?

A. The basic restriction of 10 mA/m2 is intended to protect against acute exposure effects on central nervous system tissues in the head and trunk of the body, with a safety factor of 10. ICNIRP recognises that this basic restriction may permit higher current densities in body tissues other than the central nervous system under the same exposure conditions.

It would appear from this reply that 10 mA/m2 applies only to the brain and spinal cord, and that higher basic restrictions may be tolerated elsewhere in the trunk. However, the standard does not specify what these higher values may be.

3. Temporal and spatial relationships in electrostimulation-based standards Exposure standards should be written with a view to tests for compliance. Such tests might include calculation methods, measurement procedures, or devices that could, at least in principle, demonstrate whether exposure limits of a standard are met. Two practical issues in such a demonstration would include questions of temporal and spatial averaging – what are the extents of time and space over which measurements may be taken?

3.1. Temporal relationships in compliance tests

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Electromagnetic exposure standards typically include limits in terms of rms metrics. An rms measure is a mathematical operation on a series of measurements (or a temporal sequence of data) in which the square root of the arithmetic mean of the squares of the measurements or data is taken. The mean referred to in this definition is associated with a time period over which the measurements are to be taken. For repetitive waveforms, this is simply the repetition period. For a repetitive waveform of mixed frequencies having integer relationships, the averaging period would be the repetition period of the lowest frequency component.

Another aspect of temporal averaging treats the question: is there a cumulative effect such a single instantaneous measure of exposure does not sufficiently represent the hazard? With ionising radiation, for instance, it is widely accepted that the health hazard is related to cumulative, rather than instantaneous dose. Is there an integration effect with electrostimulation?

To explore this question, I investigated the threshold effect of a train of pulses using the SENN model [Reilly, 1998a, Chapt. 4]. The excitation threshold of repeated pulses of the same polarity decreases with the number of pulses, but only if the multiple pulses occur over a time period less than a few tenths of a millisecond. The rheobase of a pulse train having a sufficiently rapid repetition rate, however, converges to that for a single long pulse. For repetition periods beyond about 0.5 ms, the individual pulses of a train appear as independent excitatory events.

A nonlinear type of integration effect is seen with sinusoidal waveforms. For sinusoidal stimulus waveforms, thresholds of nerve excitation evaluated at half-cycle increments oscillate between gradually falling maxima at odd numbers of half cycles, and minima at even number of half cycles, and converge to a single minimum threshold at about 1.3 ms of stimulus duration (Fig. 7). This suggests a maximum integration time of 1.3 ms for electrostimulation of nerve.

We lack data on integration effects with electrostimulation of muscle and nerve synapse similar to that offered by the SENN model, although single cell models have been developed for the electrodynamics of various tissues in the heart [Reilly, 2002, Chapt. 3]. Lacking appropriate data on such integration effects, I infer these properties from a comparison of the S-D time constants of nerve, muscle, and synapse. The S-D time constants for muscle and nerve synapse exceed those for nerve stimulation by factors of 20 and 168, respectively (Table 2). Consequently, a measurement averaging duration of 200 ms (≈ 168 x 1.3) would encompass the maximum integration duration needed to characterise minimum nerve, muscle, and synapse excitation thresholds.

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3.2. Spatial relationships in compliance tests

Recent exposure standards [IEEE, 2002] specify basic restrictions in terms of the in situ electric field. When determining compliance with the basic restrictions, the analyst must consider an averaging distance, da, over which the in situ electric field should be measured. A related parameter is the required distance over which the electric field must exist for efficient electrostimulation.

The relationship between the threshold of excitation and the distance over which the field exists (de) has been determined from the SENN model [Reilly and Diamant, 2002]. A minimum threshold was obtained with de of seven or more internode spaces at the terminus of the axon. With de of one internode space, the rheobase threshold was twice the minimum value of 6.15 V/m for a field existing over the affected axon (Table 1). I shall refer to this minimum threshold as a reference field, Er. With de = 2, 3, 4, and 5 internode spaces, the threshold exceeded Er by 34, 14, 7, and 3%, respectively. In the same study, we showed that when a spatially-limited field existed at a central portion of the nerve axon, the excitation threshold was greater than when it existed at the terminus.

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Figure 11 Errors in threshold measurements resulting from assumed averaging distance, da; 20 µm nerve fibre; stimulation at fibre terminus; de = distance over which the in situ field exists. [From Reilly and Diamant, 2003].

To illustrate a rationale for choosing da, consider an electric field of extent de at the terminus of a 20-µm axon and just at the threshold of excitation. Assume the field is measured with various values of da, shown on the horizontal axis of Fig. 11; the vertical axis gives the percentage, p, by which Ea deviates from Er, where p = 100[(Ea/Er) – 1]. With an averaging distance of 5 mm, p is at most 24% (with de ≈ 5.5 mm) and at least –17% (with de = 2 mm). Recall that positive p values are conservative, i.e., result in an overestimate of the threshold field, and negative values are permissive. By increasing da above 5 mm, positive p values can be reduced, but at the expense of much more deviant negative values. An averaging distance da = 5 mm is neither unduly conservative nor permissive.

Note that an overestimate of the measurement is conservative, i.e., would signal the analyst that the induced field is suprathreshold, even though it was at the threshold of excitation; an underestimate of the measurement would be permissive, i.e., would signal that a larger field could be tolerated.

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That an averaging distance of 5 mm is a reasonable choice suggests a simple metric for determining compliance with basic restrictions – the potential difference at a spacing of 5 mm within the biological medium. Such a potential difference could, in principle, be determined with computational models, phantom models, or in vivo measurements in animals exposed to electromagnetic fields.

4. Contact current and spark discharges Electric and magnetic fields induce electrical forces in conducting objects, as well as in the human body. A person who contacts such an object may conduct current while contact is made with the object, and possibly may receive spark discharges just before and after physical contact with the energised object. We call these indirect effects, to distinguish them from the fields that are induced in the body by the direct action of the environmental field. The severity of such an encounter will depend on the size of the induction object, as well as the degree of insulation from ground of the person and conducting object.

4.1. Contact current perception

Human sensitivity to electric current applied to the skin (electrocutaneous stimulation) depends on a host of factors, including: the stimulus waveform, the size of the electrode, the hydration of skin beneath the electrode, the location of contact on the body, the skin temperature, tactile force of contact, and the size of the subject. I have discussed the effects of these variables previously [Reilly, 1998a]. In addition, we observe variations in sensitivity among subjects that defy causal explanations, and for which we must resort to statistical representations.

One of the variables mentioned above is the location on the body at which the stimulus is applied. In our own research, my colleagues and I have observed that there is approximately a four-to-one variation in electrical thresholds across the points we tested on the body, and they are generally ranked with respect to the distance from the stimulus point to the brain. The threshold at the finger tip is approximately in the middle of this distribution, and this location is a natural contact point in many applied situations.

Both sensory and motor reactions with electrocutaneous stimulation are due to neuroelectric excitation. Consequently, mechanisms and waveform relationships discussed in Sect. 2, including S-D and S-F relationships, apply to electrocutaneous stimulation as well.

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The threshold current into a contact electrode varies inversely with the contact area. A touch contact area of 1 cm2 is a reasonable assumption for a light fingertip contact, whereas a much larger contact area (≈ 15 cm2) is more representative of a grasped contact.

Numerous experiments with perception of sinusoidal current reveal a strength-frequency law with a minimum plateau below a critical frequency, fe, above which thresholds converge to a frequency-proportional law when the current is of a continuous nature [Reilly, 1998a]. Based on nerve excitation models, S-D and S-F constants are connected by fe = 1/(2τe). Consequently, factors leading to small values of τe would increase fe. Experimental values of fe vary significantly, although the factors accounting for this variation are not well understood. The constant fe = 3 kHz is a reasonable estimate of the upper S-D frequency parameter within experimental and theoretical values, and that value has been used in a recent IEEE standard [IEEE 2002].

Table 3. Example rheobase contact current perception thresholds for adult men and women. Median RMS values calculated from the data of Chatterjee et a . [1986]. l

Contact method Males

(mA)

Females

(mA)

Touch

1.03

0.83

Grasp

2.77

2.67

Table 3 lists rheobase thresholds of perception for touch and grasp contacts as extrapolated from experimental data [Chatterjee et al., 1986]; the data apply to the median among a population of healthy men and women, and are consistent with other experimental data [Dalziel and Mansfield, 1950]. Chatterjee and colleagues tested thresholds only to a minimum frequency of 10 kHz; an adaptation of their threshold curve is labelled in Fig. 8 as df = 1. As expected from neuroelectric properties (Sect. 2.0), measured perception thresholds below 100 kHz and above fe are directly proportional to frequency. This fact allows us to extrapolate Chatterjee’s curve below 10 kHz to arrive at an estimate of rheobase, provided we know the S-F constant, fe. The rheobase values in Table 3 are based on fe = 3 kHz.

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Table 3 should be interpreted according to the following equations

IT = Io for f ≤ fe (10a)

IT = Io(f/fe) for f ≥ fe (10b)

where IT is the median threshold current at frequency f.

The perception thresholds listed in Table 3 for women are lower than those for men. This observation is consistent with experiments conducted in my own laboratory [Larkin et al, 1986]. However, we determined that what seemed to be gender related was actually related to body size, not to gender, per se – small individuals tend to have lower thresholds than large ones, and women tend to be smaller than men. If we compare men and women of similar body stature, their electrical sensitivity is statistically indistinguishable.

4.2. Electric field induction

Since environmental electric fields induce in situ electric fields and body currents, it might seem logical to conclude that the induced field should be limited so as to preclude direct electrostimulation effects. In practice, however, contact current and spark discharges (indirect electrostimulation) limit environmental electric fields to values significantly lower than what is required to directly induce in situ electric fields at the levels in Tables 1 and 2.

Indirect stimulation effects occur through charge transfer between a person and a conducting object within the field. With sufficiently strong fields, an individual can perceive spark discharges just prior to the moment of direct contact and just after breaking contact with conducting objects that are well insulated from ground. It is also possible to perceive current through direct contact with such objects.

The contact current component, Ic, for an erect person touching a grounded conductor in a vertically polarised electric field is [Reilly, 1998a]

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Ic = 9.0 x 10-11 h2 f E (11)

where h is the height of the person, f is the frequency of the field, and E is the environmental field strength. For fields at the relatively low frequencies treated in this chapter, and in which the environmental field magnitude varies over the area that would be occupied by the body, the field strength in Eq. (11) may be replaced with the average environmental field over the area in which the body is placed [Kaune, 1981; Deno and Zaffanella, 1982].

4.3. Spark discharges

A person contacting a conducting object within a strong electric field may experience spark discharges just before and after the moment of physical contact; during physical contact, current will be conducted by the subject at the point of contact. The intensity of spark discharges will depend on the voltage developed on the induction object and its capacitance to ground; the magnitude of the contact current will depend on the effective area of the induction object, and the frequency of the field. These effects are described by electrical induction equations [Reilly, 1998a, Chapt. 9].

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Figure 12 Perception threshold for stimuli induced within a 60 Hz electric field; electrode tapped with the fingertip. Curves for k = 1/2 and 1/4 indicate effect of leakage resistance. The curve for k = 1 applies to an induction object that is perfectly insulated from ground. [From Reilly and Larkin, 1987].

My colleagues and I studied human reactions to such stimuli at 60 Hz in relation to the electric fields produced by high voltage power transmission lines [Reilly and Larkin, 1987]. Figure 12, taken from that study, illustrates median perception thresholds among healthy adults. The stimulus in every case mimicked the combination of spark discharges and contact current that can be experienced within a 60 Hz electric field. The horizontal axis of the figure is the capacitance of the discharging object. The induction object can be any conducting object that is

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adequately insulated from ground, or it can be the human body itself. In the latter case, the capacitance of the human body standing on, but insulated from ground, it is typically in the range 100 – 150 pF. Spark discharges and contact current can occur when the insulated person touches a grounded conductor.

The parameter k in Fig. 12 takes into account leakage between the induction object and ground, where Vo is the open-circuit voltage measured on the object, Is is the short-circuit current, ω = 2πf, and Co is the capacitance of the induction object (i.e., the horizontal axis of the figure). For a well-insulated object (large leakage resistance), k = 1, which applies to the worst-case situation. The fact that thresholds are lower with reduced leakage resistance (k < 1) does not mean that reduced resistance makes the shock exposure worse. If the induction electric field is held constant, decreased leakage resistance reduces the induced voltage even more than it does the threshold. For example, comparing k = 1/4 with k = 1, the induced voltage is reduced by the multiple 0.25, but the perception threshold at Co = 3,200 pF is reduced by the multiple 0.52 (as seen in Fig. 12).

5. Statistical relationships in electrostimulation Electrical thresholds can vary significantly from one person to another. Although we can identify factors accounting for some of this variation, much of it is not well understood, and we must resort statistical formulations. The statistical distribution of electrical reaction thresholds is typically represented by a lognormal distribution, i.e., one in which the logarithm of a statistical variate has a normal distribution.

The mean of a lognormal distribution always exceeds the median. The mean-to-median ratio of a lognormal distribution, ρ, is expressed by [Hastings and Peacock, 1975]

(12)

where σ is the variance of the natural logarithm of the statistical variate. For a distribution in which the ratio of 50% to 1% values equals three, the mean-to-median ratio is 1.12, i.e., the mean exceeds the median by 12%. This relationship is useful in cases where an experimental mean is given, rather than a median.

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Experimental thresholds correspond well to the lognormal distribution in many instances of electrostimulation, although it is often necessary to replot published data on lognormal coordinates to demonstrate this. The lognormal distribution requires two parameters to describe it. I have found it convenient to describe lognormal distributions in terms of the median, and a slope parameter defined as the ratio of the median to the one-percentile rank, x50/x1.

Table 4. Example lognormal parameters for electrostimulation

Electrical Reaction Species x50/x1 citation

Forearm perception, contact current

human 3.0 a

Fingertip perception, contact current

human 2.0 a

Perception & pain time-varying mag. fields human 1.9 b Electroconvulsive therapy seizure

human 2.0 c

Contact current perception

bovine 2.3 d

Ventricular fibrillation

canine 2.0 e

a = Larkin et al. [1986]; b = Nyenhuis et al., [2001]; c = Weaver & Williams, [1986]; d = Reinemann et al., [1998]; e = Dalziel [1968];

Table 4 summarises lognormal slope parameters derived from published data from a number of sources [Dalziel, 1968; Larkin et al., 1986; Nyenhuis et al., 2001; Reinemann et al., 1998; Weaver and Williams, 1986]; the slope parameter has been determined from a lognormal fit to the published data between the median and one-percentile ranks. Note that a slope parameter of 3 represents an observed maximum slope, although a more typical condition would have a slope parameter of about 2.

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Table 5. Normalised distribution of electrical reaction thresholds using log-normal model for healthy adult population (male & female).

Percentile Rank (%)

Threshold multiplier

perception & pain

Threshold multiplier

ventricular fibrillation 99.5 3.45 2.33 99 3.11 2.14 95 2.24 1.67 90 1.85 1.51 75 1.40 1.24 50 1.00 1.00 25 0.72 0.80 10 0.54 0.66 5 0.45 0.60 1 0.32 0.47 0.5 0.29 0.43

Perception distribution based on human experimental data for arm contact. Ventricular fibrillation distributions from healthy dog hearts.

Source: Reilly [1998a]

Table 5 provides examples of log normal models (medians normalised to 1.0) applicable to sensory stimulation of the forearm of healthy adult humans, and to ventricular fibrillation (VF) in healthy dogs [Reilly, 1998a]. Experimental data for fingertip perception more closely follow the VF values. Compared with data from healthy animals, a much broader distribution of VF thresholds has been reported for direct electrode contact to the hearts of human patients undergoing open-heart surgery for valve replacement [Watson et al., 1973]. Thresholds for persons in a pathological state or under drug treatment have not been otherwise tested.

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Figure 13 Lognormal distributions. The vertical axis follows a standard normal model; the horizontal axis applies to the statistical variate normalised by its median. Lognormal distributions are represented as straight lines on this plotting format.

Figure 13 shows the form of lognormal distributions, which are represented by straight lines on the plotting format. The horizontal axis applies to the statistical variate normalised by its median value (x/x50) It is tempting to extrapolate the distribution model of Fig. 13 to arbitrarily small percentile ranks. However, experimental evidence is insufficient to support extrapolation much below the rank of about 1% due to limitations in the numbers of subjects represented in available experimental data.

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Variations in thresholds from one individual to another are not well understood. The only significant physiological parameter that has been correlated with electrical thresholds is body size and related parameters, such as gender, and age [Larkin et al., 1986; Reilly and Larkin, 1987; Reilly, 1998b]. The correlation is such that small individuals tend to have lower thresholds. A body size relationship is found in sensory reactions, let-go thresholds, and ventricular fibrillation. Experimental evidence indicates that thresholds of pain in humans, and VF thresholds in animals vary approximately with the square-root of body weight, although other relationships have been proposed [Reilly, 1998a]. Let-go thresholds in humans vary approximately in proportion to body weight [Dalziel, 1972]. Consequently, small individuals, especially children, would be most susceptible to electrical stimulation effects. On the other hand, the magnitude of current induced by electric or magnetic fields diminishes with decreasing subject size. And with contact current, the small individual typically has a greater inter-limb resistance than a larger person. Because of these compensating factors, the effect of body size is not expected to be great. Indeed, a study of the relationship between magnetic field perception thresholds and morphological factors (subject gender, girth, weight, and age) demonstrated a lack of significant correlation with any of these factors [Nyenhuis, 2001].

6. Exposure to environmental fields The fundamental dosimetric metric for electrostimulation effects is the in situ electric field. Restrictions so stated are known as basic restrictions. However, it is often simpler for an the individual who must enforce a standard to have dosimetry requirements stated in terms of the environmental field. When deriving environmental limits from basic restrictions, it is often expedient to make simplifying assumptions that are relatively stressful, but not unrealistic. Nevertheless, environmental limits may sometimes be unnecessarily restrictive. To guard against excessively restrictive environmental limits in dosimetry standards, it is desirable to provide in a dosimetry standard both basic restrictions and environmental limits and to include a statement along the following lines [IEEE, 2002]:

Lack of compliance with environmental limits does not necessarily imply lack of compliance with basic restrictions, but rather that it may be necessary to evaluate whether the basic restrictions have been met. If the basic restrictions are not exceeded, then the environmental limits may be exceeded.

To derive an environmental magnetic field from in situ E-field magnitudes, it is necessary to apply an induction model. Traditional methods used to predict whole body energy absorption during magnetic field exposure include the use of ellipsoid shapes arranged to mimic an animal or man [Reilly, 1991]. During the past several years, high-resolution anatomical models have been developed to enhance the capability to predict localised energy absorption, such as within a single organ or part of an organ [Dawson and Stuchly, 1998; Dimbylow, 1998; Gandhi and Kang, 2001; Gandhi et al., 2001].

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6.1. Magnetic field exposure

Detailed anatomical induction models

The development of high-resolution anatomical induction models has tremendously enhanced our understanding of energy absorption during electromagnetic field exposure. These models make use of high-resolution three-dimensional specifications of tissue conductivities, along with advanced computational methods. With these models it has been possible to compute the electric field and current density induced within all organs of the body and with high resolution [Dawson and Stuchley, 1998; Dimbylow, 1998; Gandhi and Kang, 2001; Gandhi et al., 2001].

Although the high-resolution model might seem the perfect tool for dosimetry studies, a number of questions about these models remain. One question concerns the issue of variability in model calculations among different analysts. A comparison of induced electric field calculations obtained by several investigators using a similarly detailed anatomical model and similar numerical techniques [Dawson and Stuchly, 1998; Dimblylow, 1998; Gandhi, 2000] showed differences of over 5:1 in the maximum field in critical organs; organ averages were usually reasonably consistent, although differences as great as 2:1 were noted. Since the basic restrictions pertaining to electrostimulation effects depend on the maximum field in particular organs, large variations in reported maximum values make it difficult to apply presently available detailed models to dosimetry standards. Although variations in the calculated induced electric field can be attributed to differences in voxel size [Mason et al., 2000], as well as to differences in assumed permittivity values of the tissues [Hurt et al., 2000], the source of the variation of results among investigators has not presently been identified.

Considering these variations, it is necessary to validate results from high-resolution computational methods with measurements in living animal models. For example, measurements of the electric field induced in the spinal cord of an intact animal might be compared with results obtained from a high-resolution computational model.

Despite these considerations, high-resolution anatomical models are extremely useful in assessing the distribution of induced fields within the body under a great variety of conditions related to the environmental field, and the position and attitude of the human body within the field. Undoubtedly, standard computational approaches will soon be defined for these models, and they will be validated with experimental results.

Before detailed anatomical models can be relied on to determine environmental limits from basic restrictions, it may be necessary to reinterpret experimental data that were originally used to

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determine in situ thresholds on the basis of simpler homogeneous conductivity models. Such reinterpretation would ensure the use of the same induction model for the calculation of in situ thresholds from exposure data pertaining to the environmental field, and environmental limitations based on basic restrictions. Some of the experimental data that may be subject to reinterpretation are discussed in the following section, as well as in Sect. 3.

Ellipsoidal induction model

Lacking results from a validated high-resolution model, a recent standard [IEEE, 2002] used an ellipsoidal induction model to associate in situ electric field specifications with an environmental field. This involves an ellipsoidal model of the head and torso of a large individual, with uniform conductivity, and a constant magnitude and relative phase of the field over the body dimensions as described in Appendix A. In all calculations, a worst-case assumption has been made for the direction of the field relative to the body.

Figure 14 Example of position of the heart within three cross-sections of the body; a and b are semi-major and –minor axes of equivalent cross-section ellipsoids. Numbered locations are identified with calculated electric field intensity (V/m) for magnetic exposure within the ellipse of dB/dt = 100 T/s. Direction of field is perpendicular to cross section. View of internal organs shows conductive paths through the heart. [From Reilly, 1998a].

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Figure 14 illustrates the ellipsoidal model applied to a large man [Reilly, 1998a, Chapt. 9]. Body dimensions in applied situations may differ from this example, and calculations would vary accordingly. Furthermore, the location of the heart within the torso will vary as the body position is changed from prone to erect, and will also move during the cardiac pumping cycle. Calculated electric fields in Fig. 14 have been carried out by representing body cross sections as ellipses. Numbered points indicate positions where the E-field (indicated in V/m) has been calculated according to Eq. (A.1) with dB/dt = 100 T/s.

Calculated E-fields generally increase as the measurement location is moved toward the periphery of the body, and are especially great along the minor axis of the ellipse. Points of maximum E-field on the perimeter of the body are points 14, 9, and 3 in the three cross sections; the maximum E-field at the heart occurs at points 10, 6, and 1.

An underlying assumption in the data in Fig. 14 is that the incident magnetic field is constant in both magnitude and phase over the torso cross-section. A somewhat more conservative calculation can be made by assuming the entire body is uniformly exposed to the magnetic field by using an equivalent body ellipse equal to the height of the person. As noted in Appendix A, such an assumption somewhat increases the calculated E-field. For instance, the calculated increase in the E-field was 11% at point (10), and 14% at point (14).

Another implicit assumption is that the internal conductivity of the body is homogeneous. In reality, the body is composed of various organs having diverse conductivities, as indicated in Fig. 14, and these variations will distort the internal E-field with respect to a homogeneous structure. For instance, in the sagittal cross section, the E-field will be enhanced between the skin and the spinal column as induced current is crowded around the low-conductance vertebrae and the curvature of the torso in the lower lumbar region. Indeed, Schaefer and colleagues[1994] report that sensory perception from magnetic stimulation is enhanced in regions where bony projections are just below the surface of the body.

It should be clear from Fig. 14 that the heart is part of a large electrically conductive circuit that includes the blood vessels, diaphragm, liver, and intestines, and it should not be treated as an isolated conducting organ suspended in a non-conductive medium (the lungs) as some have opined. With longitudinal magnetic exposure (Fig. 14c), the heart is part of a circumferential electrical circuit that includes the pericardium and muscle lining the thorax.

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Using the ellipsoidal model, an in situ field of 6.15 V/m (the assumed median threshold for nerve excitation) has been calculated to be induced in the periphery of the torso with whole-body exposure to dB/dt = 37.5T/s (see Table A.1, item 3). That theoretical value applies to conditions of exposure that minimise the excitation threshold, namely: a very large adult; constant magnitude, direction, and relative phase of the incident field over the dimensions of the body; a monophasic square-wave shape of the in situ electric field. In most cases, experimental conditions deviate from the optimal parameters resulting in greater thresholds than the minimum ones.

One of the cited optimal conditions was a monophasic square wave shape for the induced electric field. Note that the induced field follows the waveform of the time derivative of flux density, dB/dt, which is necessarily biphasic for a magnetic pulse; the mean of dB/dt is zero if the rise and fall magnitudes of flux density are equal, although the rise and fall times need not be equal. If the induced waveform is such that the phase reversal is either delayed or is gradual, then the threshold can be effectively the same as would apply to a monophasic waveform. Consequently, it is generally conservative, but not unreasonable, to assume a monophasic waveshape for dB/dt.

The conservatively derived theoretical value of 37.5 T/s may be compared with experimental thresholds conducted with pulsed magnetic field exposure of the human torso in MRI studies [Bourland et al., 1990, 1991a, 1991b, 1997; Budinger et al., 1991; Cohen et al., 1990; Mouchawar et al., 1991; Nyenhuis et al., 1990; Schaefer et al., 1994, 1995; Yamagata et al., 1991], as previously reviewed [Reilly, 1998a, Sect. 9.7]. Mean perception thresholds of 60 T/s were reported by two investigators [Budinger et al., 1991; Cohen et al., 1990], and a minimum threshold of 45 T/s was reported by another [Bourland et al., 1990]. Higher thresholds were reported by others, but, like the above cited studies, these involved sub-optimum waveforms or conditions not conducive to minimum rheobase values.

Simulated MRI fields used in experiments discussed above varied considerably in amplitude and relative phase over the dimensions of the human torso. The optimum field metric for electrostimulation is not clear when such nonuniformity exists. Recent studies report perception thresholds in terms of the spatially averaged exposure, rather than the spatial peak as in most of the studies mentioned above. Using a spatial average metric, an average rheobase value of the perception threshold was reported at 25 T/s in one study involving 65 subjects [Hebrank, 2000], and 28.8 T/s in another study involving 84 subjects [Nyenhuis et al., 2001].

Cardiac excitation thresholds using magnetic stimulation have been determined in dogs. Early results [Mouchawar et al., 1992; Yamaguchi et al., 1992] indicated dB/dt thresholds in excess of what would be predicted from the models used here (Tables 6 and A.1), although this excess

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could be explained by the use of sub-optimum exposure conditions in the cited studies [Reilly, 1993]. More recent test results with dogs [Schaefer et al., 2000] conformed well with the models used in this chapter when scaled from animal to human dimensions. It was also established that the addition of a 1.5 T static field to the time-varying excitatory field does not alter cardiac excitation thresholds [Bourland et al., 1999].

Table 6. Models for established magnetic dB/dt thresholds of reaction:

whole body exposure; median thresholds(a).

Reaction

(T/s-pk)(b)

τe

(ms)

fe

(Hz) Synapse activity alteration, brain 1.45 25.0 20

10-µm nerve excitation, brain 237 0.149 3350

20-µm nerve excitation, body 37.5 0.149 3350

Cardiac excitation 88.7 3.0 167

(a) Interpretation of Table as follows: for ; for . Also for

; for .

(b) (T/s-pk) refers to the temporal peak of the magnetic flux density.

Table 6 presents dB/dt ( ) thresholds based on the theoretical and experimental data described above. Appendix A describes the methods whereby the external field thresholds of Table 6 are

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derived from the in situ parameters of Table 2. Thresholds are computed from the parameters of Table 6 as

for (13a)

for (13b)

where tp is the phase duration of the waveform. Alternatively, the limits can be determined as

for (14a)

for (147b)

Flux density, B, listed in Table 7 can be computed from the Table 6 criteria using the relationships for sinusoidal fields

(15)

(16)

where is the minimum (rheobase) threshold value of dB/dt, and Bo is the minimum threshold value of B. Median flux density thresholds are computed from Table 7 as

for f ≥ fe (17a)

for f ≤ fe (17b)

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Table 7 Median magnetic flux density thresholds; whole body exposure.

Reaction Bo

(mT-rms)

Ho

(A/m-rms)

fe

(Hz) Synapse activity alteration, brain 8.14 6.48x103 20

10-µm nerve excitation, brain 7.97 6.34x103 3350

20-µm nerve excitation, body 1.27 1.00x103 3350

Cardiac excitation 59.8 4.76x104 167

(a) Interpretation of Table as follows: for ; for .

The flux density limits in Table 7 are based on the assumed in situ limits of Table 2 evaluated at the site of interaction. For instance, the brain exposure limits are based on the estimated field induced in the outer perimeter of the cerebral cortex; cardiac excitation applies to the field induced in the apex of the heart; and peripheral nerve limits are based on the maximum induced field in the periphery of the torso.

6.2. Static or quasi-static magnetic field exposure

Whereas Equation (17b) indicates that flux density thresholds would increase to infinity as the frequency approaches zero, an upper limit on flux density is required in dosimetry standards to avoid adverse effects from magnetohydrodynamic forces on moving charges within a magnetic field. Such movement is typically associated with the vascular system, although observable effects can also result from the rapid movement of the body or eyes within a strong static field. The physical effects are Hall voltages or Lorentz forces.

With static magnetic fields, reactions under laboratory conditions include a 17% increase in human cardiac cycle length at 2 T [Jehesen et al., 1988]. The authors opined that the observed effect is probably harmless in healthy subjects, but that its safety in dysrhythmic patients was not certain. Other observations included a 0.2 - 3% change in blood velocity between 1-10 T [Dorfman, 1971; Keltner, 1990]. A host of adverse effects were noted at 1.5 T, including: vertigo, difficulty with balance, nausea, headaches, numbness and tingling, phosphenes, and

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unusual taste sensations; much more marked reactions were noted at 4 T [Schenck et al., 1992]. Other effects include benign enhancement of the cardiac T-wave in rats at 4 T [Gaffey and Tenforde, 1981, Tenforde et al., 1983].

The studies of Schenck and colleagues report adverse effects in a significant number of subjects at 1.5 T, which is a reasonable choice as a median threshold for adverse effects. A peak value of 1.5 T is associated with a slowly varying sinusoidal field of 1.06 T-rms.

7. Adverse reactions to electrostimulation The purpose of electromagnetic field standards is to avoid adverse reactions, not just perceptible ones. A recent standard has defined an adverse effect as one that is detrimental to the health of an individual due to exposure to an electric or magnetic field, or a contact current [IEEE, 2002]. This same standard takes the position that aversive or painful electrostimulation is considered an adverse effect [IEEE, 2002].

7.1. Adverse nerve reactions

Painful sensations from magnetic stimulation of peripheral nerve are reported at multiples above perception thresholds of 1.3 [Budinger et al., 1991], 1.6 [Bourland et al., 1997], and 1.48 [Nyenhuis et al., 2001; Schaefer et al., 2000] – an average multiple of 1.45. The mean threshold for intolerable pain was observed at a perception multiple of 2.05 [Schaefer et al., 2000]. I calculate the median rheobase threshold for painful sensations as Eo = 6.15 x 1.45 = 8.92 V/m (peak). Based on a log-normal probability model of human perception thresholds of electrical stimuli (see Sect. 6), a conservative estimate of a one-percentile pain reaction threshold for healthy adults would be a factor of 3 below the median, resulting in a rheobase of 2.97 V/m.

In the case of contact current stimulation, unpleasant and painful sensations are elicited at greater multiples above perception than with magnetic stimulation. Based on experimental data from several sources [Reilly, 1998a, Table 7.3], painful cutaneous stimulation is estimated to occur at a multiple of 2.4 above the perception threshold; unpleasant sensations are estimated to occur at a multiple of 1.7; the ratio of pain to unpleasantness thresholds is about 1.4.

That smaller pain-to-perception ratios are found with magnetic stimulation than with contact current stimulation may be explained by the fact that in magnetic stimulation, the distribution of induced current varies only gradually with respect to body dimensions. Consequently, at a field level where some neurons first begin to be excited, a small increase in the field may excite

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neurons over a large area. If pain is magnetically induced in some area of the body, it is likely to be in an extended area. In contrast, cutaneous stimulation is more focal. Suprathreshold stimulation in a large area may be more painful than in a small area, and that might account for the differences in pain-to-perception ratios between magnetic induction and small-area contact current.

7.2. Adverse cardiac reactions

Cardiac excitation is not necessarily hazardous, although ventricular fibrillation (VF) is a life-threatening condition that is usually fatal unless it is quickly reversed with specialised defibrillation equipment. Minimum thresholds for VF typically exceed those for excitation by a factor of 50 or more if the excitation is a single event. However, if the heart is repeatedly excited, the VF threshold drops such that the margin between VF and excitation thresholds may be reduced to a factor as little as two when the stimulus is applied during the vulnerable period within the cardiac cycle [Reilly, 1998a, Chapt. 6].

7.3. Adverse synapse reactions

In connection with phosphene threshold experiments, Lövsund and colleagues [1980a, p. 330] state:

Virtually all the volunteers noted tiredness and some reported headaches after the experiment. Some experienced after-images which were generally of only short duration following exposure to the magnetic field. In one case, however, they persisted up to ten minutes after the experiment. Individual volunteers reported spasms of the eye muscles, probably arising from stimulation by the field.

These findings were similar to those of Silny [1986], who reported headaches, indisposition, and persistent visual evoked potential (VEP) alterations at flux density levels above phosphene thresholds, but still well below nerve excitation thresholds (by a factor of 23).

Clearly adverse reactions that may be attributable to CNS reactions (tiredness, headaches, muscle spasms, persistent after images) are reported in connection with phosphene threshold experiments. It is unlikely that the phosphenes themselves were causing the reported adverse reactions. A plausible explanation is that the adverse effects were due to electrostimulation of brain neurons in accord with the synapse mechanism discussed previously.

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The ability of sub-excitation fields to alter neuronal response has also been reported after exposure of hippocampus slices from the rat brain to magnetic fields [Bawin et al., 1984; 1986] in which induced E-field intensities were as low as 0.75 V/m peak – a factor of 16 below the threshold of 12.3 V/m for excitation of a 10-µm neuron. The rate of maze learning in living mice was significantly reduced by exposure to flux densities at and below 0.75 mT at 50 Hz [Sienkiewicz et al., 1998a,b]. Although the cited studies did not establish a synaptic mechanism, they do support the view that CNS effects, including adverse ones, are possible well below thresholds of excitation of brain neurons.

7.4. Adverse magnetohydrodynamic effects

Magnetohydrodynamic effects and forces on charges due to rapid body motion in strong static and quasi-static fields produce a variety of biological effects. As reviewed in Sect. 7.2, adverse reactions, including nausea, vertigo, and taste sensations associated with head movements, occur at 1.06 T-rms (1.5 T-peak) in approximately 50% of human subjects at frequencies below one Hz [Schenck, 1992].

8. Development of exposure standards The preceding paragraphs of this Chapter treat thresholds of human reactions to electrostimulation. This information alone is not sufficient to derive dosimetry standards; indeed, the analyst must consider additional factors, as discussed below. The discussion that follows adopts the rationale of a recent standard [IEEE, 2002], the development of which in recent years has occupied a part of my life far beyond any expectations I had when I undertook this responsibility. I must admit my bias in this approach, and restate that some of the conclusions, although they have satisfied the vast majority of IEEE committee members (now known as ICES – the International Committee on Electromagnetic Safety), are not universally accepted. I also recognise that some of the ideas discussed below cannot be demonstrated by syllogistic logic, but rather require scientific judgment.

One of the issues addressed by ICES that required judgment concerned the question of whether single or multiple tiers should be used in the standards. The committee at an early stage decided to afford protection to individuals in the general population and to groups in “controlled environments,” which were defined as:

areas accessible to those who are aware of the potential for exposure as a concomitant of employment, to individuals cognizant of exposure and potential adverse effects, or where exposure is the incidental result of passage through areas posted with warnings, or where the environment is not accessible to the general public and those individuals having access are aware of the potential for adverse effects.

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It was assumed that for the controlled environment, education and various mitigating measures could be taken to reduce the probability of adverse reactions of exposed individuals, although the exposure limits were intended to protect against adverse effects for almost all people.

The two-tier approach adopted by ICES for its low-frequency standard differed significantly, however, from the approach used by other standards [e.g., ICNIRP, 1998; IEEE, 2002]. Other standards typically define adverse reaction thresholds, and then apply a safety factor for application to exposed individuals in work environments, or in controlled environments. They then apply an additional safety factor for application to the general public.

The approach taken in the ICES low-frequency standard instead defined an acceptable level applicable to all people, in which a probability factor and safety factor had been applied to median adverse reaction levels. Under some conditions in controlled environments, higher limits were allowed. ICES considered this approach acceptable because its standard was based on avoidance of short-term reactions that would be immediately apparent to the exposed individual, rather than chronic exposure health effects as sub-perception levels, and where cumulative exposure might be significant. It was assumed that, because the short-term reactions would be apparent to exposed individuals, they could remove themselves from the environment, modify their activities, or could take other action to avoid the exposure entirely.

8.1. Basic restrictions

Table 8 Factors for converting median thresholds to maximum permissible exposure levels (MPEs) as adopted by ICES [IEEE, 2002]

A B C D E F

Safety factor

G

Rheobase basic restr.

(RMS values) Reaction Locus med. rms

rheobase

Eot

(V/m)

Adverse

mult.

Fa

Prob.

Mult.

Fp

Gen.

public

Fs

Contr.

Environ

Fs

Gen.

Public

Eob

(V/m)

Contr.

Environ.

Eob

(V/m)

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Synapse

Alter.

Brain 0.053 1.0 0.333 0.333 1.00 5.89x10-3 1.77x10-3

10-µm neuron

excite.

Brain 8.70 1.0 0.333 0.333 1.00 0.970 2.90

20-µm

neuron

pain

Body 4.35

(percept.)

1.45

(pain)

0.333 0.333 1.00 0.700 2.10

20-µm

neuron

pain

Hands,

feet,

wrists,

ankles

4.35

(percept.)

1.45

(pain)

0.333 1.00 1.00 2.10 2.10

Cardiac

excite.

Heart

apex

8.49 1.00 0.333 0.333 0.333 0.943 0.943

Table 8 demonstrates the approach that ICES used to derive basic restrictions [IEEE 2002]: column A lists the reaction under consideration; column B lists the locus of stimulation; column C lists median rheobase excitation thresholds, Eot, from Table 2, but converted from peak to rms values using the conversion E(rms) = E(peak)/ ; column D lists multipliers, Fa, applied to column C that convert from a median excitation threshold to a median adverse reaction threshold (Sect. 8); column E lists multipliers, Fp, that convert from a median threshold to a low-probability one (Sect. 6.0); columns F list safety factors, Fs, applied to the general public and in the controlled environment; columns G list rheobase in situ fields, Eob = EotFaFpFs, which are the rheobase basic restrictions.

Table 9 Basic restrictions applying to various regions of the body(a,b)

General public

Controlled

environment

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Exposed tissue fe

(Hz)

Eo

(V/m-rms)

Eo

(V/m-rms) Brain 20

5.89x10-3 1.77x10-2

Heart 167

0.943 0.943

Hands, wrists, feet & ankles 3350

2.10 2.10

Other tissue 3350

0.701 2.10

(a) Interpretation of Table is as follows: for ; for

.

(b) In addition to the listed restrictions, exposure of the head and torso to magnetic fields below 10 Hz shall be restricted to a peak value of 167 mT for the general public, and 500 mT in the controlled environment.

Source: [IEEE, 2002]

Table 9 lists basic restrictions appearing in the ICES standard. Interpretation of the table is as follows:

Ei = Eob for f ≤ fe (18a)

Ei = Eob(f/fe) for f ≥ fe (18b)

where Ei is the allowable in situ electric field at frequency f, and Eob is the rheobase basic restriction listed in columns G of Table 8.

Basic restrictions listed in Table 9 are in terms of in situ induced electric fields; the mode of induction, however, can be through the action of the environmental magnetic or electric field, or

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contact current. In addition to induced electric field specifications, it is also necessary to restrict the in situ magnetic field to avoid adverse reactions due to magnetohydrodynamic effects from very low frequency magnetic fields (see 7.2), as noted in the notes below Table 9. It is not necessary to specify magnetic field basic restrictions at greater frequencies, because potential adverse effects would be related to the induced electric field, rather than the in situ magnetic field itself.

The following paragraphs summarise the rationale for the multipliers in Table 8.

Adverse reaction factor

Pain is considered an adverse response with peripheral nerve excitation. An adverse reaction multiplier of Fa = 1.45 is applied to the nerve excitation threshold to derive a pain threshold (see 8.1). With synaptic effects, brain stimulation, and cardiac excitation, excitation itself is considered adverse as noted in 8.3; hence the adverse reaction multiplier of Fa = 1.0 is applied to the excitation threshold for these reactions.

Probability factor

A probability factor, Fp, is applied to convert from a median threshold to a low-probability one. For a lognormal distribution in which the slope parameter (median-to-one-percentile ratio) is 3, the multiplier of 0.333 applied to the median threshold corresponds to a one-percentile most sensitive subject. Whereas a slope parameter of 3 is observed in some cases (e.g., contact current perception on the forearm), with other reactions (magnetic field perception, cardiac VF, brain ECT thresholds), the slope parameter is very close to 2.0 (see Sect. 6). With a slope parameter of 2, a multiplier of 0.333 applied to the median threshold results in a 0.01% probability rank.

Safety factor

A safety factor multiplier of Fs = 0.333 provided a margin of protection for exceptionally sensitive individuals, uncertainties concerning threshold effects due to pathological conditions or drug treatment, uncertainties in the reaction thresholds, and uncertainties in the induction models. In the case of the hands, wrists, feet, and ankles, ICES chose Fs = 1 for the general public in recognition of the narrow cross sections and preponderance of low conductivity tissue that tend to enhance the in situ E-field in these areas in comparison with other areas of the body. The committee reasoned that because these regions lack critical function when compared with the vital organs, a greater localised electric field could be permitted. In the case of the controlled environment, Fs = 1 for all of the reaction types except for cardiac excitation under the

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assumption that a small probability of discomfort would be acceptable in the controlled environment for some mechanisms, but that cardiac excitation would be unacceptable for all individuals. The safety factor Fs = 1 was justified for the indicated exposures as explained above.

If the safety factor Fs = 0.333 is to be compared with that applied in standards at higher frequencies [e.g., ICNIRP, 1998; IEEE 2002], note that a divisor of 3 applied to the magnitude of the induced field is equivalent to a divisor of 9 in the specific absorption rate (SAR) because SAR is proportional to the square of the induced field.

8.2. Maximum permissible exposure levels

Sophisticated test equipment or computational capabilities may sometimes be required to assess whether basic restrictions are met. So that standards can be more easily interpreted, it is desirable to define maximum permissible exposure (MPE) levels in terms of the environmental field, rather than the induced in situ field. Most users can more easily demonstrate compliance with a standard through environmental field measurements.

Environmental magnetic field MPE limits

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Figure 15 Median thresholds for adverse stimulation from magnetic field exposure (broken lines) and recommended maximum permissible exposure limits (solid lines); whole-body exposure to spatially constant field.

Figure 15 illustrates the derivation of MPE levels for magnetic fields. The figure shows median thresholds of adverse reaction (broken lines), and MPE levels (solid lines) with whole body exposure. The MPEs have been derived from the minimum adverse thresholds at each frequency, decremented by the appropriate probability and safety factors in Table 8. The curve for synapse alteration has been extended to 1000 Hz. The MPE curves have been derived from the lowest adverse reaction threshold across the frequency spectrum as follows: 0 - 0.153 Hz, magnetohydrodynamic effects; 0.153 - 759 Hz, synapse alteration; above 759 Hz, peripheral

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nerve pain. Note that the MPEs in the controlled environment correspond to low probability reaction thresholds (≤ 1%). The limits applicable to the general public are lower by a factor of three.

Table 10 Magnetic maximum permissible exposure (MPE) levels: exposure of head and torso.

General public Controlled environment Frequency range

(Hz)

B(rms)

(mT)

H (rms)

(A/m)

B (rms)

(mT)

H (rms)

(A/m)

<0.153 118 9.39x104

353 2.81x105

0.153 - 20 18.1/f 1.44x104/f

54.3/f 4.32x104/f

20 - 759 0.904 719

2.71 2.16x103

759 - 3350 687/f 5.47x105/f

2060/f 1.64x106/f

3350 – 105

0.205 163 0.615 490

(a) Limits from 0 – 3000 Hz correspond to ICES standard, [IEEE, 2002].

(b) MPEs refer to spatial maximum.

(c) f is frequency in Hz.

Table 10 lists MPE values for whole-body exposure to sinusoidal magnetic fields. Recall that the listed values incorporate conservative assumptions such that adherence to them insures that the basic restrictions are not exceeded. Since the MPEs have been conservatively derived, it is possible that one may exceed them, and still not exceed the basic restrictions.

The upper frequency of the ICES low-frequency standard was 3 kHz. However, the frequency limits in Table 10 have been extended to 100 kHz for purposes of this chapter. The extension is consistent with the principles expressed previously in this chapter.

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Above 100 kHz, thermal thresholds are lower than those for electrostimulation when the exposure waveform is a continuous sinusoid. At these higher frequencies, published standards define MPE limits on the environmental rms field that are inversely proportional to frequency [e.g., ICNIRP, 1998; IEEE 2002]. The reason for this decrement is to ensure that the rms value of the induced field (i.e., its heat-producing capacity) remains constant. However, if the exposure waveform is a pulsed sine wave, the crossover frequency between thermal stimulation and electrostimulation increases, and it may be necessary to specify additional compliance specifications, as noted in Sect. 2.6.

Environmental electric field MPE limits

In the absence of contact current and spark discharges (Sect. 5), environmental E-fields can sometimes be perceived through vibration of body hair caused by the action of the field on charged hair follicles. With a sufficiently strong field the sensation can be annoying to some people. For instance, at 20 kV/m in an outdoor environment, 50% of standing adults can perceive a 60 Hz field, and about 5% will consider the sensation annoying [Reilly, 1978; Deno and Zaffanella, 1982]. Although 20% of subjects perceived a 60-Hz electric field at 9 kV/m, less than 5% could detect electric fields of 2 or 3 kV/m [Reilly, 1978]. With hands raised above the body, the median perception threshold is 7 kV/m.

The most significant electrostimulation effects from environmental electric fields result from contact currents and spark discharges. These effects can be most severe when a grounded person touches a large conductive object that is well-insulated from ground and is within a strong electric field. The magnitude of the effect depends on the effective area of the induction object, and its capacitance to ground in accord with well-known physical principles [Reilly, 1998a, Chapt. 9].

It is not possible to absolutely protect against all possibility of adverse stimulation without mitigating the induced charge on the object when very large (or long) objects are situated near sources that produce strong electric fields that are very extended spatially, such as is the case with high-voltage power transmission lines. For instance, one might postulate a long fence wire on insulated posts running parallel to a high-voltage transmission line. In such cases, it is preferable to avoid electrostimulation by properly grounding the conducting object, (as stated in other safety codes) rather than by limiting the electric field to an impractically small level.

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A situation that is difficult to mitigate against is where a person who is insulated from ground touches a grounded object within a strong electric field. The contact current in such a case would follow Eq. (11). The ICES standard specified its electric field MPE level for the general public so as to limit the contact current to 0.5 mA when a person of height 1.75 m touches a grounded conductor within a vertically polarised field. By solving Eq. (11) for these conditions, ICES determined an electric field limit given by

E = 1.84x106/f (19)

If this law were to extended to zero frequency, the electric field limit would approach infinity. An upper limit must be placed on the maximum permissible E-field to limit the probability of an adverse reaction to a spark discharge.

Table 11 Environmental electric field MPEs, whole body exposure as defined in the ICES standard [IEEE, 2002].

general public

controlled environment

Frequency range

(Hz)

E

(V/m-rms)

Frequency range

(Hz)

E

(V/m-rms) 1 - 368(c)

5,000(a,d) 1 - 272(c) 20,000(b,e)

368 - 3000

1.84x106/f 272 - 3000 5.44x106/f

3000 - 105

614

3000 - 105 1813

(a) Within power line rights-of-way, the MPE for the general public is 10 kV/m under normal load conditions.

(b) Painful discharges are readily encountered at 20 kV/m, and are possible at 5 -10 kV/m without protective measures.

(c) Limits below 1 Hz are not less than those specified at 1 Hz.

(d) At 5 kV/m induced spark discharges will be painful to approximately 7% of adults (well-insulated individual touching ground).

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(e) The limit of 20 kV/m may be exceeded in the controlled environment when a worker is not within reach of a grounded conducting object. A specific limit is not provided in this standard.

(f) MPEs above 3 kHz were not specified in the standard [IEEE, 2002].

The field limitations in Table 11, which were provided by ICES for protection against adverse contact current, vary in inverse proportion to frequency as in Eq. (19). A maximum permissible field of 5 kV/m is specified in Table 11 for the general public to limit the probability of painful spark discharges. ICES estimated that spark discharges would be painful to approximately 7% of individuals who are 1.8 m tall, are well insulated from ground, and who touch a grounded object within a 5 kV/m field. Unpleasant spark discharges can also occur when a grounded person touches a large conductive object that is well-insulated from ground and is situated within a strong field.

In the controlled environment where the MPE is limited to 20 kV/m, painful spark discharges, but not contact currents, can be readily encountered at the stated limit for an insulated person at ground level touching a grounded conductive object In such strong fields, ICES recommended that workers should limit the probability of painful spark discharges by appropriate use of protective clothing, grounding measures, contacting techniques, or other work practices that consider these environmental electric field effects. In the controlled environment, conductive suits could be worn that shield the body from high environmental electric fields, thereby greatly reducing indirect electrostimulation.

Power line rights-of-way fall somewhere between the definitions of "controlled" and "uncontrolled" environments for the general public in that public activity can be circumscribed by the utility, but that public access is often allowed for the public benefit. Consequently, ICES specified a limit of 5 kV/m for the general public in regions off the right-of-way, but allowed an intermediate field of 10 kV/m within the right-of-way under normal load conditions. Experimental data using spark discharge stimuli on human subjects [Reilly and Larkin, 1987; Reilly, 1998b] can be applied to this exposure. In a field of 10 kV/m, about 50% of adult subjects (1.8 m tall) who are well insulated from ground would experience painful discharges when contacting a grounded conductor. The stated probability would increase with taller subjects, and decrease with shorter ones. It is also decreased by imperfect insulation of the person with respect to ground.

Static or quasi-static electric fields

The maximum permissible environmental electric field in Table 11 is capped to limit the probability of painful spark discharges. This limit could, in principle, be extended to arbitrarily

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low frequencies since even a single discharge can be painful. However, at a sufficiently low frequency, the time constant, τh, at which a human can maintain a charge will begin to limit the magnitude of the induced charge. The time constant is given by the product of the capacitance and resistance to ground of the person. For example, consider a resistance of 1000 MΩ, which is applicable to 10% of people with normal footwear on dry ground [Reilly, 1979, 1998b, p52], and a capacitance of 150 pF. These assumptions result in a time constant of 150 ms, which is equivalent to a frequency of 1 Hz below which the induced voltage in a given field would fall, and the permissible exposure could rise. However, for people on well insulated surfaces, longer time constants would be possible. The validity of this observation is apparent considering that one may experience an unpleasant carpet spark a second or more after the charge has been acquired.

These observations may be applied to the standards of Table 11 as follows. For leakage resistance of 1000 MΩ, the allowable maximum limits below 1 Hz could be increased approximately in inverse proportion to frequency; for greater resistances, the applicable frequency would become lower.

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Appendix A: Magnetic field induction model The magnetic induction model used in developing this chapter treats an exposed cross section of the body as an elliptical shape, with homogeneous conductivity. A solution for this model, applicable to situations where the wavelength of the field is much greater than body dimensions was published by Durney et al., [1975], and expressed in applied form by Spiegel [1976]. The present form used here is the one expressed by Reilly [1991]. A general expression for the induced E-field due to an incident B-field that is constant in magnitude and relative phase over the ellipse is

(A1)

where au and av are unit vectors along the minor and major axes, respectively, (a,b) are the semi-major and -minor axes, respectively (u,v) is the location within the exposed area, and is the time rate of change of the magnetic flux density in a direction perpendicular to the cross section. In the calculations that follow, the magnitude of the induced field, E, is expressed, rather than its vector components. The coordinate system is such that the minor axis of the ellipse is along the u-direction, and the major axis is along the v-direction.

Table A1 summarises the exposure conditions used to determine data expressed in Table 6. The entries of Table A1 are interpreted as follows. The second column expresses the exposure condition. For instance, the entry in the first row is interpreted as excitation of a 10 µm neuron located in the brain, with a magnetic field perpendicular to the sagittal cross section. The third column gives the semi-minor and semi-major axes of the ellipse. The fourth column gives the location within the cross section where the E-field is evaluated. The fifth column is the assumed rheobase value of Eo (from Table 6). The last column gives the values of determined from Eq. (12). In this formulation, it is assumed that an ellipse is fitted to the torso, body, or head in one of three orientations. Consequently, the reference system (u,v) is tied to the fitted ellipse, and not to one specific reference system with respect to the body.

In items (1) and (2), the assumed ellipse is not supposed to represent the actual size of the brain, but rather the size of an ellipse that encloses its outer perimeter (the cerebral cortex) where the

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magnitude of the induced E-field is greatest. The ellipse enclosing the brain has semi-major and -minor axes that are 1.5 cm smaller than the assumed head size to account for the distance of 1.5 cm between the cortex and the scalp. Items (3) and (5) treat the exposure as uniformly covering the entire body; items (4) and (6) assume only the torso is exposed. The latter points are included to demonstrate that there is but a modest difference (about 10%) between worst-case exposure of the entire body versus exposure of only the torso with respect to peripheral nerve and cardiac stimulation.

The points u,v are selected to correspond to the worst-case exposure point for each of the assumed scenarios. In the case of the brain (Items 1 & 2), the cortex is where the induced E-field is greatest, and sagittal exposure provides the greatest magnetic induction loop. For items (3) and (5), an ellipse is fitted to the entire body viewed in the sagittal cross section. In the case of the heart, the location of greatest sensitivity to electrical stimulation is its apex [Roy et. al, 1987], and the greatest induced field at that location is found with sagittal exposure [Reilly, 1991]. The points (u,v) in items (5) and (6) correspond to the apex of the heart.

The exposure ellipses in Table A1 correspond to a large (but not extreme) body size for adults based on anthropomorphic data [SAE, 1979]. It is conservative to assume large body dimensions.

Table A1 Elliptical exposure model used to compute magnetic induction.

Item

Exposure

b,a

(cm, cm)

u,v

(cm, cm)

peak Eo

(V/m)

(T/s-pk) 1 10-µm nerve, brain, sagittal

9, 10.5 9, 0 12.3 237.1

2

synapse, brain, sagittal 9, 10.5 9, 0 0.075 1.45

3 20-µm nerve, body, sagittal 17, 90 17, 0 6.15 37.5

4 20-µm nerve, torso, coronal 20, 40 20, 0 6.15 38.4

5 heart, body, sagittal 17, 90 14, 18 12.0 88.7

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6 heart, torso, sagittal 17, 40 14, 18 12.0 98.6

7

leg 9, 42 9, 0 6.15 71.5

(a) b,a represent semi-minor and semi-major axes, respectively, of ellipse fitted to particular body part, viz: the brain in items 1 and 2, the torso in item 4, and the whole body in items 3 and 5. (u,v) represents the location within the ellipse where the induced field was evaluated, where u and v are measured along the minor and major axes, respectively.

∗ Unless otherwise noted, the term “waveform” as used in this chapter refers to the temporal variation of the in situ electric field, the in situ current density, or the conducted current – quantities that are proportional to one another. For the relatively low frequencies treated in this chapter, the induced electric field is proportional to the temporal derivative of the environmental electric or magnetic field. Consequently, the term “waveform” also applies to the time derivative of the environmental field.

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EMF DOSIMETRY HANDBOOK

INTERNATIONAL GUIDELINES FOR QUALITY EMF (RF or ELF) RESEARCH

Sheila Johnston PhD

Neuroscience Consultant

10 Queen’s Mews,

London, UK,

W2 4BZ

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Please note this is a summary document drawing on the expertise of the many international expert authors quoted directly and indirectly. I am only the gatherer of their statements. I acknowledge them fully.

Please note that each Specific Research Study Design (Epidemiological, Human Volunteer, In Vivo, In Vitro) can be read independently, but I would advise for full understanding that you read the complete document.

EMF DOSIMETRY HANDBOOK

INTERNATIONAL GUIDELINES FOR QUALITY EMF (RF or ELF) RESEARCH

Introduction

IEEE & ICNIRP

IEEE ICES TC-95, SubCommittees 3 and 4

ICES EMF Literature Surveillance

IEEE ICES EMF Standard Setting

ICES SC-3

ICES SC-4

ICNIRP, IARC and WHO EMF Project Collaboration

ELF Reviews: IARC-2002, ICNIRP-2003, WHO EMF Project-2005

RF Reviews: IARC-2006, ICNIRP-2007, WHO EMF Project-2008

Revision of the ICNIRP Guidelines 0-300 GHz -1998

COST 281

GUIDELINES FOR QUALITY EMF (RF / ELF) RESEARCH

Publication

Replication and EMF Standard Setting Criteria

General Experimental Design Criteria

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Hypothesis

Good Laboratory Practice

Methodology

EMF Exposure Systems

RF Exposure Metrics

ELF Exposure Metric

Results Analysis

Conclusion

Publication

Scientific Responsibilities

Replication

Meta Studies

General Research Priorities

Dosimetric Units

RF

ELF

Threshold Studies

SAR as a Measure of Temperature Increase

Modulation

Human RF Dosimetry

Meta Studies

Specific Research Designs

Introduction

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Traditional Evaluation of Research

New Evaluation of Research: Molecular Epidemiology

The Epidemiological Study Design

Introduction

Case Control and Cohort Studies

Case Control Studies

Cohort Studies

Spatial Epidemiology (Quoted from Elliott and Wartenberg, 2004)

Bias

Countering Bias

Dosimetry

Data Analysis

Human Volunteer Study Design

Introduction

Hypothesis

GLP

EMF Exposure Systems

RF Exposure Metrics

Human RF Exposure in the Near Field

Human RF Exposure in the Far Field

ELF Exposure Metrics

Results Analysis

Conclusion

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Publication

Scientific Responsibilities

Replication

Meta Studies

In Vivo Study Design

Introduction

Hypothesis

Good Laboratory Practice

Methodology

Traceable Dosimetry

Data collection and quality assurance

Results Analysis

Conclusions

Publication

Scientific Responsibilities

Replication

Meta Studies

In Vitro Study Design

Introduction

Hypothesis

Good Laboratory Practice

Methodology

Exposure Conditions

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Introduction

International expert evaluations of the published literature are carried out according to guidelines for quality EMF (0-300 GHz) research. As technology progresses, it is our duty as scientists and engineers to continually review and monitor new findings and to update and revise our safety standards and guidelines accordingly (Chou & D’Andrea, 2003; ICNIRP, 1998; Gajšek et al., 2002; ICNIRP, 1998).

A comprehensive and critical review of the extant scientific database of electromagnetic field (EMF) published literature is updated periodically by panels of current qualified experts, as recognized by the international scientific community.

An important task of the international expert panels is to assess the relevant extremely low

frequency (ELF) and radiofrequency (RF) accumulating papers (2200+ papers) for standard setting

for human exposure limits to protect the population against any adverse health effects.

While exposure to EMF may cause biological effects, without any known adverse consequences, the standards are based on established threshold levels for adverse health effects, i.e., levels of exposure above which adverse health effects have been established and below which adverse health effects have not been established.

IEEE & ICNIRP

There are currently 2 international expert panels that evaluate the ever-increasing EMF literature according to criteria for quality EMF (0-300 GHz) research to set the standards and guidelines.

1. The Institute of Electrical and Electronics Engineers Incorporated (IEEE)[1], International Committee on Electromagnetic Safety (ICES), Piscataway, New Jersey. ICES membership is open to all interested persons; membership of the central governing and the technical committees (TC-95 and TC-34) stands at more than 150 professionals representing 24 countries. [http://grouper.ieee.org/groups/scc28/]. ICES develops standards through an open consensus process that is transparent at every level. In addition to standards prescribing safety levels, ICES also develops standards and recommended practices for implementing these standards, e.g., safety programs, methods for exposure assessment.

2. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) Oberschleissheim, Germany (14 members from 13 countries, see board 2004-8). [http://www.icnirp.de/]. New members are appointed by the existing members.

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Although IEEE ICES standards and ICNIRP guidelines are advisory (voluntary), and carry neither a mandate for compliance nor a mechanism for enforcement, these documents frequently serve as the bases for guidelines and regulations set forth by regulatory and other agencies in diverse countries that have issued their own standards, which, in some cases, may differ substantially from those of ICES and ICNIRP.

Because of the international expertise in EMF research of IEEE ICES and ICNIRP members, it is valuable to focus attention on their processes of evaluating the relevant literature for the purpose of establishing safety criteria.

These expert panels are multidisciplinary and include, for example, epidemiologists, neurologists, biologists, toxicologists, oncologists, and psychologists who are appropriately specialized medical scientists, and physicists, engineers, and statisticians. Only peer reviewed scientific studies are included in the review. While anonymous peer review for publication adds confidence in the study results, additional review is necessary to evaluate the study design, conduct of the experiment and the statistical analyses, and to compare various aspects of the results with the results of other studies to reveal the consistency of the results. National expert reports that for the most part are not published in scientific journals may also be considered for review. But conference abstracts are of little value as they generally receive no peer review, contain sparse information, and cannot be considered as the final outcome of an experiment (Repacholi, 1998). The task of the expert panels is to assess the entire ELF and RF scientific database (2200+ papers) for standards setting for human exposure to avoid any established adverse health effects.

IEEE ICES TC-95, SubCommittees 3 and 4

ICES EMF Literature Surveillance

Subcommittee 3 and Subcommittee 4 (SC-3 and SC-4) of Technical Committee 95 (TC-95) develop standards for safety levels with respect to human exposure to electromagnetic fields. SC-3 covers the frequency range of 0 to 3 kHz; SC-4 covers the range of 3 kHz to 300 GHz. These subcommittees review the EMF literature continuously and periodically they may publish review papers in the scientific peer reviewed literature for the purpose of updating the standards (cf. BEMS Supplement). The revision process established by the ICES is a continuing rigorous and open scientific process that is transparent at all levels and includes the opportunity for input from all stakeholders.

The EMF literature surveillance covers health effects in two separate exposure frequency ranges, 0-3 kHz and 3 kHz - 300 GHz, because adverse biological effects (associated with exposures above the exposure limits) are manifested primarily as induced in situ electric field stimulation in the lower frequency range and as tissue heating in the upper range, respectively. There is some overlap of the effects associated with induced in situ electric field stimulation and with heating in the intermediate range 3 kHz to 100 kHz (or higher for pulsed fields).

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IEEE ICES EMF Standard Setting

The IEEE standards are living documents and in accordance with IEEE rules must be reaffirmed or revised every 5 years. The IEEE C95 standards are based on IEEE ICES members’ anonymous peer reviews of all the published scientific studies of health effects. They provide a wide margin of safety to workers and the public from adverse health effects from exposure to EMFs. The IEEE EMF safety limits are issued as two separate standards.[2] As indicated above, SC-3 and SC-4 have independent literature review panels who evaluate the papers in a parallel process for the ranges, 0-3 kHz and 3 kHz -300 GHz, respectively. Each subcommittee independently drafts and approves the standard that applies to its frequency range.

Each standard is first balloted by the subcommittee, which requires at least 75% of the ballots returned and at least 75% affirmative ballots following ballot resolution (during which time all negative ballots, comments and changes to the draft resulting from ballot resolution are circulated to the balloting group to allow members to comment, change or reaffirm their vote). This process is repeated at the ICES Committee level after which the draft standard is submitted to the IEEE Standards Association Standards Board (SASB) Review Committee (RevCom). The SASB RevCom has oversight to ensure that due process has been afforded to all, and that the rigid IEEE SASB Rules for standards development and balloting have been followed. When satisfied, the RevCom recommends to the SASB approval of the standard. The SASB procedures provide a mechanism for appeals based on violations of the IEEE balloting process; technical appeals are referred to the Committee and Subcommittees. Once approved by the SASB, the standards are published, usually within 2-3 months.

The ELF standard (C95.6) was approved in September 2002 and published in October 2002. When C95.6 was balloted by ICES, 93% of the ballots were returned with a 90% approval.

The current RF standard (C95.1) was approved in 1991, reaffirmed in 1997, an amendment was published in 1999 and a second amendment was published in 2004.

ICES SC-3

TC-95 SC-3 was responsible for the development of IEEE C95.6-2002. During development of this standard, SC-3 was chaired by Kent Jaffa (USA) and had about 75 voluntary ELF research members from 11 countries. The results of the SC-3 review are incorporated within IEEE Std C95.6-2002.

The SC-3 review of the literature continues with Thanh Dovan (AU) and Philip Chadwick (UK) as SC-3 co-chairs. In addition, SC-3 members have published review papers in Health Physics, 83:3, 2002, from an international conference in Brussels, 2000, entitled “The EMF exposure guidelines science workshop”.

The IEEE ICES sponsored a one-day ‘Short Tutorial Course on IEEE Standard C95.6’ in June and December 2004 in conjunction with their annual and semi-annual meetings. The same short-course was presented to the Canadian Electrical Association in March 2004 and in Dublin in June 2005. J. Patrick Reilly and Kent Jaffa, both of the USA, present this short course.

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TC-95 is working toward harmonisation of the C95.6 standard with ICNIRP and to that purpose, within the ELF range, J.P. Reilly has published a paper in Health Physics, “An analysis of differences in the low-frequency electric and magnetic field exposure standards of ICES and ICNIRP,” (Reilly, 2005).

ICES SC-4

A complete revision of the RF standard (C95.1-1991) by SC-4 is now in progress. The revision is based on the peer reviewed literature identified by the Literature Surveillance Working Group chaired by Lou Heynick (USA, 1978-2005) who continuously identified the literature for review. This process has been taken over by Joseph Morrissey and new papers are now entered on the WHO EMF Project website

The membership of SC-4 stands at 122 members representing 24 countries and is co-chaired by C-K. Chou and J.A. D’Andrea (both USA). Chou and D’Andrea coordinated the literature evaluation and, along with the literature evaluation working group chairs and other designated experts published 12 RF and health, review papers in, the Bioelectromagnetics journal, Supplement 6, 2003 (Chou and D’Andrea, 2003).

They have also incorporated a thorough literature review within the draft revision of IEEE Std C95.1-1991/1999. This standard has undergone a complete revision, which was approved by SC-4 in March and is now (August 2005) undergoing Sponsor (ICES) ballot. The SC4 literature evaluation process is explained in detail in Appendix A 1.6 of the IEEE Std PC95.1-2005 Briefly adapted from IEEE Std PC95.1-2005:

Working groups (WGs) evaluate engineering, epidemiology, in vivo, and in vitro aspects of the research. Additionally, a WG on mechanisms assesses the role of mechanisms of interaction in standard setting. The Engineering WG is tasked with assessment of the exposure systems, field characteristics and measurements, dosimetry, specific absorption rates, induced currents and fields, and temperature/humidity measurements. The sufficiency of the information provided in each publication, to allow a full understanding of how the experiment was performed, is paramount. The chair of each WG is responsible for providing copies of each paper to two independent reviewers, together with specially designed and approved review forms called Triages. These ‘Triage’ forms (available in Appendix A, below) are in a computer format that requires numerical scoring by individual reviewers for entry into a computerized database. When a review is completed, the reviewer gives the paper an overall technical merit rating on a 5-point scale. The rating scale is: Very High = 5; Moderately High = 4; Acceptable = 3; Low = 2; and Very Low = 1. For ratings of 1 or 2, a request is made for justification in writing by the reviewer. This is not requested for ratings of 3 and above, which are considered acceptable. Strong discordance between the two reviews of a given paper requires a third independent review. Periodically, the chair of each WG submits a summary of the reviews completed to the Chair of the Risk Assessment WG (RAWG). All of the reviews are performed by volunteers who are randomly selected from within each working group. The identification of each reviewer will remain confidential.

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ICNIRP, IARC and WHO EMF Project Collaboration

The ICNIRP, IARC, and WHO EMF Project expert members work in phased collaboration with each other to evaluate the EMF literature.

1. The International Agency for Research on Cancer (IARC), with their secretariat in Lyon, France, is part of the WHO. (Classifications of carcinogens: ELF, 2001). [http://www.iarc.fr/]

2. The World Health Organisation (WHO) EMF Project have their secretariat in Geneva with members from 40+ countries. [http://www.who.int/peh-emf/en/]

The ICNIRP / IARC/ WHO EMF Project, EMF literature reviews cover health effects in the two separate exposure frequency ranges of 0-100 kHz and 100 kHz - 300 GHz.

The IEEE and WHO/ IARC/ICNIRP have not yet harmonized on the intermediate division between the two ranges but they may in the near future (Reilly, 2005).

On each of the two frequency ranges IARC evaluates the cancer literature, makes a classification and writes a Cancer Monograph.

On each of the two frequency ranges the ICNIRP panel reviews the biological literature and publishes its review as a Blue Book.

Consequently the WHO EMF Project panel prepares a review on each of the two frequency ranges and writes an Environmental Health Criteria (EHC) Monograph.

Finally once they have fully reviewed and evaluated the literature on both the 0-100 kHz and the 100 kHz-300 GHz ranges, the ICNIRP panel will revise the EMF guidelines for limiting human exposure over the entire EMF range, 0-300 GHz, as a paper published in Health Physics (1998; 2008?).

ELF Reviews: IARC-2002, ICNIRP-2003, WHO EMF Project-2005 Presently IARC has evaluated the cancer literature on ELFs and published an IARC Monograph ‘Non-ionizing Radiation, Part 1: Static and Extremely Low Frequency Electric and Magnetic Fields’ (Vol. 80, 2002) and has made a cancer classification 2B for ELFs (19-26 June 2001). The IARC Panel (19-26 June 2001) consisted of 21 members from 11 countries and 4 observers (chosen by the secretariat) and the IARC secretariat.

Following this, the ICNIRP panel 2000-2004 (12 members, the Chair A. McKinlay, vice chair and chairman emeritus), along with invited consultants have written a review of the biological scientific literature concerning exposure to static and low frequency EMFs 0-100 kHz (2003) including dosimetry (2 members, 5 consultants), experimental investigations of EMF biological effects (of cellular, animal and human experiments)(7 members, 7 consultants), and epidemiology (2 members, 4 consultants). Their report is published as the ICNIRP Blue Book entitled Exposure to Static and Low Frequency Electromagnetic Fields, Biological Effects and Health Consequences (0 - 100kHz), (2003).

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The WHO EMF Project panel are completing their update of the previous Environmental Health Criteria (EHC) Monograph 35 on ELF Fields, (1992) including comprehensive risk assessment and policy recommendations, expected in late 2005.

RF Reviews: IARC-2006, ICNIRP-2007, WHO EMF Project-2008 After IARC completes the meta analyses of results of the 13 country Interphone Study (2000-2005) a similar progression of literature evaluations by the panels of IARC, ICNIRP and the WHO EMF Project will be initiated for RF exposures in the range 100 kHz to 300 GHz.

IARC will evaluate the cancer-related literature of the RF bands and make a classification of RF possibly in 2006 and publish an RF IARC Monograph.

Then ICNIRP will evaluate the biological science and publish an RF Blue book.

And WHO EMF Project will write an EHC Monograph on RF to update the previous EHC monograph 137.

Revision of the ICNIRP Guidelines 0-300 GHz -1998 Following the completed cycles of IARC/ ICNIRP/ WHOEMF Project evaluations of both the ELF and RF published literature, ICNIRP will update the ‘ICNIRP guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields up to 300 GHz’ (Health Physics 1998), possibly in 2008. Thus we may have about a 10-year cycle to update the ICNIRP EMF guidelines for limiting human exposure according to the current literature evaluation process.

COST 281

The fifth international RF literature review group is the European Cooperation in the Field of Scientific and Technical Research- Telecommunication Information Science and Technology, (COST 281), with their secretariat in Bonn, Germany. The COST 281 science members are drawn from the 25 signatory European countries. http://www.cost281.org.

The COST 244 (1996-2000) continued on as COST 281 from 2001. The COST (281) members continuously review the scientific literature on radio telecommunications and health and report to the European Commission.

Previously, COST 244 members wrote two comprehensive reviews of the RF literature [Possible Health Effects Related to the Use of Radiotelephones: Proposals for a research programme by a European Commission Expert Group, 1996 (McKinlay); McKinlay, 1997; 1999 (Veyret)].

COST 281 members are not directly involved in setting standards; however they do advise the EC on RF health effects literature for the purpose of policy and evaluating proposals for further research funding.

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GUIDELINES FOR QUALITY EMF (RF / ELF) RESEARCH

Publication

The first hurdle to demonstrate quality in research is publication; papers considered for health risk assessment must be published in a peer reviewed journal.

When evaluating submitted papers for journal publication anonymous reviewers and editors may use the following criteria to comment on the technical content of the papers: Does the TITLE accurately and briefly portray the content? Does the ABSTRACT clearly convey the problem(s), findings, and conclusion(s)? Does the INTRODUCTION clearly and concisely define the problem or issue? Is the METHODOLOGY clear, complete, and adequate for the biology and dosimetry? Are the RESULTS clearly and concisely presented, making proper use of tables? Is the DATA ANALYSIS complete and valid, using appropriate statistical methods and tests, and error analysis? Is the DISCUSSION valid, relevant, and concise? Are the CONCLUSIONS drawn fully, valid and stated concisely? Are any key citations missing in the REFERENCES? Does the list include all the references used in text? (Chou, 2003)

Replication and EMF Standard Setting Criteria

Since the paper will have already met the publication criteria, the panel members review the paper to see if it can meet replication criteria and, if replicated, can meet the EMF standard setting criteria. Review panels approach the literature in a structured way. Panels have a chairman and section heads with their committee members representing expertise in epidemiology, in vivo (animal and human laboratory studies) and in vitro research (tissue cultures) as well as engineering/dosimetry, statistics and biophysical mechanisms of interaction. The section heads summarise their areas of review for the chairman. We base guidelines on scientific data related to adverse health effects meeting the literature evaluation criteria and we require information to be consistent from multiple studies and disciplines.

Overall criteria for general experimental design, general research priorities as well as criteria for the specific areas of research (epidemiology, human acute, in vivo and in vitro) are listed below. The review criteria are derived from numerous published papers, books and standards and guidelines and the five ICES SC-4 Triages. Some key references are listed directly below:

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Papers: Hill, 1965; Blundell, 1996; Rothman et al., 1996; Cardis and Rice, 1997; Repacholi and Cardis, 1997; Repacholi, 1998; Rothman and Greenland, 1998; Repacholi and Greenebaum, 1999; Lin, 2002; Adair, 2003; Brodsky et al., 2003; Chou, 2003; Gajsek et al., 2003; Habash, 2003; Habash et al., 2003; Rushton and Elliott, 2003; Kheifets et al., 2003; Ahlbom et al., 2004; Elliott and Wartenberg, 2004; Foster and Repacholi, 2004; Feychting et al., 2005; Greenland, 2005; Johnston and Scherb, 2005.

Books: EPRI, 1994; NIEHS, 1998; IARC Vol 80, 2002; ICNIRP, 2003; Stravoulakis (Ed), 2003 (Johnston chapter, 2003); Ahrens and Pigeot, (Eds) 2005.

Standards and Guidelines: ICNIRP, 1998; IEEE Std C95.6-2002; IEEE Std C95.1-1991/1999; IEEE Std 1528-2003; Draft IEEE Std PC95.1- 2005.

Five Triages: Developed by ICES members for consistent literature review in SC-4 [listed in full in Appendix A].

General Experimental Design Criteria

Hypothesis • The project should test a clearly defined hypothesis, using a detailed protocol that would lead to

quantitative information directly or indirectly relevant to assessment of health risk from ELF and RF exposure and allow any other independent laboratory(s) to reconstruct the study and replicate the findings for validation.

Good Laboratory Practice Good Laboratory Practice (GLP) should be used throughout the design and conduct of any study where possible and practical, but especially with large and long-term studies (see, e.g., FDA, 1993, NTP 1992[3]).

• A specific protocol, consistent with the GLP guidelines, should be established and documented. Any changes instituted during the course of the study should also be documented.

• The protocol should include randomized, double blind, symmetric handling of specimens and their sources, except when precluded by the nature of the experiment or biological system.

• The protocol should include all appropriate controls (positive, negative, cage controls, sham-exposed etc.).

• Investigators should be blind to whether they are working with exposed or control materials; human subjects in laboratory experiments should be similarly unaware of their exposure status.

Quality assurance (QA) procedures should be included in the protocol, including traceable dosimetry and monitoring of the programme by both a team from within the experimental staff and an independent group, as required by GLP [Repacholi et al., 1998; Repacholi and Greenebaum, 1999; Schönborn et al., 2000, 2004; ICNIRP, 2003; IEEE Std C95.6-2002; IEEE Std C95.1, 1991/1999; IEEE Std 1528-2003; Draft IEEE Std PC95.1-2005].

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Methodology • Well-characterized biological systems or assays should be used, preferably ones that are well

established (validated) from the scientific literature that can be used reliably by other laboratories (Vijayalaxmi and Obe, 2004; Obe and Vijayalaxmi, 2005).

• The biological system used should be appropriate to the end-point(s) studied.

• In cognitive behavioural research the assessment of the metrological quality (validity and reliability and variability) of all the psychological tests (HCN, 2004); and measurements [Electroencephalograms (EEGs)(Angelone et al., 2004), Positron Emission Tomography (PETs) (Blodgett et al., 2005) Functional Magnetic Resonance Imaging (fMRIs) (Cabeza and Nyberg, 2000) etc] employed must be stated.

• Threshold responses derived from using at least 3 levels of dose and duration of exposure data where possible are sought in addition to sham-exposed controls (‘Sham/Sham’). Appropriate positive controls as well as non-RF heating controls can be very important to help assess the metric of the response of the biological system and the potential effects of RF heating should be included.

• The a priori estimated statistical power of the experiment, based on prior knowledge and the number of tests planned, should be sufficient to reliably detect the expected size of the effect (often as small as 10-20%) (Triages).

EMF Exposure Systems • It is essential for high quality research that accurate assessment of RF and ELF exposure is an integral part of all studies and that each

research team include scientists and engineers skilled in traceable EMF dosimetry, in sufficient detail for replication (Triages).

• Computational dosimetry provides the quantitative link between internal dose quantities for direct effects and external fields that can be measured. (Quoted from NRPB, 2004).

• A comprehensive uncertainty[4], variability and artifact dosimetric analysis is required to achieve consistent interpretation of the results (quoted from Kuster et al., 2004; IEEE Std 1528-2003).

RF Exposure Metrics • Current and future research should be applicable to mobile telephone systems in use. For RF

research the focus should be on signal protocols in use, e.g. second, third and fourth generation signals (2G, 3G, 4G) including their modulation patterns. For radars, the frequency and pulsing regimes should be applicable to current and emerging systems. [derived from Kuster and Balzano, 1996; Schönborn et al., 2000; IEEE Std C95.6-2002; ICNIRP, 2003; Foster and Repacholi, 2004; Kuster et al., 2004; Nikoloski et al., 2005; Andersen, and Foster Presentations, COST 281 Zurich Feb 17-18, 2005; Reilly, 2005]

• Since adverse effects are typically set on the basis of a 1°C temperature rise, it is essential in the design of local RF exposures (in vitro, in vivo and human acute experiments) that temperature rise as well as SAR is measured and verified using numerical modelling of the exposures. There are some recognized uncertainties indicated by the range of the modelling data relating temperature rise with localized SAR (Quoted from Draft IEEE Std PC95.1- 2005, Section C.2.2.2.1.2).

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ELF Exposure Metric

• In relation to dosimetry of ELF local exposures, most internal cellular effects associated with low-level ELF exposure are linked with the induced electric field stimulation and therefore the internal induced electric field should be determined in ELF research where possible [IEEE Std C95.6-2002; ICNIRP, 2003; Reilly, 2005].

• Human exposure to (external ELF) magnetic fields is measured in flux density (B) in milliTesla (mT) and magnetic field strength (H) in amps per meter (A/m) respectively (IEEE Std C95.6-2002).

Results Analysis • Results should be analysed and interpreted on the basis of the prior stated hypothesis (Triages).

• Other tests of significance post hoc should be stated as such and an appropriate ‘multiple comparisons’ correction (I.E. Bonferroni, Tukey, or Empirical Bayes Adjustments) be applied. Post hoc results can be considered hypothesis generating for new research but cannot be considered as the direct or main result of the presented research. (Keppel, 1982; Steenland et al., 2000).

• The known reliability and validity of the tests employed and the normal variance of these tests stated in the methods should be referred to in the analysis of the results. The variance in the scores due to experimental exposure needs to be stated in the interpretation of the results taking into account the normal variance of the test scores without treatment, and variance of the test scores under sham control, positive control, and negative control treatment conditions. [IEEE Std C95.6-2002; Draft IEEE Std PC95.1-2005; HCN, 2004; Foster and Repacholi, 2004; Triages].

Conclusion • The conclusion should be drawn on the basis of the prior stated hypothesis of the study

(Triages).

• Knowledgeable interpretation of the results must consider the established science (outside of the EMF area) in the field of investigation such as biophysical mechanisms of interaction, genotoxicity, cancer, cognition, sleep, etc. [Repacholi, 1998; ICNIRP, 1998; IEEE Std C95.6-2002; ICNIRP, 2003; Foster, 2003; D’Andrea et al., 2003; Vijayalaxmi and Obe, 2004; Obe and Vijalalaxmi, 2005; Draft IEEE Std PC95.1-2005]

• Results should be viewed with respect to previously accepted scientific principles before ascribing the results to new theories. Research findings pointing to previously unidentified relationships should be carefully evaluated and appropriate additional independent studies should be conducted before the findings are accepted (Repacholi, 1998; Adair, 2003; Foster, 2003).

• Theories (e.g., for mechanisms of interaction) should make sufficiently concrete predictions that they can be tested experimentally and be capable of being verified, if correct (Schwan, 1988; Repacholi, 1998; Adair, 2003; Foster, 2000; Foster, 2003; Foster and Repacholi, 2004; Draft IEEE Std PC95.1-2005).

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Publication • Well-designed and -conducted studies should be published regardless of the outcome, since

negative results are as useful as positive studies in the context of the EMF database for health risk evaluation (Kheifets et al., 2003).

Scientific Responsibilities Scientists are required to take responsibility for:

• the sensitive issue of data protection and confidentiality,

• the new opportunities, such as the increasing availability of computerized data,

• the incorporation of molecular epidemiological methods to aid the investigation of mechanistic pathways and gene-environment interactions,

• and the development and utilization of sophisticated statistical approaches.

[Quoted from Rushton and Elliott, 2003; see also Conrads et al., 2004; Dorman, 2005; Espina et al., 2005; Gilham et al., 2005; Greenland, 2005; LaPorte, 2005]

Replication • Preferably, the experiments should be repeated and the data confirmed independently, or the

claimed effects should be consistent with results of similar experiments, for which the biological systems involved are comparable (Repacholi, 1998).

Meta Studies • Since the technologies of electricity and radio telecommunications serve billions of people it is

appropriate to address the remaining health concerns where possible in large multi-centred international studies [Cardis and Kilkenny, 1999; Haarala et al., 2004; COST 281, Directive, March 11, 2004].

• Where possible, feasibility studies (pilot studies) for multi-centred studies should be undertaken with input from international experts to verify and validate every aspect of the methodology of the EMF field study under question. The feasibility studies should be open for further scientific comment and published in peer reviewed journals

• Simultaneous epidemiological and experimental replications should use the same or similar methodologies including standardized questionnaires and should report data to provide quantitative results that can be compared and or combined for meta analysis of all the independently collected data.

before these large studies are funded and carried out (Cardis and Kilkenny, 1999; COST 281, Directive, March 11, 2004).

• In meta studies the data collection and analyses should be done according to the hypothesis and methodology of the published protocol.

• Meta analyses should take into consideration prior distributions for the unidentified bias parameters used in the original sensitivity-analysis model [Quoted from Greenland, 2003; 2005; see further details in ‘The Epidemiological Study Design’ below].

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General Research Priorities Review of the new research proposals should give a high priority to studies with ELF and RF levels, frequencies, modulation and pulse characteristics relevant to human exposures from new technologies with endpoints relevant to human health. [Repacholi, 1998; Foster and Repacholi, 2004; Andersen, Foster, presentations, COST 281, Zurich, February 2005]

Dosimetric Units

RF

• SAR remains the major RF dosimetric quantity; modulation should not be added to a study unless adequate statistical power can be maintained [ICNIRP, 1998; ICNIRP, 2003; Foster and Repacholi, 2004; COST 281, Zurich, February 2005; DRAFT IEEE Std PC95.1-2005].

• Some research, not necessarily a full set of studies, would be warranted for new RF technologies that employ new modulation schemes (changes in peak relative to average signal level and changes in frequency content of a signal) if the potential for public exposure is high. This includes UMTS now being rolled out [Foster and Repacholi, 2004; Andersen, Foster, presentations, COST 281, Zurich, February 2005].

ELF

• In the low frequency range the standard is based on electrostimulation which is defined as induction of a propagating action potential in excitable tissue by an applied electrical stimulus; electrical polarization of presynaptic processes leading to a change in post synaptic cell activity [Quoted from IEEE Std C95.6-2002].

• In most biological experiments of low-frequency field (ELF) effects the induced electric fields are poorly known. It is necessary to improve macroscopic dosimetry and, particularly in the case of in vitro studies, also to examine the microscopic distribution of the induced electric field [Quoted from ICNIRP, 2003; see also IEEE Std C95.6-2002; Reilly, 2005].

Threshold Studies

• There is a lack of well-replicated studies that reveal the existence of biological effects or adverse health effects from low-level RF or ELF exposure below, at, and above public and occupational guideline limits [ICNIRP, 2003; COST 281, Zurich, February 2005].

• Similarly there is a lack of well-replicated studies that reveal the existence of biological effects or adverse health effects from low-level RF or ELF exposure at various durations of exposures similar to real life durations of exposure conditions including intermittent exposures [Ivancsits et al., 2005; Diem et al., 2005].

• We need to establish the human threshold SAR values where RF exposure has a biological and an adverse effect [COST 281, Zurich, February 2005; IEEE, ICES, COST 281: 2004/09 Thermal Physiology Workshop, Paris].

• We need to establish more precisely the ELF thresholds for human electrostimulation values where ELF exposure has a biological and an adverse effect in various tissues [IEEE Std, C95.6-2002; ICNIRP, 2003; Reilly, 2005].

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SAR as a Measure of Temperature Increase • There is general agreement that more human RF dosimetry research is needed to determine the

validity of the SAR modeling data as a measure of temperature rise (Quoted from Draft IEEE Std PC95.1-2005 Section C.2.2.2.1.2). [See also: ICNIRP, 1998; Van Leeuwen et al., 1999; Wang and Fujiwara, 1999; Bernardi et al., 2000; Schönborn et al., 2000; Wainwright, 2000; Bernardi et al., 2001; Gandhi et al., 2001; Hirata et al., 2002; Yioultsis et al., 2002; Hirata and Shiozawa, 2003; NRPB, 1997, 2004; Kuster et al., 2004; Nikoloski et al., 2005].

• A future goal is to develop appropriate techniques and thermal models for accurately predicting the thermo physiological responses of human beings who are exposed to RF fields at specific frequencies, field strengths, and field characteristics and to validate some predictions with existing human exposure data [Stolwijk and Hardy, 1977; Adair and Berglund, 1986, 1992; Adair et al., 1998, 1999, 2001, 2003, 2005; Kheifets et al., 2003; Foster and Adair, 2004; IEEE, ICES, COST 281: 2004/09 Thermal Physiology Workshop, Paris]. Much data exists that describes the regulatory response changes in the human body as a function of environmental variables, work, exercise, age, fitness, clothing insulation, and other characteristics of each individual. [WHO Geneva Workshop, 2002: Adverse Temperature Levels in the Human, International Journal of Hyperthermia, Vol 19(3), 2003]. Much of this material is amenable to comparison with data derived from RF-exposed humans and animals. [IEEE, ICES, COST 281: 2004/09 Thermal Physiology Workshop, Paris].

Modulation • We need to note the modulation of RF and ELF signals exactly. Different modulations could

have the same average value. Presently there is no convincing evidence that there is a difference between continuous and modulated RF signals in their effects. But we need to get more clear evidence of the different modulations. Are there separate classifications according to biological effects? Is lower or higher SAR more significant at producing effects? [COST 281, Zurich, February 2005]

• Experimental EMFs should be fully characterized and re-measured periodically. Waveform, pulse shape and timing, frequency spectrum, harmonics and transients from both continuous sources and from switching exposure systems on and off should all be measured where appropriate (Kuster, 1996; Schönborn et al., 2000; Nikoloski et al., 2005).

• Background fields, such as ambient, equipment-derived, and crossover fields from other exposure systems, are also important and need to be characterized. Time-varying and static components should be measured, as well as the polarization and directions of the fields.

• Field modulation introduced by experimental factors such as motion of sample shakers should be noted and measured whenever possible. Positioning of cultures or animals within exposure systems should be noted and randomized where appropriate (Repacholi, 1998).

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Human RF Dosimetry • The position of humans in exposure systems should be noted and stereotaxically (3-D) defined

for replication. (Excell et al., 1996, 1998; Vaul & Excell, 1999).

• Reports of SAR increases due to the presence of nonmagnetic electrodes during RF exposure of up to a factor of 16 have been reported (Angelone et al., 2004). During RF exposure while recording EEG, better modelling of the human head SAR is required (Huber et al., 2003). For instance you cannot use a plastic shell head phantom the same as for mobile phone compliance when measuring the effect of metal leads (for EEG recording) on SAR because of the insulating plastic phantom shell since there would be no electrical connection between the leads and the fluid inside the phantom. (CK Chou personal communication; for SAR modelling methodology see Angelone et al., 2004).

• Further modelling of SAR of children is required using realistic head and body phantoms for both near and far field exposures [ICNIRP, 2003; NRPB, 2004; Draft IEEE Std PC95.1-2005].

Meta Studies • Meta studies are required on RF effects of UMTS and 4G signals on direct and established

measures of human brain function and the possible mechanisms involved, using well validated measurements [Haarala et al., 2004; HCN, 2004; Angelone et al., 2004; Kuster et al., 2004].

• In vitro meta studies investigating DNA breaks, genomics, proteomics and molecular signalling pathways during ELF and RF exposures are a priority because of remaining positive studies in the cytogenetic literature (Vijayalaxmi and Obe, 2004; Moulder et al., 2005; Obe and Vijayalaxmi, 2005. reviews).

• Possible long-term biological effects (up to 30 years; IARC, 2002; ICNIRP, 2003; Ahlbom et al., 2004; Feychting et al., 2005) from mobile phone exposure of the public require investigation and should be given priority. Cohort studies including children are recommended. (Stewart et al., 2000; ICNIRP, 2003; Ahlbom et al., 2004; Feychting et al., 2005; Moulder et al., 2005).

• The incorporation of molecular epidemiological methods to aid the investigation of mechanistic pathways and gene-environment interactions should be included. [Rushton and Elliott, 2003; see also Conrads et al., 2004; Dorman, 2005; Espina et al., 2005].

Specific Research Designs

Introduction

Traditional Evaluation of Research

Evaluation of research literature on the effects of ELF or RF exposure is reached in a complex confirmatory interplay of human epidemiological studies, human acute studies, in vivo bioassays

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(animal lifetime studies), animal acute studies and in vitro tissue culture studies. Traditionally, investigations in human beings of associations between exposure levels and adverse health effects can utilize either human acute or epidemiological studies. And it was biologically plausible and prudent to regard EMF exposure studies in animals, as evidence of lack of risk or risk in humans. This was and is dependent on whether there is evidence for extrapolation of the science from animals to man, based on known functional and structural homologies [i.e. Spatial memory: Kandel et al., 2000; Kandel, 2001; Cell division: Lee and Nurse, 1987]. Traditionally, cellular research was considered as the possible source of a plausible mechanism for any biological effect due to exposure to RF or ELF signals [Repacholi and Cardis, 1997; Repacholi, 1998].

New Evaluation of Research: Molecular Epidemiology

Great advances in cellular research have given us a new understanding of the interplay of cellular research of genes, proteins and cell signalling in single living cells, and the functioning of the whole human body.

For instance molecular profiling for the treatment of individual patient's tumours is currently being evaluated in clinical trials at the National Institutes of Health, National Cancer Institute (Espina et al., 2005).

And high-resolution serum proteomic features are being implemented for various kinds of cancer detection (Conrads et al., 2004).

These molecular advances have an impact on EMF research priorities. Molecular profiling may become essential at all levels of research namely in epidemiology, human acute studies, animals studies and tissue cultures.

Molecular epidemiology is based on general epidemiology and utilizes the same designs (i.e., case control and cohort studies) as those employed by general epidemiology. However, molecular epidemiology utilizes molecular biology to define the distribution of disease in a population (i.e., descriptive epidemiology) and identify its potential etiologic determinants (i.e., analytical epidemiology) [Quoted from Dorman JS, Director, Collaborating Center for WHO Multinational Project for Childhood Diabetes (DiaMond), U of Pittsburgh; see also LaPorte RE, Director, Molecular Epidemiology and DNA Technology Transfer http://www.pitt.edu/~rlaporte/who.html]

The following sections are on specific research designs in epidemiology, human acute studies, in vivo and in vitro research designs respectively.

The Epidemiological Study Design

Introduction Epidemiologic investigations of possible associations of EMF exposure with risk of chronic disease pose unique and substantial difficulties. Among them are difficulties specific to an outcome studied, assessment of exposure, and interpretation of findings and long latency periods required to detect any cancer risk (up to 30 years), (IARC, Vol 80, 2002).

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• In light of the complexity of the topic (covering medicine, biology, epidemiology, neuroscience, psychology, physics, engineering, statistics) studies should be designed and implemented using expertise from all the relevant disciplines.

• Information necessary to evaluate the feasibility of a given study should be collected and analysed by experts in all the related fields and published

• The design must be maximally efficient in reaching the study objective and utilizing resources.

in a peer reviewed journal (Cardis and Kilkenny, 1999).

• The hypothesis must be explicitly and clearly stated before

• The study protocol should meet the letter and spirit of all relevant regulations and have prior approval of all relevant review boards.

the onset of the research.

• Subjects and controls must have given informed consent.

• The method of ascertaining cases of adverse health must be stated.

• Case identification must be independent of exposure. Definition of cases should be objectively, and histologically confirmed.

• Controls should be appropriate to the specific aim and design.

• Controls must be matched (to cases) individually, on age and sex, within study region, and on the basis of frequency, and be population based –i.e. ‘representative’

• Minimization of non-response or non-participation is required to achieve the necessary sample size and minimize bias through selective non-response.

• Total number of original subjects and controls must be stated as well as those removed from the study due to non-response or death.

• An adequate population sample size must be used based on previous calculation of the statistical power. Expected number of cases must be adequate in the study populations to show a relatively small effect of exposure to EMF emissions, if there is one, for instance, from electrical power devices, or mobile phones.

• Study populations must be well defined before

• The results must be calculated to evaluate the original hypothesis. It is on the basis of the stated original hypothesis that the power of the study is calculated and the size of the sample is set.

the onset of the research.

• Study designs should recognize that the exposure metric for possible effects of weak ELF and weak RF fields is uncertain and usually proxy. Subjects' exposures, particularly historical exposures that are often determined via surrogates, should be validated from specific measurements where possible. Data should include as much information relevant to alternate metrics as possible to aid future research. [Beaglehole et al., 1993; Bracken et al., 1993; Ahlbom, (ICNIRP), 1996; ICNIRP, 1998; IEEE Std C95.6-2002; ICNIRP, 2003; Ahlbom et al., (ICNIRP) 2004; Feychting et al., 2005; IARC, Vol 80, 2002; Draft IEEE Std PC95.1-2005; Neubauer et al., 2005; Reilly, 2005].

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• The authors should report the basic data on which the conclusions are drawn (IARC, Vol 80, 2002).

• Post hoc comparisons on subsets of the data may be hypothesis generating only. Appropriate corrections for multiple comparisons must be applied (Keppel, 1982; Steenland et al., 2000; Johnston and Scherb, 2005).

• Meta analyses should take into consideration prior distributions for the unidentified bias parameters used in the original sensitivity-analysis model. Accounting for uncontrolled confounding and response bias under a reasonable prior (distribution) can substantially alter inferences about the existence of an electromagnetic field effect. Analyses with informative priors (distributions) for unidentified bias parameters can help avoid misinterpretation of conventional results and ordinary sensitivity analyses [Quoted from Greenland, 2003; 2005; see also Blettner & Schlattmann, 2005].

• Studies should include collection of blood samples to create a bio-bank for molecular biological studies on brain tumours and other diseases.

• In new studies, improvement of the exposure assessment is crucial. (Feychting et al., 2005). Personal monitoring systems are being developed for base site exposures.

Case Control and Cohort Studies

• Case control and cohort studies can give a relatively close approximation to the biologic model in investigating environmental health issues because both individual person characteristics and exposures are studied in the individual environment. They can investigate for instance whether risk of cancer is raised in human populations according to their EMF exposure. In other words they investigate whether or not the possibility of an association between EMF exposure and cancer exists in human populations (IARC Vol 80, 2002; Elliott and Wartenberg, 2004).

Case Control Studies

• Case control studies provide point data for cases and a set of controls. The population is undefined in general, and cases and controls should be comparable. Cases and controls are selected according to certain criteria by the investigator. The risk measure is odds ratio (OR) (Ahrens and Pigeot, 2005).

• Case control studies are prone to selection and other biases, are moderately expensive and time-consuming to carry out, and are not feasible in all situations (Elliott and Wartenberg, 2004). [For example see Cardis and Kilkenny, 1999, The Interphone Study; and MTHR Swerdlow et al., 2002-2008, a case control study of risk of leukaemia in relation to use of mobile phones. http://www.mthr.org.uk/research_projects/funded_projects.htm]

• Epidemiological case control studies should gather toxicological information on other factors as well as EMF in cases of diseases of unknown causes to look for feasible associations that may be strongly associated with the disease (UKCCS Investigators, 2000; IARC, Vol 80, 2002). [I.E. In the UK childhood cancer study (UKCCS) childhood leukaemia has been shown to be associated with reduced exposure to infection in the first few months of life, (Gilham et al., 2005)].

Cohort Studies

• A cohort approach would allow studies with different types of outcomes.

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• Cohort studies define an exposed and a non-exposed population. Incident diseases or causes of death in the exposed and non-exposed populations are taken from a national register. The risk measure is Relative Risk (RR) (Ahrens and Pigeot, 2005):

• Cohort studies, although not subject to selection bias, are prone to other biases, including losses to follow-up, and generally more expensive and time consuming to carry out than case control studies (Elliott and Wartenberg, 2004). [For examples see Johansen et al., (2001, 2002) and Elliot et al., Cohort study of mobile phone users (pilot study), 2002-2004, in press http://www.mthr.org.uk/research_projects/funded_projects.htm]

Spatial Epidemiology (Quoted from Elliott and Wartenberg, 2004)

• Exploratory studies such as spatial epidemiology use aggregate data, such as geographic correlation studies and offer an alternative approach for generating, prioritizing, and analyzing data to address specific hypotheses of disease etiology and causation.

• Spatial epidemiology is the description and analysis of geographic variations in disease with respect to demographic, environmental, behavioural, socioeconomic, genetic, and infectious risk factors.

• Although they too are prone to biases and misclassification (Elliott and Wakefield, 2000), they are generally easier, quicker, and less expensive to conduct than case control or cohort studies.

• Sensitivity to detect areas at high risk is limited when expected numbers of cases are small.

• One example of this approach is with use of a dedicated system such as that developed by the Small Area Health Statistics Unit (SAHSU) in the United Kingdom (Elliott et al., 1992); this has recently been adopted in other European countries as part of the European Health and Environment Information System (EUROHEIS).

• One ready means of investigating the relation of RF exposure to disease is the replication of analyses in different areas based on routine data, as is done in the United Kingdom through SAHSU and in Europe through EUROHEIS [For an example see Elliott et al., 2003-2005 Case control study of cancer incidence in early childhood

Bias

and proximity to mobile phone base stations, MTHR: http://www.mthr.org.uk/research_projects/funded_projects.htm]

• Problems include the large random component that may dominate disease rates across small areas. This can be dealt with by using Bayesian statistics to provide smooth estimates of the disease risks (Elliott and Wartenberg, 2004).

• The healthy group (worker) effect can result in comparison bias.

• Recall bias occurs when cases and controls may recall exposure differently.

• Interviewer bias can arise when the interviewers may ask questions differentially.

• Selection bias could arise when sampling probability varies, and when there is loss to follow up, and there are non-responses.

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• Information bias can arise from non-differential misclassification and differential misclassification.

• Confounding could be due to mixing of effects such as age, sex, and competing exposures.

• Ecological fallacy could result from uncontrolled factors related to disease and exposure [Ahrens and Pigeot, 2005; H. Scherb presentation Feb 2005 COST281, Schriesheim].

Countering Bias

• To control for biases there should be double-blinding (subjects and investigators) where possible.

• The study should represent the population or use total ascertainment (i.e. using national registers).

• The study should use stratification of participants (for instance by age, sex, education and social economic status).

• One can counter bias by use of standardization (direct or indirect) (Ahrens and Pigeot, 2005; Scherb, 2004).

Dosimetry

• Data on different levels of exposure, its duration and temporal location should be identified (Balzano, 1999).

• The exposure must be recorded in traceable detail for replication in multiple centres of simultaneous study. Quantifiable exposure measurement is preferred over qualitative exposure data.

• Exposure gradients should be developed.

• This dosimetry should be taken into account both at the design of the study and during analysis.

Dose must be independent of control and experimental subjects, and should be measured on an individual basis (IARC, Vol 80, 2002).

• Adequate and reliable measures of exposure for each study subject are needed.

• Categorising exposure into groups can lead to misclassification, and produces a bias towards the null hypothesis -underestimating the real effects.

• More research on validation of body-worn human RF dosimeters needs to be done before studies around mobile phone base stations are feasible (Neubauer et al., 2005).

Data Analysis

• The authors should report the number of exposed and unexposed cases and controls in a case control study and the numbers of cases observed and expected in a cohort study and from this group the number used in the statistical evaluation of the primary hypothesis.

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• Both in study design and analysis, control for confounding variables is required. Data on potential confounders should be collected for statistical analysis to minimize or subtract out the confounding factors where possible. Identification of confounding factors is recognized as difficult; there is often limited knowledge about causal factors of adverse health endpoints.

• Lack of appropriate action to reduce the impact of these sources of error can decrease the credibility and the final weight given to the results of the study. (Repacholi and Cardis, 1997).

• The methods of statistical analysis should be appropriate for the evaluation of the hypothesis of the study and clearly described.

• When sophisticated or non-standard analytical procedures are used, researchers should provide a simple descriptive analysis of the data. The number of subjects and controls, and the effects of potential confounding factors that were part of the investigation should all be reported.

• Meta analyses should take into consideration prior distributions for the unidentified bias parameters used in the original model [Greenland, 2003., 2005].

• To correct for false positives, empirical or semi-Bayes methods of adjustments for multiple comparisons (post hoc) are recommended when a large number of comparisons have been made (Steenland et al., 2000).

• In summary, we look for 9 factors or viewpoints in epidemiology, as summed up by Sir Bradford Hill in 1965, namely: Strength, Consistency, Specificity, Temporality, Biological gradient, Plausibility, Coherence, Experimental, evidence, and Analogy. He stressed that from these nine different viewpoints we should study association before

we try causation.

• He states that no tests of significance can answer those (9) questions. Such tests can and should remind us of the effects that play of chance can create, and they will instruct us in the likely magnitude of these effects. Beyond that they contribute nothing to the 'proof' of our hypothesis [Quoted from Bradford Hill, 1965].

Human Volunteer Study Design

Introduction Please note there are several references to details in the ‘General Experimental Design Criteria’ and the ‘General Research Priorities’ above, in the details of the ‘Human Volunteer Study Design’ below.

The advantage of human volunteer experiments is that they indicate the likely response of other people exposed under similar conditions.

Disadvantages of volunteer studies include:

• the innocuous nature of the effects that can be investigated,

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• the short duration of investigation,

• the small number of subjects usually examined

• the ethical constraints,

• and the subjects are usually healthy adults who may not reflect the responses of potentially more susceptible members of society (Quoted from NRPB, 2004).

Hypothesis • The human study should test a clearly defined hypothesis, using a detailed protocol that would

lead to quantitative information directly or indirectly relevant to assessment of health risk from ELF and RF exposure and allow any other independent laboratory(s) to reconstruct the study and replicate the findings for validation and allow the results be combined for meta analysis where appropriate.

GLP • The protocol should meet the letter and spirit of all relevant regulations concerning experiments

using human subjects, and have prior approval of all relevant review boards. Personnel working with volunteers require special training and oversight.

• For instance, where radioactively labelled compounds could be injected into the human subject’s bloodstream [such as in Positron Emission Tomography (PET) or Single Photon Emission Computed Tomography (SPECT)] exploratory research in animals should first be considered to investigate brain areas of interest with alternate techniques, such as quantitative autoradiography (QAR) to map regional cerebral blood flow (RCBF). A RCBF study with rats exposed to RF would indicate if there is something worth looking for in humans. (Society of Nuclear Medicine Brain Imaging Council, 1996; Blodgett et al., 2005)

• Adherence to ethical rules must be indicated.

• A well-described criterion for inclusion and exclusion of volunteers is required.

EMF Exposure Systems • It is essential for high quality research that accurate assessment of RF and ELF exposure is an integral part of all studies and that each

research team include scientists and engineers skilled in traceable EMF dosimetry, in sufficient detail for replication (Triages).

• Current ELF and RF future research should be applicable to electrical and mobile telephone systems in use. For RF research the focus should be on signal protocols in use, e.g. second, third and fourth generation signals (2G, 3G, 4G) including their modulation patterns. For radars, the frequency and pulsing regimes should be applicable to current and emerging systems. [derived from Kuster and Balzano, 1996; Schönborn et al., 2000; IEEE Std C95.6-2002; ICNIRP, 2003; Foster and Repacholi, 2004; Kuster et al., 2004; Nikoloski et al., 2005; Andersen, and Foster Presentations, COST 281 Zurich Feb 17-18, 2005; Reilly, 2005]

RF Exposure Metrics • Specific Absorption Rate (SAR) in Watts per kilogram (W/kg) is the fundamental RF dosimetry

parameter.

• Since RF adverse effects are typically set on the basis of a 1°C temperature rise, it is essential in the design of local RF exposures in human acute experiments that temperature rise as well as SAR is measured and verified using numerical/thermal modelling of the exposures. There are

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some recognized uncertainties indicated by the range of the modelling data relating temperature rise with localized SAR (Quoted from Draft Std PC95.1-2005, Section C.2.2.2.1.2).

• In cognitive behavioural research the assessment of the metrological quality and uncertainties of measurement (variability reliability and validity) of all the psychological tests (HCN, 2004) and dosimetry measurements employed must be stated [Re: Electroencephalograms (EEGs) see Angelone et al., 2004 and see General Research Priorities above for more detail); Re: Positron Emission Tomography (PETs) see Blodgett et al., 2005; Re: Functional Magnetic Resonance Imaging (fMRIs) see Cabeza and Nyberg, 2000)].

• Threshold responses derived from using at least 3 levels of dose and duration of exposure data where possible are sought in addition to sham-exposed controls (‘Sham/Sham’). Appropriate positive controls as well as non-RF heating controls can be very important to help assess the metric of the human response and the potential effects of RF heating should be included.

Human RF Exposure in the Near Field

• It is necessary for replication and accurate interpretation of results that the experimental exposure setup be defined in stereotaxic coordinates for the position of the antenna in relation to the human head. It is necessary to measure, under the experimental conditions, the SAR exposure pattern from the phone antenna in the stereotaxically defined position in relation to the neuroanatomy of the (phantom) head and brain, taking into account the dielectric properties of various tissues of the typical head and neck. It is important to numerically calculate the same SAR values by the gram (1cc) through out the exposed head, brain and neck and compare these values with the experimental measurement values and thermal modelling for verification [IEEE, ICES, COST 281: Joint workshop 2004/09 Thermal Physiology Workshop, Paris].

• It is also important to measure the emissions of the exposure system and ambient room emissions in the experimental conditions. (Excell, 1996, 1998)

• Information about the internal magnetic field should be provided.

Human RF Exposure in the Far Field

• Improved assessment techniques are needed to analyse the range of RF field exposures and absorption experienced by individuals. (Repacholi, Conference Nov ’96; Repacholi, 1998; Neubauer et al., 2005).

• The IEEE C95.1-1999 Standard for RF safety calls for spatially averaged measurements of incident power density to verify compliance with maximum permissible exposure limits. Human exposure to RF power radiated by mobile base station antennas can be assessed by means of the incident power density averaged over the body. The convenience of adopting this quantity lies in the well-behaved decay away from the antenna. (Balzano and Faraone, 1999; Faraone et al., 2000)

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• Both numerical modelling and experimental measurement are important to verify whole body human exposure [Balzano and Faraone, 1999; Faraone et al., 2000; Neubauer et al., 2005].

ELF Exposure Metrics

• In the low frequency range external magnetic fields are measured as flux density (B) in milliTesla (mT) and magnetic field strength (H) in amps per meter (A/m) respectively (IEEE Std C95.6-2002).

• In the low frequency range internal electric fields are measured as in situ (in tissue) electrical forces (Volts per metre; V/m) and behaviourally as electrostimulation which is defined as induction of a propagating action potential in excitable tissue by an applied electrical stimulus; electrical polarization of presynaptic processes leading to a change in post synaptic cell activity [Quoted from IEEE Std C95.6-2002].

Results Analysis • Results should be analysed and interpreted on the basis of the prior stated hypothesis (Triages).

• No data should be discarded without valid reason (e.g. equipment failure, procedures not followed, non-participation). Reasons for this should be recorded.

• Other tests of significance post hoc should be stated as such and an appropriate ‘multiple comparisons’ correction (I.E. Bonferroni, Tukey, or Empirical Bayes Adjustments) be applied. Post hoc results can be considered hypothesis generating for new research but cannot be considered as the direct or main result of the presented research. (Keppel, 1982; Steenland et al., 2000).

• The known reliability and validity of the tests employed and the normal variance of these tests stated in the methods should be referred to in the analysis of the results. The variance in the scores due to experimental exposure need to be stated in the interpretation of the results taking into account the normal variance of the test scores without treatment, and variance of the test scores under sham control, positive control, and negative control treatment conditions. [IEEE Std C95.6-2002; Draft IEEE Std PC95.1-2005; HCN, 2004; Foster and Repacholi, 2004; Triages].

Conclusion • The conclusion should be drawn on the basis of the prior stated hypothesis of the study

(Triages).

• Knowledgeable interpretation of the results must consider the established science (outside of the EMF area) in the field of investigation such as biophysical mechanisms of interaction, cognition, sleep, etc. [Repacholi, 1998; ICNIRP, 1998; Kandel et al., 2000; Kandel, 2001; IEEE Std C95.6-2002; ICNIRP, 2003; Foster, 2003; D’Andrea et al., 2003; NRPB, 2004; Draft IEEE Std PC95.1-2005]

• Results should be viewed with respect to previously accepted scientific principles before ascribing the results to new theories. Research findings pointing to previously unidentified relationships should be carefully evaluated and appropriate additional independent studies should be conducted before the findings are accepted (Repacholi, 1998).

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• Theories (e.g., for mechanisms of interaction) should make sufficiently concrete predictions that they can be tested experimentally and be capable of being verified, if correct (Schwan, 1988; Repacholi, 1998; Foster, 2000; Adair, 2003; Foster, 2003; Foster and Repacholi, 2004; Draft IEEE Std PC95.1-2005).

Publication • Well-designed and -conducted studies should be published regardless of the outcome, since

negative results are as useful as positive studies in the context of the EMF database for the evaluation of human health risks and the setting of standards (Kheifets et al., 2003).

Scientific Responsibilities Scientists are required to take responsibility for:

• the sensitive issue of data protection and confidentiality,

• the new opportunities, such as the increasing availability of computerized data,

• the incorporation of molecular methods to aid the investigation of mechanistic pathways and gene-environment interactions,

• and the development and utilization of sophisticated statistical approaches.

[Quoted from Rushton and Elliott, 2003; see also Conrads et al., 2004; Dorman, 2005; Espina et al., 2005; Gilham et al., 2005; LaPorte, 2005]

Replication • Preferably, the experiments should be repeated and the data confirmed independently, or the

claimed effects should be consistent with results of similar experiments, for which the biological systems involved are comparable (Repacholi, 1998).

Meta Studies • Since the technologies of electricity and radio telecommunications serve billions of people it is

appropriate to address the remaining health concerns where possible in large multi-centred international studies (Haarala et al., 2004).

• Where possible, feasibility studies (pilot studies) for multi-centred studies should be undertaken with input from international experts to verify and validate every aspect of the methodology of the EMF field study under question. The feasibility studies should be open for further scientific comment and published in peer reviewed journals

• Simultaneous human experimental replications should use the same or similar methodologies including standardized questionnaires and should report data to provide quantitative results that can be compared and or combined for meta analysis of all the independently collected data (Haarala et al., 2004).

before these large studies are funded and carried out.

• In meta studies the data collection and analyses should be done according to the hypothesis and methodology of the published protocol.

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In Vivo Study Design

Please refer to the‘General Experimental Design Criteria’ and the ‘General Research Priorities’ above and the in vivo triage below for further details.

Introduction

• Animal studies provide information concerning the interaction of EMFs with living systems that display the full repertoire of body functions, such as immune responses, cardiovascular changes and behaviour.

• Individual animals in inbred strains are genetically identical, thus ensuring a relative consistency of response to the agent in question reducing the variance and improving the sensitivity of the study.

• Transgenic or gene knockout animal models of certain diseases have further increased the value of animal studies to reveal potential adverse health effects. (NRPB, 2004)

• Animal studies can be carried out over the brief lifetime of the animal (~ 2 years) to answer health questions about lifetime exposure effects.

• Animal studies can use exposures above guideline limits and thus report dose, duration and threshold data.

• The number of subjects can be increased to improve the statistical power of the experiments.

• Extrapolation of this information to humans may be considered, if there are proven homologies in structures and processes in animals and humans. For example, animal studies have been very useful in helping unravel the sequence of genetic events in the development of a number of human cancers (Balmain and Harris, 2000) (NRPB, 2004) and in the molecular mechanisms of learning (Kandel et al., 2000, 2001).

• IARC (2002) noted that, with regard to cancer ‘in the absence of adequate data on humans, it is biologically plausible and prudent to regard agents and mixtures for which there is sufficient evidence of carcinogenicity in experimental animals as if they presented a carcinogenic risk to humans’.

Hypothesis

• The project should test a clearly defined hypothesis, using a detailed protocol that would lead to quantitative information directly or indirectly relevant to assessment of health risk from ELF and RF exposure and allow any other independent laboratory(s) to reconstruct the study and replicate the findings for validation.

Good Laboratory Practice Good Laboratory Practice (GLP) should be used throughout the design and conduct of any study where possible and practical, but especially with large and long-term animal studies (see, e.g., FDA, 1993, NTP 1992).

• A specific protocol, consistent with the GLP guidelines, should be established and documented. Any changes instituted during the course of the study should also be documented.

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• The protocol should include randomized, double blind, symmetric handling of animals and specimens, except when precluded by the nature of the experiment or biological system.

• The protocol should include all appropriate controls (positive, negative, cage controls, sham-exposed etc.).

• Investigators should be blind to whether they are working with exposed or control animals or biological systems.

Quality assurance (QA) procedures should be included in the protocol, including traceable dosimetry and monitoring of the programme by both a team from within the experimental staff and an independent group, as required by GLP [Repacholi, 1998; Repacholi and Greenebaum, 1999; Schönborn et al., 2004; ICNIRP, 2003; IEEE Std C95.6-2002; IEEE Std C95.1, 1991/1999; Draft IEEE Std PC95.1-2005].

Methodology • Well-characterized biological systems (animal species) should be used, preferably ones that are

well established (validated) from the scientific literature that can be used reliably by other laboratories.

• The biological system used should be appropriate to the end-point(s) studied.

• Threshold responses derived from using at least 3 levels of dose and duration of exposure data where possible are sought in addition to sham-exposed controls (‘Sham/Sham’). Appropriate positive controls as well as non-RF heating controls can be very important to help assess the metric of the response of the biological system and the potential effects of RF heating should be included.

The a priori estimated statistical power of the experiment, based on prior knowledge and the number of tests planned, should be sufficient to reliably detect the expected size of the effect (often as small as 10-20%) (In vivo Triage).

Traceable Dosimetry

1. Current and RF future research should be applicable to electrical and mobile telephone systems in use. For RF research the focus should be on signal protocols in use, e.g. second, third and fourth generation signals (2G, 3G, 4G) including their modulation patterns. For radars, the frequency and pulsing regimes should be applicable to current and emerging systems. [Derived from Kuster and Balzano, 1996; Schönborn et al., 2000; IEEE Std C95.6-2002; ICNIRP, 2003; Foster and Repacholi, 2004; Kuster et al., 2004; Nikoloski et al., 2005; Andersen, and Foster Presentations, COST 281 Zurich Feb 17-18, 2005; Reilly, 2005]

2. Environmental conditions, such as temperature, humidity, light, vibration, sound, and background EMFs, should be measured and recorded periodically. All experimental conditions should be the same for all groups, except for EMF exposure [Schönborn et al., 2004; Engineering and In vivo Triages].

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2. Experimental EMF should be fully characterized and measured periodically. Waveform, pulse shape and timing, frequency spectrum, harmonics and transients from both continuous sources and from switching exposure systems on and off, should all be measured where appropriate. Background fields, such as ambient, equipment-derived, and crossover fields from other exposure systems, are also important and need to be characterized. Time-varying and static components should be measured, as well as the polarization and directions of the fields. Field modulation introduced by experimental factors such as motion of sample shakers should be noted and measured whenever possible. Positioning of animals within exposure systems should be noted and randomised where appropriate.

3. Experimental dosimetry should confirm any calculated values; measurements should be taken at multiple points in any in vivo model; should be reported in SAR (W/kg) - not simply as a field strength measure; should specify whether whole body or local SAR for animal studies; and should detail the method of measurement. (Repacholi, Conference, Nov ’96; Repacholi, 1998; International EMF Project/ ICNIRP, 1999)

4. In considering experimental studies more generally, a considerable problem in the interpretation of experiments is that many of them have given insufficient detail concerning exposure conditions. Moreover, in the case of pulsed fields when SAR values are quoted it is often unclear whether these refer to the average SAR or to the peak SAR during pulses. It is very important to make this distinction, since the peak SAR can be 1000 or more times the average value. Full details should be provided of experimental conditions including maximum SAR per pulse for pulsed radiation.

5. Failure to adequately characterize and control for experimental conditions (i.e. immobilization) with appropriate cage control animals, could significantly mask any potential effects mediated by the RF field on stress-related parameters (Stagg et al., 2001).

6. A comprehensive uncertainty, variability and artifact dosimetric analysis is required to achieve consistent interpretation of the results (Quoted from Kuster et al., 2004; IEEE Std 1528-2003)

Data collection and quality assurance

1. The full protocol, including QA, (including double blind conditions) should be followed strictly, as should GLP provisions for monitoring this.

2. Data should be recorded contemporaneously and back-up copies kept of all electronic data.

3. No data should be discarded without valid reason (e.g. equipment failure, procedures not followed). Reasons for this should be recorded.

4. As part of the QA programme, at least one independent reassessment of all or an appropriate sample of specimens should be made, when assays require an independent judgment by the investigator (e.g., histological evaluations).

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5. Where possible, samples should be stored for future reference.

Results Analysis • The stored data set should contain all data, and if any data are excluded from an analysis, clear,

legitimate reasons for doing so should be recorded.

• Analysis techniques should be appropriate to the data and the hypothesis and results should be analysed and interpreted on the basis of the prior stated hypothesis (In Vivo Triage).

• Other tests of significance post hoc should be stated as such and an appropriate ‘multiple comparisons’ correction (I.E. Bonferroni, Tukey, or Empirical Bayes Adjustments) be applied. Post hoc results can be considered hypothesis generating for new research but cannot be considered as the direct or main result of the presented research. (Keppel, 1982; Steenland et al., 2000).

• The known reliability and validity of the tests employed and the normal variance of these tests stated in the methods should be referred to in the analysis of the results. The variance in the scores due to experimental exposure needs to be stated in the interpretation of the results taking into account the normal variance of the test scores without treatment, and variance of the test scores under sham control, positive control, and negative control treatment conditions. [IEEE Std C95.6-2002; Draft IEEE Std PC95.1-2005; HCN, 2004; Foster and Repacholi, 2004; In Vivo Triage].

Conclusions

• Conclusions should be drawn on the basis of the hypothesis, be fully supported by the data and include all-important implications of the data set.

• Reports should include enough data and information concerning materials and methods to allow independent assessment of the conclusions and discussion.

• Knowledgeable interpretation of the results must consider the established science (outside of the EMF area) in the field of investigation such as biophysical mechanisms of interaction, genotoxicity, cancer, memory [Lee and Nurse, 1987; Repacholi, 1998; ICNIRP, 1998; Kandel et al., 2000; Kandel, 2001; IEEE Std C95.6-2002; ICNIRP, 2003; D’Andrea et al., 2003; Foster, 2003; Vijayalaxmi and Obe, 2004; Obe and Vijalalaxmi, 2005; Draft IEEE Std PC95.1-2005; Moulder et al., 2005]

• Results should be viewed with respect to previously accepted scientific principles before ascribing the results to new theories. Research findings pointing to previously unidentified relationships should be carefully evaluated and appropriate additional independent studies should be conducted before the findings are accepted (Repacholi, 1998; Adair, 2003; Foster, 2003).

• Theories (e.g., for mechanisms of interaction) should make sufficiently concrete predictions that they can be tested experimentally and be capable of being verified, if correct (Schwan, 1988; Repacholi, 1998; Adair, 2003; Foster, 2000; Foster, 2003; Foster and Repacholi, 2004; Draft IEEE Std PC95.1-2005).

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Publication • Well-designed and -conducted studies should be published regardless of the outcome, since

negative results are as useful as positive studies in the context of the EMF database for health risk evaluation (Kheifets et al., 2003).

• Timely peer reviewed publication is essential. (Repacholi, 1996; Repacholi, 1998; International EMF Project/ICNIRP, 1999; Kheifets et al., 2003).

Scientific Responsibilities Scientists are required to take responsibility for:

• the sensitive issue of data protection and confidentiality,

• the new opportunities, such as the increasing availability of computerized data,

• the incorporation of molecular methods to aid the investigation of mechanistic pathways and gene-environment interactions,

• and the development and utilization of sophisticated statistical approaches.

[Quoted from Rushton and Elliott, 2003; see also Conrads et al., 2004; Dorman, 2005; Espina et al., 2005; Gilham et al., 2005; LaPorte, 2005]

Replication • Preferably, the experiments should be repeated and the data confirmed independently, or the

claimed effects should be consistent with results of similar experiments, for which the biological systems involved are comparable (Repacholi, 1998).

Meta Studies • Since the technologies of electricity and radio telecommunications serve billions of people it is

appropriate to address the remaining health concerns where possible in large multi-centred international studies [for example Perform A EU 2000-2005; FDA study USA, 2004-2008]

• Where possible, feasibility studies (pilot studies) for multi-centred studies should be undertaken with input from international experts to verify and validate every aspect of the methodology of the EMF field study under question. The feasibility studies should be open for further scientific comment and published in peer reviewed journals

• Simultaneous experimental replications should use the same or similar methodologies and should report data to provide quantitative results that can be compared and or combined for meta analysis of all the independently collected data.

before these large studies are funded and carried out

• In meta studies the data collection and analyses should be done according to the hypothesis and methodology of the published protocol and the guidelines for complex statistical analyses (Keppel, 1982).

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In Vitro Study Design

Introduction Please refer to ‘General Experimental Design Criteria’ and the ‘General Research Priorities’ above and the in vitro triage below for details as well.

Traditionally, cellular research was considered as the possible source of a plausible mechanism for any biological effect due to exposure to RF or ELF signals [Repacholi and Cardis, 1997; Repacholi, 1998].

Great advances in cellular research have given us a new understanding of the interplay of cellular research of genes, proteins and cell signalling in single living cells, and the functioning of the whole human body in health and disease [Lee and Nurse, 1987; Kandel et al., 2000; Kandel, 2001; Conrads et al., 2004; Dorman, 2005; Espina et al., 2005; Laporte, 2005].

These molecular advances have an impact on EMF research priorities. Molecular profiling may become essential at all levels of research namely in epidemiology, human acute studies, animals studies and tissue cultures. We now see the incorporation of molecular methods to aid the investigation of mechanistic pathways and gene-environment interactions.

Hypothesis

The in vitro study should test a clearly defined hypothesis, using a detailed protocol that would lead to quantitative information directly or indirectly relevant to assessment of health risk from ELF and RF exposure and allow any other independent laboratory(s) to reconstruct the study and replicate the findings for validation and allow the results be combined for meta analysis where appropriate.

Good Laboratory Practice

Good Laboratory Practice (GLP) should be used throughout the design and conduct of any study where possible and practical (see, e.g., FDA, 1993, NTP 1992).

Quality assurance (QA) procedures should be included in the protocol, including traceable dosimetry and monitoring of the programme by both a team from within the experimental staff and an independent group, as required by GLP [Repacholi, 1998; Repacholi and Greenebaum, 1999; Schönborn et al., 2000; ICNIRP, 2003; IEEE Std C95.6-2002; IEEE Std C95.1, 1991/1999; Draft IEEE Std PC95.1-2005]

Methodology

• Well-characterized biological systems or assays should be used, preferably ones that are well established (validated) from the scientific literature that can be used reliably by other laboratories (Murchan et al., 2003; Vijayalaxmi and Obe, 2004; Obe and Vijayalaxmi 2005).

• The biological system used should be appropriate to the end-point(s) studied.

• Threshold responses derived from using at least 3 levels of dose and duration of exposure data where possible are sought in addition to sham-exposed controls (‘Sham/Sham’). Appropriate positive controls as well as non-RF heating controls can be very important to help assess the metric of the response of the biological system and the potential effects of RF heating should be included.

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• Methods of staining and scoring for tests including DNA breakage [such as micronuclei (MN), chromosome aberrations (CA) and sister chromatid exchange (SCE)] should be standardised, and their variability, reliability, and validity stated.

• Procedures should be double-blinded and scored by more than 1 independent laboratory before breaking the code.

• The a priori estimated statistical power of the experiment, based on prior knowledge and the number of tests planned, should be sufficient to reliably detect the expected size of the effect (often as small as 10-20%) (In vitro Triage).

• Sufficient numbers of each sample for staining and scoring for tests including DNA breakage [for instance, MN, CA, SCA] should be taken to insure the power of the study is large enough to find a significant effect of exposure if there is one. [COST Action 281 Recommendation on an Internationally Co-ordinated Research on Genotoxic Effects of Electromagnetic Radiation from Mobile Communication Systems, http://www.cost281.org/activities/Gentox-recomm-090304AW.doc; Brüske-Hohlfeld et al., 2001; Vijayalaxmi and Obe, 2004; Obe and Vijayalaxmi, 2005].

• Further experiments should include techniques for genomics, proteomics and transcriptomics to give us information about genes and the state of protein such as whether it is activated or inactivated. The power of newer protein arrays is that they give us a window into the microcircuitry of the cell, revealing which pathways are being used at a particular moment [Conrads et al., 2004; Espina et al., 2005]. They may provide crucial information that could even be due to (IR) radiation therapy [Sreekumar et al., 2001] or possibly EMF exposure.

Exposure Conditions There should be well-defined exposure conditions with improved control of the exposure parameters, continuous monitoring and control of the environmental parameters, support of a double blinded study protocol, carefully characterized dosimetry including the average specific absorption rate (SAR), standard deviation of the SAR, monitoring of the temperature load, determination of the local SAR and possible temperature hotspots. (Nikoloski et al., 2005)

1. Temperature, atmosphere in CO² incubators, vibration, and stray fields from incubator heaters and fans are sources of asymmetry (differences between exposed and control samples) that are often overlooked in cell and tissue culture experiments. These must be measured with appropriate instrumentation and every effort made to ensure that any differences are minimized, except for EMF exposure of the "exposed" samples.

• The environmental requirements (e.g., stabilized temperature, atmospheric control, sterility) must be strictly fulfilled.

• All relevant technical and biological parameters must be monitored during the experiment.

• The most important technical data should be logged in order to track possible malfunctions of the system.

• All controlling and monitoring devices should be rigorously checked for interference under worst- case considerations.

• Non-disturbance of commercial services must be ensured [Burkhardt et al., 1996; Schönborn et al., 2000; Schuderer and Kuster, 2003; Nikoloski et al., 2005].

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2. Contemporaneous positive controls for the assay and for non-RF thermal effects are recommended and negative controls, both maintained under identical circumstances to exposed cultures, sham-sham comparisons of multiple exposure systems, randomized handling of cultures, and blinding, forced cooling and repeatable positioning of the flasks, should form part of the study, as appropriate. It is strongly recommended that any pathology or histopathology be performed in a blinded fashion.

3. To characterize electric fields or induced currents in cultures, electrode geometry and materials (including agar bridges, etc.), dish shape and dimensions, depth of medium and specimen dimensions, conductivity (RF and ELF) and dielectric constant (RF only) of medium are important.

4. Comparisons of E field, SAR and temperature measurements should result in good agreement.

5. Signal characteristics and field strengths inside the medium must be relevant to risk assessment. Therefore, the system should allow for field strengths that are at least as high as the relevant limits defined by the regulatory bodies [ANSI, 1992; CENELEC, 1998; ICNIRP, 1998] or the highest exposure occurring in real life situations.

6. The field distribution at the location of the cell culture should be as homogeneous as possible. The target value shall be better than ±30% [Kuster and Schönborn, 2000; Schönborn et al., 2000; 2001; Schuderer and Kuster, 2003; Nikoloski et al., 2005].

7. The signal characteristics must be well defined. In particular, this requires a signal source that is well defined with respect to the frequency, modulation scheme, power stability, and noise level (Schönborn et al., 2000]. Field values should be measured directly. Electrophoretic products should be considered and measured, where possible, when electrodes are used.

8. ELF magnetic field experiments should consider the factors above as they apply to induced current. The angle between applied field and medium, as well as the angle between applied ELF fields and the local DC field, should be measured.

9. When using media, serum or other reagents that may have variation from batch to batch, serious consideration should be given to purchasing and storing sufficient stocks in a single batch for the duration of the experiment. Similarly, the characteristics of cell lines derived from a standard source should not be allowed to diverge over time. There should be backup stocks from the original source.

10. For experiments lasting more than a few days and in all cases where samples or stock cultures are maintained for extended periods or data are gathered or stored electronically, backup systems must be installed to protect the work against equipment or power supply failure.

11. The costs for design, construction, and maintenance of the system should be reasonable. The system should be easy-to-use and as foolproof as possible. (Quoted from Schönborn et al., 2000).

Results Analysis • Results should be analysed and interpreted on the basis of the prior stated hypothesis (In vitro

Triage).

• Other tests of significance post hoc should be stated as such and an appropriate ‘multiple comparisons’ correction (I.E. Bonferroni, Tukey, or Empirical Bayes Adjustments) be applied. Post hoc results can be considered hypothesis generating for new research but cannot be

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considered as the direct or main result of the presented research. (Keppel, 1982; Steenland et al., 2000).

• The known reliability and validity of the tests employed and the normal variance of these tests stated in the methods should be referred to in the analysis of the results. The variance in the scores due to experimental exposure needs to be stated in the interpretation of the results taking into account the normal variance of the test scores without treatment, and variance of the test scores under sham control, positive control, and negative control treatment conditions. [IEEE Std C95.6-2002; Draft IEEE Std PC95.1-2005; Foster and Repacholi, 2004; In vitro Triage].

Conclusion • The conclusion should be drawn on the basis of the prior stated hypothesis of the study (In vitro

Triage).

• Knowledgeable interpretation of the results must consider the established science (outside of the EMF area) in the field of investigation such as biophysical mechanisms of interaction, genotoxicity, DNA, RNA and protein replication, cell cycle, reproduction, apoptosis, free radicals, molecular mechanisms, and cell signalling [Repacholi, 1998; ICNIRP, 1998; IEEE Std C95.6-2002; ICNIRP, 2003; Foster, 2003; Vijayalaxmi and Obe, 2004; Obe and Vijalalaxmi, 2005; Draft IEEE Std PC95.1-2005].

• Results should be viewed with respect to previously accepted scientific principles before ascribing the results to new theories. Research findings pointing to previously unidentified relationships should be carefully evaluated and appropriate additional independent studies should be conducted before the findings are accepted (Repacholi, 1998; Adair, 2003; Foster, 2003).

• Theories (e.g., for mechanisms of interaction) should make sufficiently concrete predictions that they can be tested experimentally and be capable of being verified, if correct (Schwan, 1988; Repacholi, 1998; Adair, 2003; Foster, 2000; Foster, 2003; Foster and Repacholi, 2004; Draft IEEE Std PC95.1-2005).

Publication • Well-designed and -conducted studies should be published regardless of the outcome, since

negative results are as useful as positive studies in the context of the EMF database for health risk evaluation (Kheifets et al., 2003).

Scientific Responsibilities Scientists are required to take responsibility for:

• the sensitive issue of data protection and confidentiality,

• the new opportunities, such as the increasing availability of computerized data,

• the incorporation of molecular epidemiological methods to aid the investigation of mechanistic pathways and gene-environment interactions,

• and the development and utilization of sophisticated statistical approaches.

[Quoted from Rushton and Elliott, 2003; see also Conrads et al., 2004; Dorman, 2005; Espina et al., 2005; Gilham et al., 2005; Keppel, 1982; LaPorte, 2005]

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Replication • Preferably, the experiments should be repeated and the data confirmed independently, or the

claimed effects should be consistent with results of similar experiments, for which the biological systems involved are comparable (Repacholi, 1998).

Meta Studies • Since the technologies of electricity and radio telecommunications serve billions of people it is

appropriate to address the remaining health concerns where possible in large multi-centred international studies [COST 281, Directive, March 11, 2004].

• Where possible, feasibility studies (pilot studies) for multi-centred studies should be undertaken with input from international experts to verify and validate every aspect of the methodology of the EMF field study under question. The feasibility studies should be open for further scientific comment and published in peer reviewed journals

• Simultaneous in vitro replications should use the same or similar methodologies including standardized tests and should report data to provide quantitative results that can be compared and or combined for meta analysis of all the independently collected data.

before these large studies are funded and carried out (COST 281, Directive, March 11, 2004).

• In meta studies the data collection and analyses should be done according to the hypothesis and methodology of the published protocol.

This is a living document and research methods are updated continually as are the IEEE ICES SC-4 Triages listed in Appendix A below.

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Appendix A

IEEE ICES SC-4 TRIAGES

1. In Vivo Working Group Triage, 16 screens

2. In Vitro Working Group Triage, 17 screens

3. Epidemiology Working Group, Triage, 13 screens

4. Engineering Working Group Triage, 17 screens

5. Chair Working Group Triage 7 screens

The 5 Triages (listed below) were developed by Martin Meltz and ICES SC-4 colleagues to regularise and automate the compilation of anonymous peer review results of the literature identified by the Literature Surveillance Working Group (See also ICES SC-4, page 7).

For Example IEEE SC-4: In Vivo Triage: 16 Screens Are Summarized Below: Questions on the form include the date the paper is received by reviewer, the paper accession number, and the reviewer code number. The rating scale on each criteria is: high 3; acceptable 2; low 1; not acceptable= 0.

Briefly, there 9 criteria rating scales:

A. Reviewer rating for clarity of statement if objective, specific goals and/or hypothesis. 0-3

B. Reviewer rating for completeness of description of biological system exposed. 0-3

C. Reviewer rating for completeness of description of the time/duration of exposure and biological response. 0-3

D. Reviewer rating for completeness of description of the organs systems studied and endpoints examined. 0-3

E. Reviewer rating for confidence in the methodologies employed. 0-3

F. Reviewer rating for confidence in the completeness of the data reporting and the merit if the data analysis. 0-3

G. Reviewer rating for confidence in the conclusion of the authors. 0-3 (give a statement of the author’s conclusions)

H. Conclusion Presentation (Does the title and the abstract accurately reflect what was measured?). 0-3

I. Overall technical merit rating of the in vivo bioeffect review. 5-1

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I request that the chair have this paper reviewed by the statistics WG: yes/no

Overall rating, if it is 2 or 1 your are requested to give reasons for the score.

Category score A..B..C..D..E..F..G.. Overall Score. 0-3

Study will become important to the standards setting process if independently replicated. yes/no

Relevance for human standard setting. 0-3

All the 5 Working Group Triages are attached below for your use:

1. In Vivo, 2.In Vitro, 3. Epidemiology, 4. Engineering, 5. WG Chair.

[1] The IEEE is today the world’s largest technical professional society, with more than 365,000 members in over 150 countries.

[2] IEEE Std C95.6-2002, “IEEE Standard for Safety Levels with Respect to Human Exposure to Electromagnetic Fields, 0 to 3 kHz,” and IEEE Std C95.1-1991/1999, “IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz.”

[3] Detailed guidelines on the conduct of high quality laboratory research can be found in the good laboratory practice guidance of the US Food and Drug Administration (FDA, 1993) and in the specifications of the US National Toxicology Program (NTP, 1992).

[4] General uncertainty in measurement is defined as: The estimated amount by which the observed (measured) or calculated value of a quantity may depart from the true value. Ordinarily taken as the sum of the random errors at the 95% confidence level and the estimated upper limit of the systematic error. NOTE—The uncertainty is often expressed as the average deviation, the probable error, or the standard deviation. IEEE Std 1528-2003

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Use of magnetic field dosemeters for occupational exposure assessment

Philip Chadwick, EMF Dosimetry Handbook Project Co-ordinator

This summary should be read in conjunction with John Swanson's chapter on Power frequency EMF measurements

In 1997 the UK Health and Safety Executive published Contract Research Report CRR 1997:128 on the suitability and use of an EMDEX II magnetic field dosemeter for the assessment of the magnetic field exposure of induction heating workers.

This is a report of good general usefulness as the induction heating environment can give rise to some of the highest occupational exposures to magnetic fields at a range of frequencies, from les than 10 Hz to greater than 10 kHz. The waveforms of the magnetic fields can be complex, with a high harmonic content, and exposures can be transitory and very dependent on the details of working practices.

This brief summary is provided for the EMF Dosimetry Handbook by permission of HSE

The HSE report contains some very useful general conclusions about the use of personal dosemeters in industry:

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• Dosemeters will give better estimates of true operator exposure than can be derived from spot measurements and observations of working practices

• The use of dosemeters on multiple workers or for prolonged periods will minimise the perturbation of working practices

• Exposures which occur infrequently and which would not be predicted from routine observation may be captured by a dosemeter

• Variations in emitted magnetic flux density during a long production cycle will be accounted for

Several aspects of magnetic field dosimetry in industrial environments were emphasised:

• The exposure environment must be characterised, to allow comparison with exposure standards and to ensure that the magnetic flux densities encountered are within the frequency and dynamic ranges of the dosemeter

• The dosemeter data should be examined carefully for evidence of bad data and the range of recorded exposures compared with those predicted from spot measurements. Discrepancies should be investigated further

• The functionality of the dosemeter should be checked immediately before it is issued and after it has been returned

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• It is a feature of many induction heating environments that the magnetic flux density varies across the dimension of the human body. It must be recognised that the position at which the dosemeter is worn may affect the maximum magnetic flux density that it records.

Particular conclusions related to practical aspects of the use of dosemeters included:

• Most of the operators wore dosemeters for one or two days only, and it is conceivable that their working practices may have been affected, consciously or otherwise.

• Some of the dosemeter data are not consistent with the spot measurements of magnetic flux density made at each site and it is likely that the magnetic field emissions vary markedly between different stages in the production process

• Reliable personal dosimetry would require the wearing of a dosemeter for many days.

• The exposure of induction heater operators is characterised by transient exposure maxima as they approach the coils and the dosemeter should be set to the highest sample rate possible to avoid problems of aliasing. Where aliasing does occur, the amplitude of the maxima is likely to be underestimated but warning of this is given by the presence of the bad data flag in the key byte of the EMDEX data file. Workers' exposure records should be checked for the presence of this flag and if it is present, the data treated with caution.

• Sample rate is often limited by the available memory of the dosemeter, and memory constraints also preclude routine measurement of the separate vector components of magnetic flux density.

• It is vital that the battery not be allowed to run down before the data file is transferred to a computer or the exposure information will be lost.

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• If the dosemeter is dropped, one or more sensing coils may be damaged. This may not be obvious to the user but could seriously affect the validity of the data gathered. It is important that the integrity of each data channel is checked with a test source before and after each exposure assessment is made.

Dosemeter record from operator of induction furnace

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Power frequency EMF measurements

John Swanson, Technology. & Science Labs., National Grid Transco Research. & Development. Centre, Leatherhead, UK

1 General principles Measurements of electric and magnetic fields can be made for various purposes. Depending on the purpose, different techniques will be appropriate. There is no single “correct” procedure; the correct procedure will depend on the reason the measurements are being made and the use that will be made of the results.

When embarking on a measurement programme, therefore, it is important first to identify clearly the objective. This objective must be used to decide on the procedures to be followed. This chapter discusses the choices and issues involved, and gives suggestions for appropriate procedures, but is not a substitute for intelligent, informed choice on the part of the person specifying or performing the measurement.

The main international standard relating to measurements at power frequencies is a 1998 IEC standard IEC-61786, “Measurement of low-frequency magnetic and electric fields with regard to exposure of human beings – Special requirements for instruments and guidance for measurements”. The provisions of this standard are referred to where appropriate in the following material.

2 Types of measurement The following are some of the main reasons for performing measurements. This is not, however, an exhaustive list.

Simple characterisation of the field in a building

The objective is to characterise the field in a home, a work location, or similar, by a single number. Depending on resources and time available, this can be done by a single measurement at a single location; a single measurement at each of a series of locations; a sequence of measurements over time at a single location; or a sequence of measurements over time at multiple locations.

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Identification of sources

The objective is to make measurements of the field at a location, specifically how it varies over space or time, to enable the source of the field to be identified.

Comprehensive characterisation of the field in a building

The objective is to collect more data on the field in a building than just a single number, so as to permit extraction of desired information at a later date.

Characterisation of sources

The objective is to perform measurements that relate to a particular source of field (eg a power line, an item of equipment) rather than to the field in a particular place, so as to characterise that source.

Personal exposure

The objective is to measure the exposure of a person over a period of time during which they are exposed to fields from various sources.

Compliance with exposure limits

The objective is to assess whether a given set of EMF exposure limits are exceeded. This can be done either by assessing the fields in an area in which people will be present, or by monitoring the exposure of the people, or a combination.

Laboratory measurements

The objective is to measure the field within experimental apparatus in a laboratory, eg the field produced by coils used to expose an experimental system to magnetic fields.

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3 Choice of instrument for magnetic field measurements

3.1 Sensor technology

There are three technologies for measuring magnetic fields: fluxgate magnetometers, Hall effect devices, and search coils.

3.1.1. Fluxgate magnetometers

These comprise a ferromagnetic core which is driven in and out of magnetic saturation in opposite directions by a high-frequency current generated within the instrument. The addition of an external field creates an asymmetry, and the instrument uses this asymmetry to measure the field. Fluxgate magnetometers are sensitive to all external fields up to a certain frequency, static fields as well as alternating fields. The static and alternating components of the field can be separated and recorded separately by the instrument circuits.

Fluxgate magnetometers are usually the preferred choice when it is desired to measure static fields as well as alternating fields. The fluxgate sensor itself can be made reasonable small and is usually remote from the bulkier remainder of the instrument, so they can have uses where it is necessary to probe the field in difficult locations. Other than these specific applications, their use is limited, as they are more expensive and have higher battery consumption than search coil instruments. Care should be taken if using them in laboratory settings, as they produce a finite high-frequency field themselves as part of their operation which could perturb an experimental setup.

3.1.2. Hall effect devices

These devices pass a current through a suitable semiconductor, and detect the field via the voltage produced across the element perpendicular to the current. Like fluxgate magnetometers, they detect the instantaneous total field, the sum of the static and any alternating fields. They are often used in other applications because they can measure high fields at the Tesla level and above. However, they are usually not very sensitive to low fields and suffer from zero-point drift, and the probes are often fragile, which means they have few applications in EMF dosimetry.

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3.1.3. Search coils

Search coils are simply coils of wire. An alternating magnetic field induces a voltage in the coil. Except for the specialised instances where fluxgate magnetometers are appropriate, search coils are the preferred technology for EMF dosimetry where it is not desired to measure the static field as well.

Characteristics of search-coil instruments

• Size of coil

To obtain a large enough signal, search-coil instruments tend either to have relatively large coils (10-20 cm), or small coils (less than 1 cm) with ferrous cores. Larger coils are perfectly acceptable as long as the field itself does not vary significantly over the area of the coil. Thus they will often be acceptable under power lines or in the middle of rooms, but will be less appropriate close to conductors or to equipment. IEC specifies that the coil should have area 0.01 m2 or less.

It is sometimes recommended to use large coils so as to measure the average field over an area comparable to part of the human body. Usually, however, where the field varies over space this rapidly, it is preferable to measure the variation of the field with a small probe and then to apply any desired averaging to the results of those measurements.

Small, ferrous-cored coils are often preferred and produce a smaller, more versatile instrument. The one disadvantage is that the ferrous core produces non-linearities at high fields, greater than 1 mT. For most purposes this is irrelevant as fields are rarely that high, but care should be exercised if high fields are to be measured.

• Number of coils

Each coil measures the component of field in one direction. Instruments have either one or three orthogonal coils. The choice depends on the purpose of the measurement. When identifying and investigating different sources of field, a single coil can be useful, as the extra information on the

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direction of the field can assist in identifying the source. For most other applications, however, three coils are more useful.

The resultant field from multiple sources at the same frequency always traces out an ellipse. There are two alternative ways of quantifying an elliptically polarised field, shown in figure 1.

Figure 1 Alternative measures of an elliptically polarised field. 1:

“maximum” field. 2: “resultant” field

Vector 1 gives the rms of the field along the major axis of the ellipse, which is the direction of maximum field. This is known as the “maximum” of the field. Vector 2 gives the actual rms of the field, known as the “resultant field”.

The “maximum” field, vector 1, can be measured by rotating a single coil until the maximum value is obtained. The “resultant” field, vector 2, can be measured using three orthogonal coils. The rms of the signals from the three coils are combined as root-sum-of-squares to give the resultant field. This applies regardless of the orientation of the coils relative to the field.

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Three orthogonal air-cored coils can be arranged so that, to a good approximation, their centres are coincident and therefore they measure the field at the same point in space. Figure 2a shows an example. Ferrous-cored coils are nearly always physically separate. This still gives satisfactory results as long as the field does not vary over the distance scale of the separation of the coils. As this can be small (of the order of a cm) this is usually acceptable. Figure 2b shows three such ferrous-cored coils.

Figure 2 Examples of instruments using

(a) three orthogonal coils arranged to be centred on a single point

(b) three separate orthogonal coils

If only a single-axis instrument is available, the resultant (total) field can be measured by performing three successive orthogonal measurements. This can have limited accuracy, however, if the field varies over the time taken to perform these measurements. It can also be difficult to locate the sensor at the same point each time; if accuracy is required, a jig to locate the instrument should be used.

3.2 Type of display

Increasingly, most displays are digital. The main occasion when an analogue display is more useful is when investigating a source, when variations in the field as the meter is moved need to be readily observed, but even here a digital display can be used.

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3.3 Range and resolution

Average background fields in homes (ie fields in the general volume of the home, not close to equipment) range from a few tens of nanotesla in some European countries to over a hundred nanotesla in North America. If performing measurements in a high-field country, and particularly if the emphasis of the measurements is on high fields in that country (eg on identifying homes with fields greater than 200 or 400 nT), a resolution of 10 nT will be adequate. In many other instances for measurements in homes, however, a resolution of 1 nT is desirable.

The desired range of the instrument depends on the maximum field likely to be encountered. Background fields in homes are rarely greater than 1 µT; fields in industrial settings or near domestic equipment can be 1 mT or more. Occupational exposure limits are typically of the order of a mT, so a range of at least this is necessary to assess compliance with such limits.

3.4 Storage abilities

If using measurements of field to investigate a source, no storage ability is necessary; the changes in field as the instrument is moved are observed in real time. For some other purposes, storage within the instrument is not needed; the reading can be written down or entered into a database. For many purposes, however, it is helpful or necessary to store results within the instrument. The choice of measurement interval and total duration is a compromise limited by the total storage capacity available in the instrument.

3.5 Frequency response

The output of a search coil is proportional to frequency. Most instruments adapt this frequency response within the instrument to give a final output which is either broadly flat between a lower cut-off (typically 20-30 Hz) and an upper cut-off (typically 500 Hz – 5 kHz), or is sensitive only to the power frequency, 50 or 60 Hz.

The choice of frequency response is determined by the characteristics of the field to be measured and the purpose of the measurements. In many cases, the source of the field is the power system. Then, the main component of field will be at 50 or 60 Hz, with smaller harmonics at multiples of this, principally three times or “third harmonic”, 150 or 180 Hz. Most flat responses will be adequate for this. However, if there is particular interest in harmonics, or if the sources is such that higher harmonics (eg from some ac-dc power conversion processes) or lower frequencies (eg from some railways at one-third the power frequency) are present, then an instrument with a particular frequency response may be appropriate.

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Note that search coils produce a signal when rotated in a static field. To avoid this being erroneously recorded as a power-frequency signal, many instruments have filters to reduce the response at low frequencies. If an instrument sensitive to lower frequencies is used (eg because railways are to be measured), extra care must be taken to avoid this interference.

Exactly how flat a frequency response is depends on the sophistication of the instrument. However, achieving a flatter frequency response also tends to increase the power consumption and the weight of the instrument. Therefore, instruments designed to be small, light and with long battery life may have poorer frequency responses. This is part of the choice that has to be made, but in many instances, the flatness of the frequency response will not affect the result much.

3.6 rms and other measures

Increasingly, instruments are made with true rms detection of the field, and this is regarded as preferable for most purposes.

Alternatives are rectified average and peak detection. There may be occasions when each of these is desirable. However, in general, these are a legacy of less developed instrument technologies, and should be avoided. They cause particular problems when harmonics are present.

3.7 Accuracy

The accuracy required of an instrument should, in principle, be determined by the measurement purpose. However, IEC specifies an overall uncertainty in the measurement of a uniform field of ±(10% of reading +20 nT), and most commercial instruments can be regarded as accurate to at least this 10% value.

3.8 Other factors

IEC specifies that an instrument should function over a temperature range of 0 to 45 °C and from 5% to 95% relative humidity.

If measurements are to be done in the high electric fields close to high-voltage equipment, the instrument should be immune to these fields. IEC specifies that fields of 20 kV m-1 should produce no

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more than 20 nT change in the reading. Most instruments will have satisfactory EMC performance; IEC specifies immunity to 3 V m-1 from 150 kHz to 1 GHz assessed in accordance with standard IEC 61000 (IEC 1995).

4 Choice of instruments for electric-field measurements Most practical electric-field measurements are made by measuring the voltage (or current) between two parallel plates perpendicular to the electric field. Other technologies are available, but have few advantages. Such a meter is a “free body” meter; the alternative is a “ground reference” meter, which measures the voltage between a single electrode and ground, but this is less common.

Parallel-plate sensors for electric fields tend to be physically larger than magnetic-field sensors. Therefore, electric-field instruments are more often single-axis. However, three-axis meters are available, with the three separate parallel-plate assemblies either on three orthogonal faces of a cube or spread over the surface of a sphere.

One common measurement scenario is near or under power lines. In this instance, the electric field is essentially vertical for the first metre or two at least above ground level. Therefore, in this instance, a single-axis meter, aligned to measure the vertical field, is perfectly satisfactory.

Many of the issues discussed under magnetic fields apply to measurements of electric fields as well: frequency response, storage, type of display, etc. There is, however, one major issue with electric fields that does not apply to magnetic fields.

4.1 Perturbed and unperturbed electric fields

Electric fields are perturbed by any conducting object. This includes the person making the measurement and any conducting supports for the sensor.

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Electric-field measurements should be either of the unperturbed field or of the perturbed field and it should be clear which is being measured. Measurements made by a meter worn by a person are unavoidably of the perturbed field. In most other cases, however, it is preferable to attempt to measure the unperturbed field. This can sometimes be an ambiguous definition, where the field is perturbed by an object not connected with the measurement process. The usual objective is to measure the field that would be present in the absence of the person, but including any perturbation caused by fixed objects that would still be present when the person is not.

To obtain an unperturbed measurement, the instrument sensors must be supported in a way that does not perturb the field, and the person performing the measurement must be sufficiently distant.

The instrument can be supported either on an insulating pole held horizontally, or on a vertical insulating support (a tripod or similar). In both cases, if readings in real time (as opposed to stored and read only later) are desired, the sensors are connected to the rest of the instrument by a radio link or a fibre-optic, or the instrument is self-contained with a display large enough to be read from a distance.

A vertical tripod or similar support is often mechanically easier and makes it easier for the operator to be distant. However, in humid conditions, moisture can settle on both a tripod or a pole, making them less good insulators and perturbing the reading. As most electric fields are vertical near ground level, the vertical support, which is parallel to the field lines, leads to greater perturbation than the horizontal pole, which runs perpendicular to the field lines, along an equipotential, and therefore is less sensitive to small amounts of moisture. This is a reason for preferring horizontal poles when some degree of perturbation is regarded as inevitable.

The person making the measurements should be 2 m distant from the instrument to ensure that perturbation is negligible. Measurements at 1 m may be acceptable but less accurate. Hand-held measurements should be avoided.

The perturbation caused by the person making the measurement is, simplistically, to increase the field towards the top of their body, and to reduce it towards the bottom of their body. There is therefore a height of measurement, around 1.5 m above ground and therefore higher than the 1 m above ground which is recommended as a suitable measurement height for other reasons, where the perturbing effect of the person is neutral. This means in practice, the distance of the person from the instrument may not

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be so critical. However, this should not be relied on, and for good measurements, the person should always be 2 m distant.

Measurements of the perturbed electric field are made by attaching a meter to a person’s body. However, the reading obtained is extremely dependent on the amount of perturbation, which in turn is extremely dependent on the exact location of the meter on the person’s body. For certain simple geometries and for certain locations on the body (eg the top of the head of a person standing upright in a vertical field, where the factor is approximately 20) it is possible to calculate the conversion factor from the perturbed to the unperturbed field. In most situations, however, this conversion factor cannot be calculated, and varies as the person moves anyway. Therefore, such perturbed measurements have limited use, and should never be compared directly to unperturbed fields.

The need to measure unperturbed fields undoubtedly adds considerably to the effort and cost of making electric-field measurements, but in most situations, it is the unperturbed rather than the perturbed field which is the most useful.

5 Calibrations It is necessary to be sure that any instrument used is giving an acceptably accurate reading; this is achieved by calibration. IEC requires the overall uncertainty of the calibration process to be no greater than ±(5% +10 nT).

Most instruments will be supplied calibrated by the manufacturer. Periodic re-calibration will be needed. This can be performed either by the user or by sending the meter to a specialist calibration service or to the manufacturer. Calibration systems are not simple, and probably only users with multiple instruments will be able to justify maintaining their own calibration systems.

Magnetic fields are calibrated by placing them in a known field produced by a coil system of known geometry. It is common to refer to these coils as a Helmholtz arrangement. In fact, a Helmholtz pair, two equal coaxial circular coils separated axially by one radius, is a legacy of the time when fields had to be calculated analytically; its main advantage is the ability to express analytically the field and its first few derivatives at the centre. There are alternative systems which are more efficient at producing a uniform field (for a review, see Kirschvink 1992). The simplest system is a single square coil; the best results are achieved from three or four coaxial coils, of carefully chosen numbers of turns and axial

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separation. The size of the coil is determined by the requirement that the field at the centre must be reasonably uniform over the area of the search coil in the instrument being calibrated. IEC requires less than 1% (or 1.5% for larger probes under certain circumstances) departure of the field anywhere over the area of the search coil from the field at the centre. For a 0.1 m diameter search coil, this could be achieved by a single square coil of side 1 m, or a Merrit four-coil system of side 0.4 m.

IEC requires the field in the calibration coil to be known to ±3%. In most practical situations, the geometry of the coil system is known to rather better than this, so the limit on the accuracy of the calibration system is the accuracy of the measurement of the current flowing in the coils.

Electric fields are calibrated by placing them between two parallel plates across which a known voltage is applied. The uniformity of the field is improved by grading rings round the edges of the system, but to avoid perturbation, the system of plates should still be placed a minimum distance away from objects that might perturb the field. The limit on the size of the plates is then proximity effects between the meter and the plates. IEC specifies that a meter of maximum dimension 0.23 m requires plates of 1.5 m square separated by 0.75 m.

Calibration at high electric fields may involve applying voltages to the calibration plates sufficiently high such that corona becomes an issue and corona guard rings become necessary.

For both electric and magnetic fields, when a calibration is performed, there are three alternatives. One is simple to accept the instrument as passing the calibration provided it is within a certain margin of the correct field. A second is to adjust the instrument so it reads the correct value, and a third is to record a correction factor which should be applied to any readings taken to give the correct reading. IEC allows all three, and the choice is determined by the ease of adjusting the meter and the accuracy required. The option of recording and subsequently applying a correction factor gives the highest accuracy, but is best avoided in many practical measurement situations where it increases the potential for mistakes.

All calibrations should be performed as part of a quality-controlled calibration system with traceable records. Exactly how this is done and what is required depends on the quality regime in operation.

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Calibrations should be performed at set intervals. It is common to make this yearly, but if documented experience shows that the meter retains its calibration acceptably for longer than this, as will often be the case with commercial instruments, then a longer calibration interval is acceptable.

6 Sample measurement procedures This chapter has emphasised that there are no universal rights and wrongs in EMF measurements; the choices made depend on the purpose of the measurements. The following are therefore suggestions for the particular scenarios identified at the start of this chapter and should not be regarded as definitive.

• Simple characterisation of the field in a building

The objective is to characterise the field in a home, a work location, or similar, by a single number. Depending on resources and time available, this can be done by a single measurement at a single location; a single measurement at each of a series of locations; a sequence of measurements over time at a single location; or a sequence of measurements over time at multiple locations.

Probable measurement procedure: use a battery-powered instrument with logging facilities and three ferrous-cored coils. Leave it in a standard location for 24 hours or longer. The location should be clearly specified in the study protocol, eg “1 m above floor level at the middle of the side of the child’s bed”, and should be distant (at least 1 m) from any items of electrical equipment. This usually means the centre of a room, or as near it as is compatible with the occupants’ use of the home. Standardisation may be improved by providing a stand to hold the meter. The logging interval will be determined by the need for the logging capacity to last the required time. An instrument with 1 nT resolution will be preferable in homes but 10 nT resolution may be acceptable.

Similar measurements for electric fields are possible but will be more problematical because of perturbations by people walking near to the meter.

• Identification of sources

The objective is to make measurements of the field at a location, specifically how it varies over space or time, to enable the source of the field to be identified.

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Probable measurement procedure: use a hand-held, battery-powered instrument with a clear read out (analogue will be easier but digital is acceptable as well). Walk round the location observing the reading so as to identify areas of high field, and track these to their source. It may be helpful to have a single-axis readout as the direction of the field helps determine the direction of the current producing it.

• Comprehensive characterisation of the field in a building

The objective is to collect more data on the field in a building than just a single number, so as to permit extraction of desired information at a later date.

Probable measurement procedure: use several instruments to measure further characteristics of the field. For example: to characterise the field at more than one location over a house, one approach is to leave several instruments logging for 24 hours at locations spread over the house. Another approach is to measure a profile of field (using a single instrument but making the measurements as close together as possible) from one side of the house to the other.

• Characterisation of sources

The objective is to perform measurements that relate to a particular source of field (eg a power line, an item of equipment) rather than to the field in a particular place, so as to characterise that source.

Probable measurement procedure to characterise a power line: use a three-coil magnetic field meter. The size of the coils is less important. Choose one or a few standardised locations, eg directly under the centreline of the power line at mid-span, and possible some standard distances perpendicular to the line, eg 10 m, 25 m, 50 m. Perform measurements at 1 m above ground. For electric fields, use a single-axis meter, aligned to measure the vertical field, held on a horizontal insulating pole at 1 m above ground, or a clean, dry vertical insulating stand. If extra detail is desired, measure the field at more than one height: ground level, 1 m and 2 m above ground. Measurements at one point in time obviously give the field only at that time. If the load on the line is available, the measured field at one load can be scaled to other loads; or the measurements can be logging measurements over an extended period of time.

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• Personal exposure

The objective is to measure the exposure of a person over a period of time during which they are exposed to fields from various sources.

Probable measurement procedure: use a small three-axis meter. The priority is to prolong battery life and to reduce the bulk, so factors such as the flatness of the frequency response may be sacrificed. Get the subjects to wear it at a standard position on their body, possibly in a pouch on a belt, or for children, in a child-friendly back-pack or similar. Preferably arrange for any display to be blanked to reduce the temptation for the subject to experiment with approaching high-field sources. Give clear instructions on what to do at night time, ie where to place the meter when it is taken off.

There are some very specific issues related to personal exposure measurement (dosimetry) for the assessment of occupational exposures to magnetic fields. They are discussed here.

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References Diplacido J, Shih C H and ware B J 1978 Analysis of the proximity effects in electric field measurements IEEE Transactions on Power Apparatus and Systems PAS-97 2167-2177

IEC 1995 Electromagnetic compatibility (EMC) IEC 61000

IEC 1998 Measurement of low-frequency magnetic and electric fields with regard to exposure of human beings – special requirements for instruments and guidance for measurements IEC 61786

Kirschvink J L 1992 Uniform magnetic fields and double-wrapped coil systems: improved techniques for the design of bioelectromagnetic experiments Bioelectromagnetics 13 401-411

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Guidelines for the RF exposure assessment of metallic implants

This document available as a pdf here

November 2006

Dr Vitas Anderson

L3, 170 Pacific Hwy St Leonards (Sydney) NSW 2065

AUSTRALIA [email protected]

Dr Robert McIntosh

400 Burwood Road Hawthorn (Melbourne) VIC 3122

AUSTRALIA [email protected]

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Abstract

This chapter of the EMF Dosimetry Handbook provides guidance for assessing whether persons bearing metallic implants inside their bodies should be restricted from exposure to the upper tier limits of the radiofrequency (RF) safety guidelines (1998) published by the International Commission for Non-ionising Radiation Protection (ICNIRP) and the C95.1 (2005) standard of the Institute of Electrical and Electronic Engineers (IEEE). The recommendations presented here are based on original research by the authors, investigations of specific implant cases by the Telstra Research Laboratories in Melbourne, Australia and various publications in the scientific literature. Wherever possible, rules-of-thumb have been developed to provide simple and practical ways for assessing implants and for some external body worn metallic objects. Nonetheless, there remain some assessment scenarios that will still require detailed analysis.

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Document history

Release number Date issued Details of revision

R1 21 Nov 2006 First released issue from Anderson and McIntosh, reviewed by the ACRBR

Acknowledgements

This material is based on research sponsored by the Air Force Research Laboratory, under agreement number FA4869-06-1-0115. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation thereon. The authors also acknowledge the following organisations for their sponsorship and support of the research and literature review that was required for the drafting this chapter:

• Telstra Corporation Ltd, Australia • Kordia Pty Ltd, Australia ( http://www.kordiasolutions.com/ )

Lastly, the authors thank Mr Raymond McKenzie for his helpful reviews of this chapter on behalf of the Australian Centre for Radiofrequency Bioeffects Research ( www.acrbr.org.au ).

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Index

1 Background 4 1.1 Types of metal implants in the body 4 1.2 RF heating and human exposure limits 4 1.3 Safety targets for RF tissue temperature increases 4 1.4 RF safety assessments of metal implants 5

2 RF and thermal factors affecting implant assessments 6 2.1 RF factors 6

2.1.1 RF absorption in the body 6 2.1.2 RF absorption around the metal implant 6

2.2 Thermal factors 7 3 Canonical studies of implant rods and infinite plates 9

3.1 Introduction 9 3.2 Canonical modelling of plane waves travelling through layered tissues 9 3.3 Canonical modelling of plane waves reflected off metallic planar boundaries 13 3.4 Canonical modelling of rod implants in infinite medium 15

3.4.1 General approach 15 3.4.2 Use of VAR instead of SAR 15 3.4.3 Model setup 16 3.4.4 Peak VAR vs. frequency for E polarization exposure of a 40 mm rod in infinite bone 18 3.4.5 Peak VAR variation with rod length 20 3.4.6 Peak VAR variation with shape of rod tip 22 3.4.7 Peak VAR variation with field orientation 23 3.4.8 Peak VAR variation with dielectric value of medium 23

4 Specific implant assessments 25 4.1 Linear implants (e.g. pins, rods, and long narrow plates) 26 4.2 Screws 27 4.3 Arterial stents 27 4.4 Wide plates 28 4.5 Pacemakers 28 4.6 Loops 29 4.7 Cochlear implant systems and auditory brainstem implant (ABI) systems 30 4.8 Spectacles 31 4.9 Jewellery 31 4.10 Tooth fillings, caps, and orthodontic braces and plates 31 4.11 Implanted retinal stimulators 32 4.12 Implanted radiators 32 4.13 Spinal fusion systems and cervical fixation devices in MRI 33

5 General rules-of-thumb and observations for implant assessments 34 5.1 General observations 34 5.2 Thermal effects 34 5.3 Rods and other linear objects 34 5.4 Screws 35 5.5 Arterial stents 35

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1Background 1.1Types of metal implants in the body Many people carry pieces of metal implanted within their bodies, which vary in their origin. These metal implants can for example be unwanted remnants of shrapnel, or more commonly, screws, rods, wires, pins or plates that are implanted by orthopaedic surgeons to repair broken bones and worn joints. Other metallic implant types include arterial stents and implanted electronic devices such as cardiac pacemakers and cochlear implants. External body-worn metallic objects, such as spectacles, jewellery, and the outer components of the cochlear implant system, are also considered in this chapter.

1.2RF heating and human exposure limits Tissue heating is a well established effect of exposure to electromagnetic radiofrequency (RF) fields due to the absorption of RF power from fields induced inside the body. The common metric for RF heating is the Specific energy Absorption Rate, or SAR in W/kg, which is simply related to the internal RF electric field at any point by:

where Eint is the magnitude of the internal electric field (V/m), σ is the electrical conductivity of the tissue (S/m), and ρ is the mass density of the tissue (kg/m³). Metallic implants can sometimes concentrate the RF heating effect around them by the way they scatter the incident RF field. This possibility has been recognised in various RF safety guidelines and standards (ICNIRP, 1998, IEEE, 2005, ARPANSA, 2002) which caution that the potential for exceeding allowable exposure limits for localised SAR around metal implants should be assessed for persons exposed up to upper tier limits, i.e. the occupational limits in the ICNIRP Guidelines for electromagnetic exposures (1998) and the controlled environment limits in the IEEE C95.1 RF safety Standard (2006). The lower tier limits prescribed for general public exposures in these documents incorporate substantial additional safety margins that are generally regarded as providing sufficient protection for implant RF field enhancements. The localised SAR limits in the ICNIRP Guidelines (1998) and the IEEE C95.1 standard (2006) are assessed by averaging the point SAR over a mass of 10 g, usually in the shape of a cube, in recognition of the thermal diffusion properties of tissues. Different upper tier limits apply to different parts of the body as indicated in Table 1 below:

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Table 1 Upper tier limits for localised SAR in the ICNIRP Guidelines (1998) and the IEEE C95.1 (2006) standard for human exposure to RF fields.

ICNIRP Guidelines (1998) IEEE C95.1 standard (2006)

Head and torso 10 W/kg Head (except pinna), torso, upper arms, elbows, thighs and knees

10 W/kg

Arms and legs 20 W/kg The pinnas and limbs distal to the elbows and knees

20 W/kg

The upper tier RF limits for exposure to ambient electric (E) and magnetic (H) fields in both the ICNIRP Guidelines (1998) and the IEEE C95.1 Standard (2006) have been primarily formulated to restrict whole body average (WBA) SAR absorption to less than 0.4 W/kg for standing children and adults exposed to uniform plane wave fields. There is a general presumption that these E and H-field limits will also ensure that the localised 10/20 W/kg SAR limits are not exceeded for all circumstances.

1.3Safety targets for RF tissue temperature increases Tissue temperature rise is a more fundamental indicator of RF heating hazard than localised SAR as it includes the effect of the body’s capacity to dissipate RF heating. A conservative safety target is to restrict RF tissue heating to no more than 1°C in the head and torso (ICNIRP 1998). For other parts of the body that are more tolerant of temperature increases and have less critical functions (i.e., where the higher 20 W/kg SAR limit of the ICNIRP Guidelines or the IEEE C95.1 standard is applied), then a target temperature rise of 2°C would seem appropriate.

1.4RF safety assessments of metal implants Detailed assessments of SAR concentrations and temperature rises around metal implants generally require complex analyses and specialised skills that are beyond the reasonable capabilities and resources of the great majority of affected persons and organisations. Furthermore, due to enormous diversity in the size, shape and location of metal implants, as well as the many different possible scenarios for RF exposure, it has been difficult to make generalizations from one particular implant assessment to another. As a result, and despite warnings from RF safety standards and guidelines, most persons bearing personal metal objects and working in high RF fields have not been assessed for the potentially adverse RF heating of those metal objects. To address this lack, this chapter offers practical and accessible guidelines, or rules-of-thumb, for making such assessments for as many implant scenarios as possible. In devising these rules-of-thumb, the authors have drawn upon many sources including their own research, previous assessments conducted by the Telstra Research Laboratories in Melbourne, Australia, as well as published scientific papers on this topic, and have assumed that:

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• The rules-of-thumb only apply to persons who are not exposed above the upper tier limits

of the ICNIRP Guidelines (1998) or the IEEE C95.1 standard (2006). • A metallic implant in the head or torso can be considered safe if the localised SAR

(averaged over a 10 g cube) in tissue around the metal object does not exceed 10 W/kg or if the RF induced temperature rise in tissue around the metal object does not exceed 1°C.

• A metallic implant in the limbs and pinna can be considered safe if the localised SAR (averaged over a 10 g cube) in tissue around the metal object does not exceed 20 W/kg or if the RF induced temperature rise in tissue around the metal object does not exceed 2°C.

• The rules should hold for all orientations of the metal objects with respect to the incident field since an individual will generally move about in the field.

• There is no RF heating of the implant itself, i.e. the implant is assumed to be a perfect electrical conductor and the only RF heating occurs in the tissue around the implant.

As this topic is still in a relatively early stage of development, it should be expected that at least some of the rules-of-thumb offered in this chapter will require further amendment as more research is accumulated. Nonetheless, it is hoped that the publication of even imperfect rules-of-thumb will at least be a useful starting point and impetus for the development of better guidelines, and preferable to making no assessments at all.

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2RF and thermal factors affecting implant assessments 2.1RF factors

2.1.1RF absorption in the body The factors influencing the electromagnetic interaction between metal implants and the RF exposure field external to the body are varied and complex. Firstly, one must consider the general interaction of the body with the RF exposure field, as this affects the local incident field exposure around the implant. The main factors that affect the efficiency and distribution of RF absorption in the body are:

1. The frequency of the RF source 2. The polarisation of the incident RF field with respect to the body and its parts 3. The position of the RF source to the body which may lead to partial body exposures and

near field coupling effects 4. The size, shape and grounding of the body 5. The dielectric properties (permittivity and conductivity) of body tissues

The dependence of SAR distribution on the RF exposure frequency offers a number of avenues for developing rules-of-thumb. For certain frequency ranges, the internal SAR may be too low to exceed peak allowable limits in particular parts of the body, even with RF field concentrations around the implants. Thus, it would be useful to identify those frequency ranges where metallic implant assessments are not necessary in all or parts of the body. Conversely, in certain frequency ranges, there may be parts of the body where localised SAR levels are relatively high, and where the additional SAR enhancement effect of the implant is more critical. Frequency ranges that are particularly worth noting include:

1. Frequencies above ~4-6 GHz At frequencies above this range the small skin depth of absorption, δ, can provide effective RF shielding of implants buried in the body. At one skin depth, the point SAR will diminish by a factor of 0.14 relative to point SAR at the surface.

2. Frequencies up to the MF band (300 kHz – 3 MHz) In this range RF coupling to the body is weak, and E-field limits in the ICNIRP Guidelines (1998) and the IEEE C95.1 Standard (2006) are predicated on the more stringent requirements of protecting against external shock and burns arising from contact with passively charged conductors.

3. Frequencies in the HF (3–30 MHz) and VHF (30–300 MHz) bands In this range, whole and partial body resonances occur. Induced RF currents in the ankle and neck are of particular interest due to the concentration of RF current flows in these narrowed conduction areas.

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2.1.2RF absorption around the metal implant Having established a base level of RF exposure in the body, the next step is to determine how the metal object perturbs and possibly concentrates the SAR around it. This should include a consideration of the following factors:

1. The size of the metal object 2. The shape of the metal object 3. Any gaps in the metal object 4. Location of the metal object within the body 5. Dielectric values of tissues around implanted metal objects 6. Whether the implant traverses local tissue boundaries 7. Orientation of the implant with respect to the local induced in vivo fields. 8. Distribution of the in vivo field around the implant (more important for large implants)

It should be noted that passive metal objects cannot of themselves generate any additional RF energy in accordance with the thermodynamic law for conservation of energy. However, due to RF field scattering, they can redistribute the incident RF energy around them, leading to SAR concentrations at some points and corresponding SAR reductions in other areas. There are at least four basic mechanisms of SAR enhancement around implants as displayed in Figure 1:

1. SAR enhancement at the ends of implants, particularly when the long axis is parallel to the in situ electric field

2. SAR enhancement in gaps of linear implants 3. SAR enhancement in the gaps of broken loops that are cut by changing magnetic flux

density (B) 4. Constructive interference in surface layers with underlying metallic plates

Figure 1 Four basic mechanisms of SAR enhancement around metallic implants

2.2Thermal factors Thermal factors that can influence the local temperature rise around RF exposed implants include:

1. The thermal conductivity and physical structure of the implant which influences the ability of the implant to redistribute temperature variations around it through internal heat transfer.

2. The size and specific heat capacity of the implant which alters the thermal mass of the implant, and affects the transient response to heating.

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3. The heat transfer environment around the implant including: the thermal conductivity of surrounding tissues; the micro blood perfusion of surrounding tissues, and; the proximity of implant to large blood vessels.

4. The proximity of the implant to the body surface, where heat transfer from the skin to the ambient environment becomes important.

To a good first approximation, heat transfer inside the body can be numerically modeled using the classic bioheat equation (Pennes, 1948):

where T is the tissue temperature (°C), c is the specific heat capacity (J/kg°C), K is the thermal conductivity (W/m°C), A0 is the metabolic heat production (W/m³), b is the heat-sink strength from each tissue volume by blood perfusion (W/m³ °C), and Tb is the temperature of the perfusing blood. The desired solution for T is obtained when the system reaches steady-state thermal equilibrium. Heat transfer at the surface of the body can be modelled as a convective boundary:

where h is the convection coefficient (W/m²°C), Ta is the ambient temperature (°C), qe is the evaporative heat loss (W/m²) and n is the direction of the unit normal to the surface. The convection coefficient may include a linearised component for heat radiation. A favoured method for computational analysis of the bioheat equation in human bodies is the finite difference (FD) technique whereby complex heterogeneous models of the human body and implants can be represented by voxels in a regular rectangular mesh. A particular advantage of this approach is that the finite difference mesh can be made to coincide with the voxel mesh of an RF model based on the Finite Difference Time Domain (FDTD) technique, thereby allowing easy transfer of the calculated RF SAR data to the FD thermal analysis. For a detailed example of this approach see, for example, McIntosh et al. (2005).

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3Canonical studies of implant rods and infinite plates 3.1Introduction This chapter section is drawn from a project report by the authors (Anderson and McIntosh, 2004) for a study on metallic implants that was sponsored by the Asian Office for Aerospace Research and Development (AOARD) of the United States Air Force Office of Scientific Research (AFOSR). It also includes observations gathered from earlier implant modelling for specific assessments that were conducted at the Telstra Research Laboratories in Melbourne, Australia. Results and conclusions about implants are provided from canonical studies of the following areas:

• Calculations of SAR attenuation in planar multilayer skin/muscle/bone/metal models exposed to a plane wave; and;

• A canonical assessment of rod implants exposed to a plane wave in infinite medium that investigated the influence of size, tip shape, rod orientation and the dielectric properties of the surrounding tissue medium.

3.2Canonical modelling of plane waves travelling through layered tissues Radios waves are attenuated as they travel through lossy materials such as human tissues. At high frequencies, above 4-6 GHz, the rate of attenuation with depth is so pronounced for human exposures that most of the RF power is absorbed at the body surface. At these frequencies, metallic objects located deeper in the body may be effectively shielded from RF exposures. The extent of this shielding can be gauged by examining a simple canonical model of a plane wave travelling through multiple planar layers representing skin, muscle and bone, as represented in Figure 2 below. This scenario has been modelled in a commercial RF analysis package, FEKO v4.1 (EMSS, 2003), using Greens functions for planar multilayered substrates. The final bone layer extends infinitely in the z direction.

Figure 2 Model setup for examination of a plane wave incident on infinite multilayers of skin, muscle and bone.

Skin and muscle thickness over bone varies in different locations of the body. In some places, there is effectively no muscle layer at all, e.g. on the forehead and shins. To cover these different scenarios, the following scenarios were analysed:

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• 3 mm skin layer, infinite bone • 5 mm skin layer, infinite bone • 7 mm skin layer, infinite bone • 5 mm skin layer, 10 mm muscle layer, infinite bone • 5 mm skin layer, 30 mm muscle layer, infinite bone

The models were examined in the frequency range of 1-10 GHz using dielectric values for dry skin, skeletal muscle and cortical bone from Gabriel (1996) as shown in Table 3.

Table 2 Tissue dielectric values for multilayer models

Frequency (GHz)

Dry skin Skeletal muscle Cortical bone

εr σ (S/m) εr σ (S/m) εr σ (S/m)

1 40.9 0.900 54.8 0.978 12.4 0.156

2 38.6 1.265 53.3 1.454 11.7 0.310

4 36.6 2.340 50.8 3.016 10.5 0.727

6 34.9 3.891 48.2 5.202 9.6 1.203

8 33.2 5.824 45.5 7.798 8.8 1.680

10 31.3 8.014 42.8 10.626 8.1 2.136

Results of the multi tissue layer model analyses are shown in Figures 3 to 7. The curves for each frequency have been normalised so that the point SAR = 1 at the air skin surface.

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Figure 3 Normalised point SAR in a multilayer tissue model (3 mm skin, infinite bone) exposed to a plane wave over the frequency range 1-10 GHz.

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Figure 4 Normalised point SAR in a multilayer tissue model (5 mm skin, infinite bone) exposed to a plane wave over the frequency range 1-10 GHz.

Figure 5 Normalised point SAR in a multilayer tissue model (7 mm skin, infinite bone) exposed to a plane wave over the frequency range 1-10 GHz.

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Figure 6 Normalised point SAR in a multilayer tissue model (5 mm skin, 10 mm muscle, infinite bone) exposed to a plane wave over the frequency range 1-10 GHz.

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Figure 7 Normalised point SAR in a multilayer tissue model (5 mm skin, 30 mm muscle, infinite bone) exposed to a plane wave over the frequency range 1-10 GHz.

A number of general trends are evident from these results:

1. SAR decays more rapidly with depth as the frequency of exposure increases. For all cases studied, the point SAR at a depth of 10 mm had diminished by at least a factor of 10 for exposures above 6 GHz. This observation lends support to the treatment of RF exposures above 6 GHz as a surface heating phenomenon.

2. The large disparity in dielectric values between bone and skin or muscle causes a reflected wave from the bone interface. This can lead to a significant standing wave pattern in the skin and/or muscle exhibiting constructive or destructive interference depending on the layer thickness and the exposure wavelength, λ, which is dependent on frequency. A constructive interference pattern occurs when the skin/muscle layer is approximately a quarter wavelength thick, resulting in enhanced SAR. This phenomenon was most pronounced at 1-2 GHz for the models studied.

3. At the depth of the bone layer, the point SAR is substantially diminished compared to SAR at the surface in all of the studied cases.

3.3Canonical modelling of plane waves reflected off metallic planar boundaries An obvious thread to follow up from the observation of standing waves described in the preceding section is the constructive interference patterns that can result from RF waves reflected off a planar metal surface underneath the skin. Classical transmission line theory indicates that the maximal constructive interference occurs when the thickness of the skin between the air and plate is a quarter of the RF wavelength, λ, in the skin. Using a FEKO model as indicated in Figure 8, the calculated field pattern in a 3 mm layer of skin in front of a metal boundary is shown in Figure 9. It shows the maximal SAR levels occur at 4 GHz where the wavelength in skin is approximately 12 mm, in accordance with the λ/4 expectation.

Figure 8 Model setup for examination of a plane wave incident on an infinite layers of skin overlaying a perfect electrical conducting (PEC) plane. The averaging cube for calculating 1 g or 10 g SAR is positioned against the air/skin surface and extends behind the metal plane where SAR equals zero.

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Figure 9 Point SAR distribution for a 1 mW/cm² plane wave normally incident on a 3 mm thick layer of skin overlaying a metallic plane. Results were calculated in a similar FEKO model as described in the previous section.

For other skin thicknesses, the frequency at which maximal quarter wave enhancement occurs is indicated in Table 3 as gauged by the 10 g average SAR over the shape of a cube at the surface. The point SAR in the skin layer for exposures at these frequencies at the upper tier ambient E-field limit in the ICNIRP Guidelines (1998) is shown in Figure 9.

Table 3 10 g average SAR (in the shape of a cube) in a skin layer overlaying a metal plane exposed to a normally incident plane wave at the upper tier limit for ambient E-field exposure ICNIRP Guidelines (1998), as shown in Figure 10.

skin thickness 3 mm 4 mm 5 mm 6 mm 7 mm 8 mm

λtissue = 4 * skin thick 12 mm 16 mm 20 mm 24 mm 28 mm 32 mm

Freq (GHz) 4.10 3.04 2.41 1.99 1.70 1.47

ICNIRP E-field limit (W/m²) 50 50 50 50 42.5 36.8

Max 10 g avg SAR (W/kg) at ICNIRP E-field limit 1.126 1.130 1.126 1.116 0.944 0.804

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Figure 10 Point SAR distribution in a layer of skin (3 – 8 mm thick) overlaying a metallic plane. Exposure consisted of a normally incident plane wave of intensity equal to the ICNIRP Guidelines (1998) upper tier (occupational) E or H field reference levels at the quarter wave frequency for each skin thickness. SAR is zero behind the metal plane interface.

Table 3 indicates that the ICNIRP 10 g localized SAR limits (10 W/kg for head and torso, 20 W/kg for the limbs) are not exceeded for ambient exposures at the occupational field limits. The quarter wave enhancement effect appears to monotonically decrease for increasing skin thickness greater than 4 mm (see figure 10).

3.4Canonical modelling of rod implants in infinite medium

3.4.1General approach As depicted in Figure 1, SAR can be enhanced at the tips of linear metal structures, particularly when the E-field is oriented parallel to the longest dimension of the implant. This phenomenon has been investigated by the authors in a series of canonical models for rods exposed to a plane wave in an infinite dielectric medium, and with particular regard to the following factors:

1. The length of the rod 2. The shape of the rod tip 3. The orientation of the rod with respect to the incident E-field exposure 4. The dielectric medium around the rod

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3.4.2Use of VAR instead of SAR Rather than calculate mass averaged SAR around the rod, it was decided that the Volumetric Absorption Rate (VAR) in W/m³ averaged over a fixed sized cube was a more appropriate metric for comparing the relative RF field enhancements. The VAR at any point is calculated as VAR = σ|E|², c.f. SAR = σ|E|²/ρ. The RF power calculated by integrating point VAR over a 10 cm³ cube is equivalent to the RF power obtained by integrating SAR over a cube of 10 g mass if the density of the medium is 1000 kg/m³, as is commonly assumed for most tissue types. The decision to choose volume averaged VAR as the comparison metric was based on its greater ease of calculation and because it is more directly related to tissue temperature rise. On the first point, the density of metals (steel ~ 8000 kg/m³) is much higher that the density of tissues (~ 1000 kg/m³) which can substantially affect the size of a constant mass averaging cube when it intersects a metal implant and hence greatly complicates the calculation of mass averaged SAR compared to a constant size VAR averaging cube. Moreover, this variability in the size of a SAR averaging mass also introduces an arbitrary variation in the level of RF power deposited in the cube which makes mass averaged SAR less directly related to temperature rise than volume averaged VAR. The more direct coupling between VAR and tissue temperature can be plainly seen in the bioheat equation for steady state RF heating shown below:

ρSAR VAR

3.4.3Model setup The analyses were performed using Method of Moment (MoM) analysis in the FEKO v4.1 software (EMMS, 2003). The 10 cm³ volume averaged VAR was calculated by averaging point VAR in a 24 x 24 x 24 cubic array as depicted in Figure 13. The rod models consisted of a perfect electrically conducting (PEC) round rod exposed to a plane wave in an infinite tissue medium. The volume averaged VAR over a 10 cm³ cube was calculated along the length of the rod as shown in Figure 14.

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Figure 11 The 10 cm³ averaging cube for VAR was subdivided into a cubic array of 24 x 24 x 24 cuboids. The 10 cm³ VAR was obtained by averaging the point VAR at the centre of each of the13,824 cuboids.

Figure 12 FEKO model of a PEC rod implant exposed to a plane wave in an infinite tissue medium. The 10 cm³ VAR was evaluated along the length of the rod.

In these canonical analyses, the rod was immersed in an infinite tissue medium of either bone or muscle. The analyses were conducted over a frequency range of 0.1 MHz to 10 GHz with uniform logarithmic spacing of 5 points per decade (1, 1.6, 2.5, 4, 6.3, 10). The tissue dielectric values were obtained from Gabriel et al. (1996) as shown in Table 4.

Table 4 Relative permittivity, εr, and conductivity, σ, of cortical bone and skeletal muscle used in the canonical analyses of a rod exposed to a plane wave in infinite tissue medium. The plane wave wavelength, λ, and the skin depth, δ, in the tissues are also shown.

freq (MHz)

Cortical bone Muscle

εr σ (S/m)

λ(mm) δ (mm) εr σ (S/m)

λ(mm) δ (mm)

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0.1 2.28 E+2

2.08 E-2

67274 11379 8.09 E+3

3.62 E-1

15624.2 2815.1

0.16 2.11 E+2

2.10 E-2

52218 9088 6.95 E+3

3.75 E-1

11893.4 2230.8

0.25 1.97 E+2

2.12 E-2

40694 7366 5.76 E+3

3.96 E-1

9092.6 1769.7

0.3 1.91 E+2

2.14 E-2

36633 6765 5.23 E+3

4.07 E-1

8137.3 1602.1

0.4 1.82 E+2

2.18 E-2

30889 5917 4.34 E+3

4.28 E-1

6835.0 1360.7

0.63 1.66 E+2

2.27 E-2

23298 4775 2.97 E+3

4.65 E-1

5227.7 1038.4

1 1.45 E+2

2.44 E-2

17230 3793 1.84 E+3

5.03 E-1

4032.0 785.2

1.6 1.19 E+2

2.70 E-2

12571 2935 1.07 E+3

5.35 E-1

3128.8 594.6

2.5 9.33 E+1

3.03 E-2

9331 2251 6.40 E+2

5.59 E-1

2470.6 460.7

3 8.32 E+1

3.19 E-2

8278 2012 5.22 E+2

5.68 E-1

2244.2 416.2

4 6.87 E+1

3.44 E-2

6870 1682 3.85 E+2

5.81 E-1

1927.7 355.4

6.3 5.01 E+1

3.86 E-2

5144 1272 2.49 E+2

5.99 E-1

1513.7 278.5

10 3.68 E+1

4.28 E-2

3837 969 1.71 E+2

6.17 E-1

1179.3 218.8

16 2.79 E+1

4.70 E-2

2831 748 1.25 E+2

6.34 E-1

909.6 172.4

25 2.25 E+1

5.09 E-2

2096 597 9.93 E+1

6.51 E-1

705.5 138.6

30 2.09 E+1

5.25 E-2

1844 548 9.18 E+1

6.58 E-1

634.2 127.1

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40 1.89 E+1

5.51 E-2

1497 483 8.26 E+1

6.69 E-1

533.7 111.4

63 1.67 E+1

5.94 E-2

1057 403 7.25 E+1

6.88 E-1

401.0 91.6

100 1.53 E+1

6.43 E-2

722 343 6.60 E+1

7.08 E-1

293.1 76.7

160 1.43 E+1

7.05 E-2

479 295 6.17 E+1

7.31 E-1

206.7 65.9

250 1.37 E+1

7.84 E-2

318 255 5.90 E+1

7.57 E-1

143.7 58.5

300 1.34 E+1

8.27 E-2

268 239 5.82 E+1

7.71 E-1

122.8 56.1

400 1.31 E+1

9.13 E-2

204 213 5.71 E+1

7.96 E-1

95.0 52.6

630 1.28 E+1

1.13 E-1

132 169 5.58 E+1

8.58 E-1

62.3 47.3

1000 1.24 E+1

1.56 E-1

84.7 121 5.48 E+1

9.78 E-1

40.0 40.7

1600 1.19 E+1

2.42 E-1

53.9 76.2 5.38 E+1

1.24 E+0

25.3 31.7

2500 1.14 E+1

4.04 E-1

35.3 44.6 5.27 E+1

1.77 E+0

16.4 21.9

3000 1.11 E+1

5.06 E-1

29.8 35.2 5.21 E+1

2.14 E+0

13.7 18.0

4000 1.05 E+1

7.27 E-1

22.8 24.0 5.08 E+1

3.02 E+0

10.4 12.7

6300 9.46 E+0

1.27 E+0

15.2 13.03 4.78 E+1

5.57 E+0

6.79 6.68

10000 8.12 E+0

2.14 E+0

10.3 7.27 4.28 E+1

1.06 E+1

4.48 3.34

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3.4.4Peak VAR vs. frequency for E polarization exposure of a 40 mm rod in infinite bone In an initial exploration of linear resonance mechanisms, a model of a round rod (40 mm long, 4 mm diameter) was exposed to a plane wave in an infinite bone medium. The incident E-field was set to 1 V/m at the centre of the rod, with E parallel to the rod’s axis as depicted in Figure 12. The 10 cm³ VAR along the length of the rod was normalized with respect to the unperturbed 10 cm³ VAR at the same location when the rod is not present. The peak relative VAR enhancement along the rod is plotted in Figure 13 over the frequency range 0.1 MHz to 10 GHz.

Figure 13 Peak relative VAR enhancement caused by the presence of a 40 cm long rod exposed to an E polarised plane wave in infinite bone medium.

The graph depicted in Figure 13 displays some interesting features. Firstly, a resonant response is clearly evident at around 630 MHz with a peak VAR enhancement of around 4.2. Below this frequency, there is a lower and constant enhancement of 2.1. Above the resonance, the VAR enhancement drops to unity, i.e., there is no enhancement of 10 cm³ average VAR. Figure 14 displays field plots of point VAR at frequencies in each of the three frequency regions just described. At resonance (630 MHz), the peak VAR enhancement is clearly seen at the tips of the rods. A relative reduction in VAR can also be observed around the middle of the implant, illustrating the important general principle of power conservation. In other words, the rod cannot create additional VAR, but simply redistributes the available RF power around it provided by the incident exposure. At sub-resonance, (1 MHz in Figure 16), a similar pattern of field enhancement/reduction around the rod occurs as at resonance. This similarity was evident for all frequencies below resonance. However, above resonance, there is a clear qualitative shift in the pattern of VAR distribution. At 6.3 GHz, where λ is 15.2 mm and small compared to the rod length, the rod acts as an electrically large scatterer, producing a pattern of standing waves immediately in front of it (note also the attenuation of the E-field to the left of the picture as the field propagates towards the implant). Over the 21.5 mm side length of the 10 cm³ averaging cube (which is large relative to λ/4), the constructive and destructive peaks of these standing waves average out to unity. The logarithmic attenuation of the plane wave as it travels through the medium is also quite evident at these high frequencies.

Figure 14 Field plots of point VAR around the 40 cm long cylinder implant at sub-resonance (1 MHz), resonance (630 MHz) and supra-resonance (6.3 GHz) for an incident plane wave E-field of 1 V/m at the centre of the rod. All three plots use the same linear scale for point VAR.

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3.4.5Peak VAR variation with rod length The influence of rod length on 10 cm³ VAR enhancement was studied by analysing 4 mm diameter round rods of varying lengths (20, 30, 40, 80, 160 and 320 mm long) exposed to a plane wave in an infinite bone medium with E parallel to the rod axis. The relative enhancement of 10 cm³ VAR is shown in Figure 15. The plots for each rod length display the same general resonant features as described previously, though with some interesting differences. Firstly, the frequency of the resonant peak changes with rod length. In particular, the resonant peak appears to occur when the rod length is around one third of the exposure wavelength, as previously reported by Fleming et al. (1999). Secondly, rod length influences the magnitude of the relative 10 cm³ VAR enhancement at the resonant peak and at sub-resonance. Figure 16 shows the peak 10 cm³ VAR enhancement at resonance. Only a modest enhancement is seen for small rods (×1.57 for a 20 mm length), but the enhancement is quite substantial for larger rods (×37.4 for 160 mm length). The increase in 10 cm³ VAR enhancement appears to be fairly linear for rod lengths greater than 40 mm. Figure 17 shows 10 cm³ VAR enhancement in the plateau sub-resonance region, with similar trends as described for the resonant VAR enhancement.

Figure 15 Variation in the peak relative enhancement of 10 cm³ VAR for 4 mm diameter round rods of varying length exposed to an E polarised plane wave in infinite bone medium.

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Figure 16 Variation in the peak enhancement of 10 cm³ VAR with rod length at resonance in an infinite bone medium.

Figure 17 Variation in the peak enhancement of 10 cm³ VAR with rod length for plane wave exposure at sub-resonance frequencies in an infinite bone medium.

3.4.6Peak VAR variation with shape of rod tip The influence of rod tip shape on the peak relative enhancement of 10 cm³ VAR is shown in Figure 18 for 80 mm long round rods (4 mm diameter) exposed to an E polarized plane wave in

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infinite bone medium with either flat or conical tips. They indicate that the flat tip rod exhibited a very similar 10 cm³ VAR enhancement response compared to the rod with conical tips, indicating that the rod tip shape is not a significant cause of variation.

80 mm

Figure 18 Variation in the peak enhancement of 10 cm³ VAR with frequency for flat and pointed end tips of an 80 cm round rod exposed to an E polarised plane wave in an infinite bone medium..

3.4.7Peak VAR variation with field orientation An important determinant of RF field enhancement around long linear structures is the orientation of the E-field with respect to the implant. Maximum coupling at frequencies around or below resonance occurs when the E-field is parallel to the long axis of the structure. This is clear in Figure 19 which illustrates the peak relative VAR enhancement around an 80 mm long round rod (4 mm diameter, flat ends) exposed to a plane wave in infinite bone medium. In contrast to the maximum enhancements when E is parallel to the rod, virtually no enhancement is seen when E is perpendicular to the rod. Halfway between these orientations at 45°, the peak sub-resonant enhancement is around 59% of the parallel enhancements, but the resonant response is much more subdued being only 32% of the parallel enhancement. A second smaller resonant peak is also evident.

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Figure 19 Variation in the peak enhancement of 10 cm³ VAR with frequency for different polarisations of a plane wave E-field incident on an 80 cm round rod in an infinite bone medium.

3.4.8Peak VAR variation with dielectric value of medium In the final series of canonical rod implant models, the influence of the tissue dielectric properties was examined. In these analyses, flat tip rods of various lengths were exposed to E polarized plane waves in both infinite bone and infinite muscle medium. The results of these analyses are shown in Figure 20 with comparison to the results obtained in the bone medium.

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Figure 20 Variation in the peak enhancement of 10 cm³ VAR with frequency for different rod lengths exposed to an E polarised plane wave in infinite bone and muscle medium.

These results reveal some very obvious and interesting trends that are consistent for all of the different length rods. Firstly, the 10 cm³ peak VAR enhancement in the sub-resonance frequency range is exactly the same for rods immersed in muscle as for rods in bone. Likewise, in the supra-resonance region, the 10 cm³ peak VAR enhancement trends to unity for rods in both mediums. However, in the resonant region there are clear differences wherein the enhanced resonant response seen in bone, is completely damped out when the rod is placed in muscle. It would seem reasonable to speculate that this damping effect is due to the increased RF power losses arising from the higher conductivity of muscle, noting that VAR (and SAR) is directly proportional to conductivity. Note also that the actual value of VAR and SAR may be higher in muscle than in bone, even though the relative enhancement as shown in Fig. 20 is less. For rods of all lengths immersed in muscle, the peak 10 cm³ VAR enhancement is negligible (i.e., < ×1.4) for frequencies above 500 MHz.

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4Specific implant assessments In this section we describe specific implant assessments that have been published in the literature as well as unpublished assessments conducted by the Telstra Research Laboratories for Telstra RF workers.

The majority of these studies employed numerical modelling (primarily FDTD and MoM), with metal objects placed in or near human body models or canonical models to perform the assessments. In some cases this was followed by the use of thermal modelling (primarily FD) to assess the resultant thermal changes.

For a separate review of the literature and for a general discussion on metallic implants see Virtanen et al. (2006).

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4.1Linear implants (e.g. pins, rods, and long narrow plates)

Type Dimensions Placement Exposure Comments Source

Pin/rod 7 - 28 mm long and 0.5 - 8 mm diameter

Different orientations of pin/rod located on the skin or in muscle, fat or bone of a cylinder model of the body

250 mW, 900/1800 MHz mobile phone type exposure simulated by a monopole on metal handset box 10 mm from skin surface

Resonance found around λ/3 (14 mm for 900 MHz). Enhancement of average SAR found to be 2 to 3 times but study concludes that the “… enhancement is unlikely to be problematic.”

Virtanen et al. (2005)

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Long narrow plate

220 mm long and ~ 20 mm thickness

Attached to humerus with six screws and surrounded by muscle in whole body FDTD model

RF plane wave exposure

Resonance around 50–100 MHz. Peak 10 g SAR = 1.03 W/kg at bottom of rod for 10 W/m2 input and hence complies with ICNIRP Guidelines (1998)

Telstra individual assessment (2005)

Intramedullary nail

440 mm long and 15 mm diameter

Inside femur (attached with screw 65 mm from top) in whole body FDTD model

900 MHz plane wave exposure

Negligible effect

Telstra individual assessment (2001)

Pin Wire 5 - 25 mm long (peak at ~λ/2 = 20 mm). Diameter either 0 (filament) or 1 mm (pin).

In a spherical model of a head, parallel to dipole and 10 mm from surface

250 mW, 900 MHz dipole 15 mm from head (GSM900 mobile phone type exposure)

Peak 10 g SAR increases no more than 30% compared to tissue without implant. Peak 1 g SAR is around 3.9 W/kg for filament 20-25 mm in diameter

Cooper and Hombach (1996)

Summary: The assessments conducted at 900 MHz indicate that short (5-28 mm long) and long (440 mm) implants do not cause excessive SAR, which is in line with the results of the canonical modelling of rods (see figure 20). The assessment of the long narrow plate (220 mm) in the humerus (upper arm) provides some limited evidence that implant assessments in

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that location are not likely to be problematic at any frequency.

4.2Screws

Type Dimensions Placement Exposure Comments Source

Screws ~ 15 mm long and ~ 4 mm wide

In elbow in whole body FDTD model

RF plane wave exposure

Resonance around 900 MHz but effect negligible

Telstra individual assessment (2003)

Summary: This assessment lends support to the notion that small implants do not cause excessive SAR concentration.

4.3Arterial stents

Type Dimensions Placement Exposure Comments Source

Coronary artery stent

34 mm long and 2 mm diameter

Top of the heart in the middle artery (modelled as cylinder) in whole body FDTD model

100-900 MHz plane wave exposure

At 100 MHz the peak 1 g SAR near the stent was 0.35 W/kg for 10 W/m2 exposure.

Telstra individual assessment (2001)

Coronary artery stent

6 mm and 25 mm diameter stents

The stent is modelled as a cylinder in infinite muscle tissue with blood inside. Measurements were made in egg-white.

Heating furnaces - 6.25 and 92.6 kHz RF and MRI considered

Conclude that the ANSI/IEEE C95.1-1992 standard limits provide adequate protection. Paper also confirms convective cooling effects of blood vessel.

Foster, Goldberg and Bonsignore (1999)

Summary: Studies show no cause for concern. The potential for adverse heating around

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stents is substantially mitigated by the convective cooling from blood flow within the artery.

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4.4Wide plates

Type Dimensions Placement Exposure Comments Source

Circular cranioplasty plate

50 mm diameter, curved around forehead, and 1.5 mm thick

Implanted on the front of the cranium around 5-6 mm under the surface of the forehead in whole body FDTD model

100 MHz-3 GHz plane-wave from the front to the back of the body

Resonant response occurred at around 200-300 MHz (peak 10 g SAR is around 0.8 W/kg for 10 W/m2 input power at the occupational limit) and cumulative interference in the scalp at around 2100-2800 MHz (4.9 W/kg for 50 W/m2) (where the scalp thickness was λ/4). The resultant temperature increase is less than 1 °C.

McIntosh et al. (2005)

Circular Disk

15 - 22 mm diameter

In a spherical model of a head, 10 mm from the surface

250 mW, 900 MHz dipole (15 mm from head) mobile phone type exposure

Peak 1 g SAR is around 3.5 W/kg for disk around 18 mm in diameter

Cooper and Hombach (1996)

Summary: In comparison to the linear enhancements for rod tips, the peak linear SAR enhancement is generally more spread around the circumference of a plate which reduces the peak 10 g average SAR. The cumulative interference effect (with fields in the skin-plate interface giving rise to increased SAR in front of the plate) has been shown to not lead to SAR limits being exceeded (in line with the canonical modelling of section 3.3).

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4.5Pacemakers

Summary: There have been many studies in the area of the interaction between electromagnetic fields and pacemakers but the main topic of interest has been interference issues (see, for example, Hrabar et al. (2001)).

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4.6Loops

Type Dimensions Placement Exposure Comments Source

Loops 15 - 50 mm diameter

Different orientations of pin/rod located on the skin or in muscle, fat or bone of a cylinder model of the body

250 mW, 900/1800 MHz mobile phone type exposure (from monopole), 10 mm from surface

Resonance found when loop is λ/3 to λ/2 in diameter. Relative enhancement for 1 g SAR up to 2.7× at 900 MHz for 30 mm diameter ring, although at typical mobile phone power levels “… enhancement is unlikely to be problematic.” Averaged SAR highest when loop is in muscle.

Virtanen et al. (2005)

Wire loop and tie

~ 20 - 25 mm diameter with ties around 7-12 mm long

Ties around the sternum. Heat transfer analysed in finite element (FE) model.

Plane wave exposure at 3, 9.5, 80,1650-3000 MHz

Temperature rise estimated to be 1.3 °C for 50 W/m2 at 80 MHz. The occupational limit at this frequency in ICNIRP (1988) is 10 W/m2 so all max temperature increases < 1 °C.

Hocking, Joyner and Fleming (1991)

Summary: Assessments here for a variety of dimensions and exposure conditions show no cause for concern. There are no published studies of SAR enhancement in the gaps of loops – further research is needed.

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4.7Cochlear implant systems and auditory brainstem implant (ABI) systems

Type Placement Exposure Comments Source

Cochlear implant system

Models of cochlea (assessed using the Finite Integration Algorithm (FIT))

900 MHz mobile phone

Local increases about electrode arrays of SAR and temperature

Franzoni et al. (2006)

Cochlear implant system (with metal hook over the ear)

Standard placement for internal and external components, in whole body FDTD model

Mobile phone type exposure (from dipole) at 900 MHz/250 mW and 1800 MHz/125 mW. Dipole placed 10 mm from ear. Different orientations considered.

At 900 MHz (1800 MHz) peak 10 g averaged SAR reached near the implant was calculated to be 1.31 W/kg (0.93 W/kg) and the peak temperature increase was 0.33 °C (0.16 °C).

McIntosh et al. (2006[1,2])

Cochlear implant system (no external component)

Standard placement for internal component, in whole body FDTD model

50 Hz, 50 kHz, 5 MHz and mobile phone frequencies 900 MHz, 1750 MHz and 1950 MHz

Satisfied ICNIRP exposure limits

Bahr and Boltz (2006)

ABI and cochlear implant system (no external components in either)

Receiver-stimulator placed on outside of head and leads placed across central slice of head in phantom

MRI imaging (RF component at 63.8 MHz)

No heating due to devices observed

Chou, McDougall and Chan (1995)

Summary: These studies are fairly comprehensive for cochlear implant systems covering a wide variety of exposure conditions and show no cause for concern.

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4.8Spectacles

Dimensions Placement Exposure Comments Source

Several common frame shapes

About human head FDTD model

1.5 – 3 GHz plane wave at 50 W/m2 and 1.8 GHz dipole

SAR averaged over eye increases by up to 160% (compared to when spectacles not present) and decreases up to 80% depending upon type of spectacles. Safety standards satisfied.

Edwards and Whittow (2005)

Frame 166 mm wide, lens aperture 68 x 40 mm

About human head FDTD model

450 MHz, 1 W monopole placed in front of face

Minor decrease in 10 g average SAR with spectacles (compared to when spectacles not present)

Troulis (2003)

About human head FDTD model

Mobile phones 915 MHz GSM and 1.9 GHz DECT

SAR levels comply with Austrian safety standard ÖNORM S1120 (4 W/kg for 1 g mass)

Yelkenci and Magerl (2000)

About a phantom

835 MHz, 600 mW, handset with pull-out antenna

Unaveraged SAR in eye increased up to 29% with spectacles present but still below RF safety standard limits

Anderson and Joyner (1995)

120 mm wide, lens circumference 150 mm, wings 150 mm long

About a phantom

2 – 4 GHz TEM waves

Spectacles give “… either a shielding or enhancement effect …” but “No serious human health effect is conclusively revealed …”.

Griffin (1983) and Griffin and Davias (1983)

Summary: , The fairly thorough consideration here has been driven by the concern of

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possible enhancement of RF energy in the eyes by the spectacles. In particular, the study by Edwards and Whittow (2005) was quite exhaustive in the number of frames and exposure scenarios examined.

4.9Jewellery

Type Dimensions Placement Exposure Comments Source

Ear-rings: 1. Band 2. Three studs

1. Band 86 mm long, 5 mm thick, and 10 mm wide 2. One 25 mm long and two around 10 mm diameter

1. Band along back side of pinna. 2. Studs along back side of pinna

Placed in and about sphere and head models (assessed using FIT and FDTD techniques) and inside and outside of a phantom shell.

900 MHz dipole simulating mobile phone

Increased point SAR values were observed but no significant differences were found when considering the 10 g volume averaged SAR.

Fayos-Fernández et al. (2006)

Summary: The study presented raises no concerns, even with the jewellery in close proximity to the RF source, and one item of significant size (86 mm long). Even though jewellery can be worn on the body surface the dimensions of jewellery is usually small (< 20 mm).

4.10Tooth fillings, caps, and orthodontic braces and plates

Type Placement Exposure Comments Source

Teeth caps

In human head FDTD model

Mobile phones 915 MHz GSM and 1.9 GHz DECT at maximum levels

SAR levels comply with Austrian safety standard ÖNORM S1120 (4 W/kg for 1 g mass)

Yelkenci and Magerl (2000)

Summary: The Yelkenci et al. (2000) study raises no concerns, as could be expected for very small metal implants. Moreover, given that the oral cavity is routinely subjected and adapted

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to substantial heat loads (e.g. from a hot cup of coffee), then RF heating at upper tier limits would seem to be comparatively trivial.

4.11Implanted retinal stimulators

Type Placement Exposure Comments Source

Implanted retinal stimulator

In eye, adjacent to retina, in a 2-D FDTD head model

46 mW, 2 MHz extraocular transmitting multiturn coil around 20 mm from eye

Peak temperature rise of 0.6 °C, without blood flow, and 0.4 °C, with blood flow assumed

DeMarco et al. (2003), Lazzi et al. (2003)

Implanted retinal stimulator

In eye, adjacent to retina, in human head FDTD model

50 mW, 1.45 and 2.44 MHz extraocular transmitting multiturn coil

Peak 1 g SAR is 1.59 W/kg at 2.44 GHz and 0.83 W/kg at 1.45 GHz

Gosalia K and Lazzi G (2003)

Summary: These studies raise little concern. Would need to consider each case with improvements and changes in the technology.

4.12Implanted radiators

Type Dimensions Placement Exposure Comments Source

Implanted radiator

Loop 5 mm x 10 mm

Normal to chest wall with hip-mounted monitor, in human body FDTD model

403 MHz, 25 µW radiated power

Peak 10 g SAR is 1.79 W/kg

Scanlon (2004)

Summary: Particular study gives example that satisfies safety standards. Such devices can be expected to become more prevalent in society with the increase in medical monitoring systems and would need to be reviewed for each new technology.

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4.13Spinal fusion systems and cervical fixation devices in MRI

Type Dimensions Placement Exposure Comments Source

Cervical fixation devices

In standard position in phantom

MRI imaging (RF component at 63.8 MHz)

Study confirmed pain to patient caused by significant heating

Chou, Hover, McDougall and Ren (2004)

Spinal fusion stimulator (two wire leads and flat metal case) around the lumbar vertebrae

Not stated In standard position in phantom

MRI imaging (RF component at 63.8 MHz)

Temperature increase less than 2 °C unless broken electrode present which gave rise to 14 °C increase

Chou, McDougall and Chan (1997)

Spinal fusion implant (two wires and a flat metal case)

Wires 11.8 cm long. Case 3.67 cm by 2.3 cm.

In central position in box shaped torso

MRI switched-gradient magnetic fields. Exposure 600 Hz magnetic field

Highly localised increase in the E-field up to 197 times compared to when implant not present

Buechler, Durney and Christensen (1997)

Summary: Studies show situations of significant concern for MRI patients, but are difficult to generalise to assessments of persons exposed to the upper tier limits.

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5General rules-of-thumb and observations for implant assessments Listed below are rules of thumb and general observations which may be useful in determining whether an implant requires detailed assessment for a person exposed up to the upper tier limits of the ICNIRP Guidelines (1998) or the IEEE C95.1 standard (2006). These recommendations are based on the canonical modelling in section 3 and the specific implant assessments reviewed in section 4.

5.1General observations

1. RF field enhancements around an implant are affected by the frequency of exposure, the shape and size of the implant, its orientation with respect to the polarization of the in situ field and the dielectric properties of the surrounding tissue medium.

2. The absolute level of the SAR around an implant will also depend on the incident RF field levels in the body area around the implant. Thus, the potential for excessive localized SARs around an implant is only likely in parts of the body where in situ fields are already relatively high. Conversely, implants located in parts of the body which are relatively well shielded would not generally require assessment, especially in in low conductivity tissues like bone.

3. A metallic implant is a passive re-radiator, and of itself cannot create additional RF power absorption in the body. Thus the overall RF heating in the general vicinity of the implant will remain about the same. One possible exception to this rule is the case of a large implant in one leg (e.g., a metal rod in the tibia), which by providing a lower impedance conductance path preferentially diverts additional current flow to that leg for exposure frequencies around and below whole body resonance frequencies.

4. Constructive and destructive interference effects can enhance or diminish the RF field level in a skin or skin/muscle layer above bone depending on the thickness of the layers and the frequency of exposure, thereby affecting the incident exposure of an implant located there. See Figures 3-7 for point SAR plots vs frequency for various tissue layer thicknesses.

5. SAR attenuation at the skin surface is very substantial at frequencies above 6 GHz and thereby provides RF shielding protection against metallic implant enhancements in the body (see Figures 3-7).

6. 10 cm³ average Volumetric Absorption Rate (VAR) is a better metric for assessing RF heating effects around implants than 10 g average SAR as it is not affected by mass density changes between the metal implant and the surrounding tissue and is more closely related to temperature rise.

5.2Thermal effects

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7. Some implants are located in a thermal environment where efficient heat transfer mechanisms will greatly mitigate any localized heating around parts of the implant. For example, the temperature of an arterial stent is strongly controlled by the convective heat transfer of the arterial blood flow passing through it. Metal plates located close to the skin (e.g., plates on the outside of the cranium) are another example, as are all forms of body-worn metallic objects (e.g. spectacles, jewellery, and the external component of a cochlear implant system).

5.3Rods and other linear objects

8. In low loss tissues such as bone, a maximal resonant response for E parallel rods occurs when the rod length is equal to one third of the exposure wavelength (see Figure 15). This resonant enhancement increases linearly with the length of the rod (see Figure 16). Long rods can cause very substantial field enhancement at their tips, and hence are more likely candidates for detailed assessments. This resonance effect is damped out in tissues with higher electrical conductivity such as muscle (see Figure 20).

9. A common mechanism for RF enhancement around a metallic implant is the field concentrations that appear at the opposite ends of an implant where the projected length of the implant against the incident RF electric field vector is the longest. For rods and other linear structures, the enhancement mostly occurs at the end tips (see Figure 14). The level of enhancement depends on the size of the implant with respect to the wavelength of the exposure, which in turn is inversely proportional to the exposure frequency.

10. Short rods less than 20 mm in length do not cause any significant field enhancement around the implant, which may in part be due to the averaging effect of the 10 cm³ VAR volume (or 10 g SAR mass). It is probably reasonable to infer from this that all objects with a maximum dimension of 20 mm or less will not require assessment.

11. The diminished enhancement of SAR at the tips of implants for supra-resonance removes the need to assess a large class of implants of certain lengths above certain frequencies which are located at certain distances below the skin (use Figure 20 in combination with Figures 3-7 for assessment guidance).

12. For rods of all length immersed in muscle, the 10 cm³ VAR enhancement is low (< ×1.4) for frequencies above 500 MHz (see Figure 20). Similar observations may apply in other high loss tissues. The re-radiated fields around an implant tend to decay very quickly in a lossy dielectric tissue environment.

13. The RF field enhancement at the ends of an implant is constant for frequencies below resonance. The level of this enhancement increases with the rod length (see Figure 17) and is independent of the dielectric properties of the surrounding tissue (see Figure 20).

14. There is no significant RF enhancement at the ends of an implant in the supra resonant frequency range. The supra resonant range occurs at lower frequencies for longer implants (see Figure 20).

15. The 10 cm³ VAR enhancement at the tips of linear implants diminishes substantially for non parallel E polarizations. No field enhancement is seen for E polarizations that are perpendicular to rods. Hence, a person moving with respect to the exposure source would likely reduce implant SAR enhancements when averaged over time.

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16. The tip shapes on rods have negligible impact on localised RF heating. This is probably a consequence of the small size of tips relative to the 10 cm³ VAR averaging volume. Likewise, RF field enhancements that occur around any sharp point in the implant would be so localized that their influence would not be noticeable in a 10 cm³ VAR of 10 g SAR averaging mass.

5.4Screws

17. No assessment required for screws up to 20 mm in length. Longer screws may require assessment depending on the frequency of exposure and the level of RF shielding at the implant body location. The pointed end of a screw (where the localised E-field can be elevated) does not require specific assessment when determining 10 g mass averaged SAR (see Figure 18).

5.5Arterial stents

18. Detailed studies published so far show no cause for concern for stents up to 34 mm in length (see section 4.3). In addition, the potential for adverse heating around stents is substantially mitigated by the convective cooling from blood flow within the artery. Hence no assessments are required for all stents.

5.6Wide Plates

19. Metal plates that lie directly beneath the skin may enhance SAR in the skin at microwave frequencies due to constructive interference which is maximized when the thickness of the skin is equal to a quarter wavelength of the RF exposure in that tissue. For skin thicknesses between 3 to 8 mm, the quarter wave resonance for a normally incident exposure ranges from 4.1 to 1.5 GHz (see Table 3). This enhancement however does not appear to cause the 10 g average SAR to exceed the 10 W/kg upper tier limit for ambient field exposures below the upper tier reference levels for E or H of the ICNIRP Guidelines (1998).

20. The peak linear SAR enhancement is generally more spread around the circumference of

a wide plate compared to a rod which reduces the peak 10 g average SAR. As seen in figure 20, the level of enhancement rises with increasing implant length. For exposure frequencies less than 200 MHz, plates larger than 50 mm in diameter may need to be assessed for linear enhancement effects if situated close to the skin where shielding is low (see sections 3.2 and 4.4).

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5.7Pacemakers 21. The body of the pacemaker should not be of concern based upon the studies listed above

for wide plates, particularly as they are usually located deep inside the body. Further studies are required in regards to the influence of the pacemaker leads.

5.8Loops

22. A loop shaped metal implant which is oriented normal to the in situ H-field may produce enhanced SAR in any gap in the loop. This phenomenon requires further investigation.

5.9Cochlear Implant Systems

23. No assessment required. See section 4.7 for further details.

5.10Spectacles

24. No assessment required. See section 4.8 for further details.

5.11Jewellery

25. Even though there have been limited assessments performed on jewellery (see section 4.9) there should be no requirement for an assessment. Jewellery is not usually positioned near vital body tissues, the exception being piercings near the eye but these are typically small (< 20 mm) and do not require assessment. Heat transfer mechanisms will easily dissipate any localised heating.

5.12Tooth fillings, caps, and orthodontic braces and plates

26. No assessment required. The oral cavity is naturally adapted to higher heat loads (e.g. from a hot cup of coffee) which in part is due to the convective and evaporative cooling from respiration air flow. This may reasonably be expected to provide a larger margin of safety from RF heating around metallic implants in the mouth. Tooth fillings and caps are also small in size.

5.13Shrapnel and shotgun pellets

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27. No assessment required for pieces up to 20 mm in length. Longer shrapnel pieces may require assessment depending on the frequency of exposure and the level of RF shielding at the implant body location.

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6References Anderson V, McIntosh RL. 2004. First stage report on AOARD metallic implant project. Tokyo:

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Virtanen H, Huttunen J, Toropainen A, Lappalainen R. 2005. Interaction of mobile phones with superficial passive metallic implants. Physics in Medicine and Biology. 50:2689-2700

Virtanen H, Keshvari J, Lappalainen R. 2006. Interaction of radio frequency electromagnetic fields and passive metallic implants - A brief review. To appear in Bioelectromagnetics.

Yelkenci T, Magerl G. 2000. Die Berechnung der von Mobiltelefonen im menschlichen Kopf hervorgerufenen spezifischen Absorptionsraten. Elektrotechnik und Informationstechnik 117:744-749.

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Radiofrequency/Microwave Safety Standards

R. C. Petersen

Introduction Radiofrequency (RF)/microwave safety standards generally refer to standards, regulations, recommendations and guidelines that specify basic restrictions and exposure limits for the purpose of protecting human health. Contemporary standards are based on the results of critical evaluation and interpretation of the relevant scientific research – ideally, all laboratory and epidemiology research that relates any biological response, from short-term and long-term exposure, would be included. From this evaluation, a threshold is established for the most sensitive confirmed response that could be considered harmful to humans. To account for uncertainties in the data and to increase confidence that the limits are well below the levels at which an adverse effect could occur, the resulting threshold is lowered by a somewhat arbitrary safety factor. RF safety standards have evolved over several decades from a simple single value that is applicable over a broad band of frequencies e.g., 10 MHz to 100 GHz, to sophisticated frequency and time dependent limits that cover a much greater frequency range, e.g., 3 kHz to 300 GHz. The early single-value limits were usually expressed in terms of incident power density and were based on simple models predicting whole-body heating; contemporary standards address effects associated with electrostimulation at low frequencies, effects associated with whole-body heating, effects associated with surface heating and usually include limits on induced and contact, exposure to pulses of high peak but low average power, and localized exposure. The evolution of the development of the standards and guidelines developed by committees of the American National Standards Institute (ANSI), the International Commission on Non-Ionizing Radiation Protection (ICNIRP), the Institute of Electrical and Electrical Engineers (IEEE) and the National Council on Radiation Protection and Measurements (NCRP) is described below.

The early years Although the interest in the potential effect on humans exposed to RF energy goes back almost a century, it was only toward the end of World War II that a concerted effort was made to try to understand the interaction of RF energy with biological systems and, from this understanding, establish criteria to protect against effects that could be considered harmful. The effort in the United States mainly stemmed from anecdotal reports of various effects by radar technicians and others who came in contact with various military radars, e.g., temporary male sterility from exposure to radar beams, the induction of opacities in the lens of the eye. Although by this time the heating effects of RF energy was well-understood and the technology had been applied in medicine for decades, e.g., RF diathermy, the anecdotal reports in conjunction with studies reporting lens opacities in the eyes of subject animals exposed to microwave energy (e.g., Richardson, et el., , Clark et al. , Daily, et al. ) and a report of cataracts in a radar technician (Hirsch and Parker ), resulted in a coordinated effort to understand the interaction of RF/microwave energy with biological systems and to establish safety limits. As clearly evidenced by the several orders of magnitude differences between the protection guides initially adopted by different organizations worldwide during the mid to late 1950’s, there was little agreement as to suitable protection criteria or an appropriate rationale for establishing these criteria.

Organized efforts to seek an understanding of the possible interaction mechanisms and the effects on human beings of exposure to electromagnetic energy at RF/microwave frequencies began in the United States with a number of meetings and symposia. Such meeting included the “Symposium on Physiologic and Pathologic Effects of Microwaves” held at the Mayo Clinic in 1955, the “First Annual Tri-Service Conference on Biological Hazards of Microwave Radiation” and the “Second Annual Tri-Service Conference on Biological Hazards of Microwave Radiation,” in 1957 and 1958, respectively . The purpose of these meetings was to bring together key researchers in the radiation hazards area in order to discuss ongoing research and identify needed research and, ultimately, establish science-based safety limits. As pointed out by Mumford , during the time period of these symposia, and even before, a number of widely different exposure limits were recommended and used by different organizations in the US. These recommendations, expressed in terms of incident power density, ranged from 100 µW/cm2 to

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100 mW/cm2. The upper level was based on an apparent threshold for opacities in the lens of the eye (cataracts), which was estimated by Hirsch and Parker to be of the order of 100 mW/cm2. Others, e.g., Williams, et al. and Ely et al. , found higher thresholds but there was general agreement that 100 mW/cm2 should be considered an approximate threshold for biological damage, i.e., levels above this value were considered hazardous and should be avoided. In 1953, one Department at Bell Telephone Laboratories added a 30 dB safety factor to the level considered hazardous and adopted 100 µW/cm2 as a safe level. In 1954 General Electric adopted 1 mW/cm2 as a safe level, some organizations informally adopted 10 mW/cm2 as a potentially hazardous level, and still others merely adopted 100 mW/cm2 as a hazardous level without specifying a safe limit. Based on a number of animal studies and discussions at the symposia noted above, it became apparent by the late 1950’s that 100 µW/cm2 was too conservative, 100 mW/cm2 was probably not conservative enough and most organizations adopted an exposure limit of 10 mW/cm2 as recommended by Schwan and Li . This value was based on a simple thermal model that limited the rise in core temperature of an exposed individual to less than 1ο C, assuming that about half of the incident energy was absorbed. The frequency range was 10 MHz to 100 GHz.

USAS C95.1-1966 and ANSI C95.1-1974 In 1960, the first formal RF safety standards project was approved in the US when the American Standards Association (ASA)1 approved the initiation of Radiation Hazards Standards Project C95 and the establishment of a committee (C95), which was charged with developing standards through an open consensus process. The scope of the committee was “Hazards to mankind, volatile materials, and explosive devices which are created by man-made sources of electromagnetic radiation. The frequency range of interest extends presently from 10 kHz to 100 GHz. It is not intended to include infrared, X-rays or other ionizing radiation.” The C95 Committee, co-sponsored by the Department of the Navy (Bureau of Ships), and the American Institute of Electrical Engineers,2 was chaired by Schwan; there were six members on the committee, including the chairman. The committee deliberated for approximately six years and in 1966 the first C95.1 standard, USAS C95.1-1966, was published . The exposure limit was presented as a “Radiation Protection Guide” (RPG), defined as the radiation level which should not be exceeded without careful consideration of the reasons for doing so. The RPG for whole-body exposure was 10 mW/cm2 across the frequency spectrum from 10 MHz to 100 GHz. Included were an averaging time of 6 minutes and a corresponding energy density limit of 1 mWh/cm2. The 6 min averaging time appears to have come from the diathermy literature, although this is not stated in the standard. It is noted that the RPG is applicable in moderate thermal environments and that under conditions of moderate to sever heat stress the RPG should be reduced accordingly. The entire standard is less than one and one-half pages in length. Although some at the time considered the RPGs only applicable in the occupational environment, nowhere in the standard is this stated or implied.

A revision of USAS C95.1-1966 was published in 1974 by the American National Standards Institute (ANSI) as ANSI C95.1-1974 . The normative part of the standard was still less than two pages in length; the RPGs for continuous whole body exposure, expressed in terms of incident power density and energy density, remained at 10 mW/cm2 and 1 mWh/cm2, respectively. Because it was then recognized that important exposures could occur in the near field, particularly in the workplace, limits were also given separately in terms of mean squared electric and magnetic field strengths. Although the mean-squared electric and magnetic field strengths were each based on an equivalent power density of 10 mW/cm2, it was considered important to assess each independently, at least at frequencies below 300 MHz. It is noted in the standard that the RPGs were based on the currently available literature, it was the consensus of the committee that effects associated with tissue heating remain dominant, and the RPGs should protect against such effects. It is also noted that at the time, sufficient information concerning modulation effects, peak power effects and frequency dependent effects was not adequate to substantiate adjustments to the RPGs to account for these effects . The frequency range over which the RPGs applied remained 10 MHz to 100 GHz. During the eight year development of the 1974 revision the working group (Subcommittee 4)3 had grown considerably in size, totaling almost 70 members.

ANSI C95.1-1982 Standard and the National Council on Radiation Protection and Measurements (NCRP) Recommendations (1986) Each revision of the C95.1 standard was more scientifically sound, albeit more complex than its predecessor, with major changes appearing in ANSI C95.1-1982 (the revision of ANSI C95.1-1974). These changes were related to the significant advances that occurred in the 1970s in instrumentation and the techniques for measuring complex

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electromagnetic fields and, most important, in the field of RF dosimetry. Advances in dosimetry included the use of numerical techniques to study energy absorption patterns in simple spheroidal and block models of humans and animals and the use of thermography to study the absorption characteristics of complex realistic models of animals and anthropomorphic models of humans. These studies led to a clearer understanding of the frequency-dependent absorption by objects in an RF field, in particular the pronounced resonance over a narrow range of frequencies, the extent of which depended on the geometry and orientation of the object in the field. Under optimal exposure conditions, i.e., conditions yielding maximal absorption, it was found that the absorption cross section at resonance could be 2-3 times greater than the geometrical cross section (see Figure 1). From this understanding, it became apparent that realistic future protection guides should be frequency-dependent, something that was studied by Soviet scientists in the early 1960s, cf. Pressman . Thus, while the RPGs in the 1966 and 1974 C95.1 standards were independent of frequency, by the mid 1970’s it was recognized that the amount of RF energy absorbed by an object in the field would be frequency dependent—as should the RPGs.

RF dosimetry studies , i.e., the study of RF absorption in models of humans and animals, were carried out by Guy , , Guy et al. , Gandhi, et al. , , Durney , Durney et al. , Hagmann and Gandhi , and others, using thermographic techniques and numerical modeling. These studies led to an understanding of how the incident and internal electromagnetic fields are related as a function frequency, field polarization, and size, geometry, orientation and composition of the object in the field. They also led to the recognition of the need for a dosimetric quantity to relate the incident fields to the internal fields, a quantity that would be more directly related to a biological effect than the incident fields alone. This need became very apparent during the literature evaluation that led to the 1982 standard where the general criticism of the growing body of literature was a complete lack of consistency in reported results, particularly with respect to the field parameters necessary for determining the internal field distributions or energy absorbed from the field. In many cases only the incident power density was reported without mention of other parameters necessary for estimating these quantities and, hence, made the comparison of studies difficult at best. This lack of consistency and completeness also helped explain large differences in the incident power density reported for the same biological effect in different animal species, and in the same animal species under different exposure conditions. It was agreed that an appropriate quantity for establishing meaningful thresholds and allowing comparison across frequency and animal species should be analogous to “dose,” and “dose rate” used by the ionizing radiation community. This then would be the basic parameter that should be reported so that the results of studies at different frequencies, using different animal species and widely different exposure conditions e.g., plane wave, TEM cell, cylindrical cavity, could be compared. Once a threshold for an adverse effect is determined in terms of the “dose rate,” i.e., the rate at which energy is absorbed from the field, the growing understanding of RF dosimetry would provide the means for relating this threshold to the incident fields and, with a suitable safety factor, to realistic frequency-dependent RPGs.

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Figure 1—Calculated whole-body average SAR versus frequency for simple models of the average man for three standard polarizations. The incident power density is 1 mW/cm2. Curves E and H refer to exposure geometries where the major axis of the body is aligned with the electric field (E) and the magnetic field (H), respectively; K refers to the geometry where the direction of propagation is in the direction of the major axis of the body. (From Durney, et al. )

Various quantities and terms were proposed for an appropriate dosimetric quantity including “absorbed power density” expressed in units of W/cm3 or W/kg, and “dose, and “dose rate,” i.e., the energy imparted to a unit mass of biological material (dose) and the rate at which energy is imparted to unit mass (dose rate). After considerable discussion and debate within C95 Subcommittee 4 (SC4) during the 1970’s, there was consensus that dose and dose-rate were appropriate. To avoid confusion and the connotation associated with terms traditionally used in ionizing radiation protection, “dose” and “dose rate” were named “specific absorption” (SA), defined as the incremental energy absorbed by (dissipated in) an incremental mass, and “specific absorption rate” (SAR), defined as the time rate of incremental energy absorbed in (dissipated in) an incremental mass—specific meaning that it is unique to RF/microwave frequencies.. The units of SA and SAR are J/kg and W/kg respectively. Although SA and SAR first appear as the defining RF dosimetric quantities in the 1981 NCRP Scientific Committee 39 Report (No. 67) , it had already been accepted by C95 SC4 in the late 1970s during the development of C95.1-1982 and was used effectively to compare the results of studies in the database in order to determine an SAR threshold for effects considered adverse. From this threshold and the results of the increasing number of dosimetry studies, frequency-dependent limits expressed in terms of the incident fields were derived. These limits were called Radiofrequency Protection Guides (RFPG) in order to mitigate confusion with the term RPG used by the ionizing radiation community. Compliance with the RFPGs, ensures that the SAR remains below the threshold (with an adequate margin of safety) under various exposure conditions and for various size humans from infants to adults.

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As indicated above, the 1966 and 1974 C95.1 standards were based on the assumption that effects to protect against are related to gross thermal effects associated with elevations in core temperature. By 1980, however, a number of studies reporting effects that occurred at levels where significant temperature increases were not observed or expected (i.e., “athermal effects”) began to appear in the scientific literature. These studies warranted careful examination and were included in the list of citations considered by SC4 during the development of the 1982 C95.1 standard. (It is pointed out in the 1982 standard that “classification and judgment of findings were made without prejudgment of mechanisms of effects,” i.e., the intent of the subcommittee was to protect exposed humans against harm from adverse effects associated with any interaction mechanism, including effects associated with an elevation in body temperature” ). During the literature selection process that led to ANSI C95.1-1982, several hundred experimental studies reporting effects associated with RF energy were reviewed and a select list of 32 studies was compiled in accordance with the following criteria: demonstrability (positive effects), relevance, reproducibility and dosimetric quantifiability (i.e., was the SAR reported or was there enough information in the report regarding the exposure setup to allow determination of the SAR).

Studies that demonstrated general evidence of morbidity or debilitation, chronic or acute, were emphasized . The bias toward positive findings added a degree of worst-case conservatism to the resulting exposure limits. When positive results were demonstrated for a specific biological endpoint by several laboratories, those studies that demonstrated the effect at the lowest SAR and longest exposure duration were selected. Biological endpoints were grouped in the 15 categories shown in Table 1 along with the number of studies that met the selection criteria in each category. Reports of specific effects induced by low frequency amplitude-modulated RF carriers, e.g., calcium efflux from chick brain tissue, were included but were not considered adverse for the following reasons: inability of the SC4 members to relate the effect to human health; the narrow range of effective modulation frequencies; the study author’s finding that the effect is reversible. The studies were reviewed by the biologists on SC4 and also by the physically trained scientists and engineers with emphasis on reliability, evidence of adverse effects, and whether the study had been independently replicated in another laboratory. The engineers also determined the SAR for each of the studies.

Following the critical evaluation of the selected studies, the subcommittee agreed that the most sensitive, reliable confirmed biological response that could be considered potentially harmful to humans is disruption of food-motivated learned behavior. Even though this effect is, modest, transient and represents an adaptive response, it serves to identify a threshold for potentially harmful effects . It was also assumed that while behavioral disruption was demonstrated to be transient and reversible after acute exposure, chronic exposure could lead to irreversible injury. The threshold for behavioral disruption was found to reliably occur within a narrow range of whole-body-averaged SARs between approximately 4 to 8 W/kg, across animal species from rodents to primates, frequencies from 600 MHz to 2450 MHz, and incident power densities that ranged from 10 to 50 mW/cm2. Thus it was agreed by SC4 that the appropriate biological endpoint for acute exposures should be disruption of behavior, and the corresponding threshold, in terms of whole-body-average SAR, should be set at 4 W/kg. That is, SAR values above 4 W/kg could produce adverse effects while SARs below 4 W/kg were not shown to result in effects that could be considered hazardous.

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Table 1—Category of exemplary reports selected from the experimental literature by SC4 for the

development of ANSI C95.1-1982

Environmental factors (effects of temperature on the specific endpoint – 3 studies

Behavior and physiology – 6 studies

Immunology – 4 studies

Teratology – 1 study

Central nervous system/blood-brain barrier – 4 studies

Cataracts – no reliable studies were found reporting cataracts at levels ≤ 10 mW/cm2

Genetics (no reliable studies were found reporting genetic effects at levels ≤ 10 mW/cm2

Human studies – no reliable human studies were found

Thermoregulation and metabolism – 5 studies

Biorhythms – 1 study

Endocrinology – 3 studies

Development – 3 studies

Evoked auditory response (RF hearing) – no studies

Hematology – 2 studies

Cardiovascular – 1 study

There was considerable deliberation during the development of ANSI C95.2-1982 as to an appropriate margin of safety and whether a single frequency-dependent RFPG should apply to exposures of the public and the worker. In order to ensure an adequate margin of safety, a safety factor of 10 was incorporated, which was considered adequate to protect members of the public and the worker because of the conservatism already built into the 4 W/kg threshold. Thus, a whole-body-averaged SAR value of 0.4 W/kg was adopted as the basis of the standard (basic restriction) from which the frequency-dependent RFPGs (also called derived limits, investigation levels, reference levels) would be derived.

By 1980 the field of RF dosimetry had advanced to the point where reliable techniques were available to determine the incident power density that would limit the whole-body-averaged SAR to a specific value. Theoretical analyses were carried out to determine the magnitude of the incident fields that would limit the whole-body-averaged SAR to 0.4 W/kg under worst-case exposure conditions, i.e., conditions that would maximize energy absorption. The results of these analyses demonstrated that under plane wave exposure conditions, energy absorption in models of humans, ellipsoids, animals, etc., is generally maximal when the major axis of the exposed object is aligned with E-field vector of the incident field and, under these exposure conditions, absorption increases with the square of frequency, reaches a maximum (resonance), then decreases linearly with increasing frequency over a limited range of frequencies, and then remains relatively constant. Work by Gandhi and others showed that under these conditions, maximum absorption (resonance) occurs when the length of the long axis of the exposed object is approximately 0.36 to 0.4 wavelengths. For example, the resonant frequency varies from about 79 MHz to 54 MHz, respectively, when the height of the body ranges from 1.52 m to 1.98 m. Moreover, it was found that when the object is in contact

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with a ground plane, the resonant frequency is about one-half the value found when it was not in contact. Data were complied from a number of dosimetry studies and a family of resonance curves and plotted as a function of whole-body-average SAR versus frequency for humans ranging in size from infants to tall adults, both in and not in conductive contact with a ground plane and with the long axis of the body parallel to the E-field vector of the incident field. The result of this exercise, normalized to an incident power density of 1 mW/cm2 (which limits the maximum SAR at resonance to 0.4 W/kg) is shown in the Appendix of C95.1-1982, . The RFPGs were obtained by rearranging these data to determine the maximum incident power density that would limit the whole-body-averaged SAR to 0.4 /kg across the frequency range of interest. The results showed that the incident power density required to maintain an essentially constant SAR in humans of all sizes could be approximated by a broad resonance curve that decreased as 900/f2 up to 30 MHz, above which it remained constant up to 300 MHz, then rose as f/300 to 1500 MHz, above which it remained relatively constant at 5 mW/cm2. Although the limiting incident power density continues to increase with decreasing frequency for frequencies below 30 MHz, the RFPG was limited to 100 mW/cm2 for frequencies below 3 MHz to prevent reactions at the body surface caused by the relatively high E-fields (> 600 V/m), e.g., perception and electric shock. The averaging time remained 6 min over the entire frequency range. There was some concern by members of the subcommittee about the 6 min averaging time as it applies to pulses of high peak power but low average power because time averaging single pulses of extremely short duration, e.g., a few microseconds, leads to unrealistically high exposure limits. No agreement was reached on how to treat this situation and, therefore, there are no explicit peak power limitations in the 1982 standard.

In addition to RFPGs for whole-body exposures, the 1982 standard contained the following exclusions: 1) The RFPG for whole body exposure at frequencies between 300 kHz and 100 GHz could be exceeded if it could be shown using laboratory techniques that the resulting SAR averaged over the whole body would not exceed 0.4 W/kg and the peak spatial average SAR could not exceed 8 W/kg as averaged over any one gram of tissue. The 8 W/kg was based on the peak to average SAR values reported in a number of animal studies where it was found that typically the peak to average SAR ratio was 20 to 1; 2) At frequencies between 300 kHz and 1 GHz, the RFPGs could be exceeded if the RF input power to the devices is 7 W or less, which is based on limiting the peak spatial-average SAR to 8 W/kg.

Finally, the standard contained the following caveat: “Because of the limitations of the biological effects database, these guides are offered as upper limits of exposure, particular for the population at large. Where exposure conditions are not precisely known or controlled, exposure reduction should be accomplished by reliable means to values as low as reasonably achievable [ALARA] .” This last sentence often has been quoted out of context by applying it to RF exposure in general.

NCRP Report No. 86 A number of important events occurred during the interval between approval of ANSI C95.1-1982 and IEEE C95.1-1991, including comprehensive reviews of the extant RF bioeffects literature by a scientific committee of the National Council on Radiation Protection and Measurements (NCRP).4 Although NCRP is concerned mostly with ionizing radiation, in 1973 Scientific Committee 53 (SC53 – now SC89-5) was convened to carry out a comprehensive review the scientific literature and make recommendations for limiting exposures to RF energy. SC53 consisted of 6 members, 5 advisory members and 5 consultants (NCRP )—8 of whom were at the time also members of SC4 of the ANSI C95 committee. Whereas SC4 adopted criteria for selecting studies specifically relevant to standard setting (e.g., demonstration of positive effects, relevance, reproducibility, dosimetric quantifiability), and consequently reviewed in detail a relatively small number of reports, SC53 carried out complete review of the literature and close to 1000 studies were included in the NCRP literature evaluation (the literature cutoff date was 1982 but a few 1983 references are included). With the quality of the selected reports ranging from excellent to poor, including some which appear to be nothing more than anecdotal reports, value judgments had to be made in interpreting and assessing the quality of each of the studies. Reports were divided roughly by biological endpoint into the categories shown in Table 2.

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Table 2—Category of reports reviewed NCRP SC53 SC4 during the development of NCRP Report 86

Macromolecular and cellular effects

Chromosomal and mutagenic effects

Carcinogenesis

Effects on growth, reproduction and development

Effects on the hematopoietic and immune systems

Effects on the endocrine system

Effects on cardiovascular function

Interaction with the blood-brain-barrier

Interactions with the central nervous system

Behavioral effects

Cataractogenesis

Human studies

Thermoregulatory response in human beings

Mechanisms of interactions

As did SC4 of the C95 committee, the members of SC53 also concluded that the most sensitive and statistically significant biological endpoint was behavioral disruption. Although the carrier frequencies for behavioral disruption ranged from 225 to 5800 MHz, across animal species from laboratory rats to rhesus monkeys (see Table 3), the incident power densities ranged from 8 to 140 mW/cm2 and the exposure conditions included near field, far field, planewave, multipath, CW and modulated RF, the SAR threshold for behavioral disruption narrowly ranged from 3 to 9 W/g, which is in fair agreement with the threshold reported in ANSI C95.1-1982.

Table 3—Comparison of power density and SAR thresholds for behavioral disruption in trained

laboratory animals (from Osepchuk and Petersen )

Species and Conditions

225 MHz (CW) 1.3 GHz (Pulsed)

2.45 GHz (CW) 5.8 GHz (Pulsed)

Norwegian Rat Power Density SAR

10 mW/cm2 2.5 W/kg

28 mW/cm2 5.0 W/kg

20 mW/cm2 4.9 W/kg

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Squirrel Monkey Power Density SAR

45 mW/cm2 4.5 W/kg

40 mW/cm2 7.2 W/kg

Rhesus Monkey Power Density SAR

8 mW/cm2 3.2 W/kg

57 mW/cm2 4.5 W/kg

67 mW/cm2 4.7 W/kg

140 mW/cm2 8.4 W/kg

For the frequency range where surface effects predominate, SC53 went further than the C95 committee and recommended lowering the RFPG if there is a likelihood of coming into contact with grounded metallic objects. To prevent RF burns at the point of contact, the recommendation was to lower the RFPG such that the induced RF current does not exceed 200 mA. This is to be done on a case by case basis.

Recommendations, based on the low-frequency modulation-specific effects literature, e.g., calcium efflux studies, were also included. It was pointed out that it is not known whether these affects lead to a risk to human health, but the reliability of the studies and their independent confirmation in avian and mammalian species dictates the need for caution . The recommendation is as follows: “If a carrier frequency is modulated at a depth of 50% or greater at frequencies between 3 and 100 Hz, the exposure criteria for the general population shall also apply to occupational exposures.” The incorporation of this caveat, which was based on reported frequency and intensity “windows,” was extremely controversial and has not been accepted by other standard-setting bodies or incorporated into contemporary science-based standards and guidelines.

Although the RFPGs in the 1982 C95.1 standard and the NCRP Report are far more realistic and sophisticated than those used before 1982, both suffer serious shortcomings, including the following: 1) There are no limitations on peak power for pulses of high intensity but low average power—the 6 min averaging time allows exposure to short pulses in excess of those known to cause burns at frequencies where the energy is deposited superficially, i.e., frequencies above a few GHz; 2) There is no explicit guidance to limit induced current at the lower frequencies, i.e., frequencies below a few MHz, to prevent electric shock and RF burns; 3) The magnetic field strength limits, which correspond to the equivalent plane wave power density of the RFPG, is unrealistic at the low frequencies where the magnetic field is inefficiently coupled to the body; 4) No clear distinction is made between whole-body and partial-body exposure (Petersen ). Many of these issues were addressed in the next revision of the 1982 C95.1 standard, i.e., IEEE C95.1-1991.5 Table 4 is a comparison of the rationale between NCRP Report 86 and ANSI C95.1-1982.

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Table 4—Comparison of rationale: ANSI/NCRP (from Petersen )

Parameter ANSI C95.1-1982 NCRP Report No. 86

Recognition of whole-body resonance Yes Yes

Incorporation of dosimetry(SAR) Yes Yes

Database of experimental literature Relatively small (32 citations) Large

Most significant biological endpoint Behavioral disruption Behavioral disruption

Whole-body-averaged SAR associated with behavioral disruption

4-8 W/kg 3-9 W/kg

Limiting whole-body-averaged SAR 0.4 W/kg 0.4 W/kg 0.08 W/kg*

Averaging time 6 min 6 min 30 min*

Criterion for limits below 3 MHz Surface effects, e.g., perception, electric shock (E field))

Surface effects, e.g., perception, electric shock (E field).

Criterion for localized exposure Whole-body-averaged SAR < 0.4 W/kg Peak spatial average SAR < 8 W/kg

Peak spatial average SAR < 8 W/kg Peak spatial average SAR < 2 W/kg*

Special criterion for modulated fields No Yes (for occupational exposure)

Specific limits for high peak, low average power pulses

No No

*General population

Institute of Electrical and Electronics Engineers (IEEE) C95.1-19916 Almost immediately after ANSI C95.1-1982 was published, Subcommittee 4 of the ANSI C95 Committee began work on the next revision with emphasis on addressing some of the recognized shortcomings mentioned above. As with the earlier revisions, the literature was culled for relevant studies and a total of 321 papers were identified by the Literature Surveillance Working Group. (See Figure 2 for a graphical depiction of the literature evaluation process.) Although most of the selected reports were published before 1985, several reports, particularly those relating to shock, burns and peak-power effects, were published after 1985. Those peer-reviewed studies that reported effects at whole-body averaged SARs less than 10 W/kg, and which also met the criteria of the Engineering, Biological and Statistical Evaluation Working Groups, were sent to the Risk Assessment Working Group, whose charge was to determine the threshold SAR above which potentially deleterious effects are likely to occur in humans, even if the effects are reversible . As in the case of ANSI C95.1-1982 and the NCRP Report, the working group concluded that a threshold SAR of 4 W/kg is appropriate to protect against behavioral disruption, which was once again found to be the most sensitive and reliable biological endpoint. Without saying that behavioral disruption is a “thermal” effect, it was noted that behavioral disruption in laboratory animals was

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accompanied by a core temperature increase of approximately 1 °C and the effect, regardless of the interaction mechanism, was reversible. Effects reported to be non-thermal, e.g., modulation specific effects such as changes in calcium efflux from chick brain tissue, were again considered but it was the consensus of the Risk Assessment Working Group that such effects were inconsistent, could not be related to human health, and, therefore, not useful for standard setting. It was also the consensus of the Risk Assessment Working Group that a safety factor of 10 to account for dosimetric, biological and other uncertainties would provide an adequate margin of safety, thereby yielding a basic restriction of 0.4 W/kg in terms of whole-body-averaged SAR.

Figure 2—Graphical depiction of the ANSI/IEEE literature evaluation process

Unlike C95.1-1982, however, which consisted of a single tier that was considered protective of all, the SC4 Societal Implications Working Group recommended following NCRP and including a separate lower tier for exposures that take place in uncontrolled environments. There recommendation was based on the following argument : “To some, it would appear attractive and logical to apply a larger, or different, safety factor to arrive at the guide for the general public. Supportive arguments claim subgroups of greater sensitivity (infants, the aged, the ill, and the disabled), potentially greater exposure durations (24 hours/day vs. 8 hours/day), adverse environmental conditions (excessive heat and/or humidity), voluntary versus involuntary exposure, and psychological/emotional factors that can range from anxiety to ignorance. Non-thermal effects, such as efflux of calcium ions from brain tissues, are also mentioned as potential health hazards .” However, this is followed by “The members of Subcommittee 4 believe the recommended exposure levels should be safe for all, and submit as support for this conclusion the observation that no reliable scientific data exist indicating that

1. Certain subgroups of the population are more at risk than others; 2. Exposure duration at ANSI C95.1-1982 levels is a significant risk; 3. Damage from exposure to electromagnetic fields is cumulative; or 4. Non-thermal (other than shock) or modulation-specific sequelae of exposure may be meaningfully related

to human health.”

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and “No verified reports exist of injury to human beings or of adverse effects on the health of human beings who have been exposed to electromagnetic fields within the limits of frequency and SAR specified by previous ANSI standards, including ANSI C95.1-1982.” Thus, any scientific justification for the lower tier is tenuous at best. However, the C95 standards are developed through an open consensus process and the majority of the voting members agreed that a lower tier is appropriate.

The lower tier was derived by reducing the upper tier value by a factor of 5, at least in the resonance region where SAR is important resulting in a whole-body-averaged SAR of 0.08 W/kg. However, unlike other standards and guidelines that set limits based on population groups, i.e., an upper tier for occupational exposure and a lower tier for exposure of the general population, the committee concluded that it would be more meaningful to address the exposure environment rather than the exposed population to help clarify the assignment of an appropriate set of limits to personnel, particularly in the workplace. Thus, the derived limits of the upper tier, now referred to as maximum permissible exposure values (MPE) to be consistent with the use of the term in other standards relating to non-ionizing radiation protection, e.g., the laser safety standards ANSI Z136.1 and IEC 60825, apply to exposures in controlled environments; the MPEs of the lower tier apply to exposures in uncontrolled environments. Controlled environments are considered locations where exposures may be incurred by individuals who are aware of and have control of their potential for exposure, e.g., as a concomitant of their employment, or by other cognizant persons; uncontrolled environments are locations where there is exposure of individuals who have no knowledge or control of their exposure (in living quarters, offices or in workplaces where there are no expectations that exposure levels may exceed the MPE recommended for lower tier). (See Figure 3 for graphical representation of the IEEE C95.1-1991 MPEs.)

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F

igure 3—Graphical representation of the C95.1-1991 MPEs expressed in term of the E-field equivalent plane

wave power density

In addition to a lower tier and the use of exposure environments rather that exposed populations, a number of other significant changes appear in the 1991 revision of ANSI C95.1-1982. These include the following:

1. Increased frequency range: The frequency range of the 1982 C95.1 standard is 300 kHz to 300 GHz; the frequency range of the 1991 standard is 3 kHz to 300 GHz.

2. Magnetic field limits: The magnetic field limits, which correspond to the equivalent free-space power density of the RFPGs in the 1982 standard, were relaxed in the 1991 standard at frequencies below 3 MHz in order to more realistically reflect the contribution of the magnetic field to the SAR.

3. Power density limits at quasi-optical frequencies: The MPEs in terms of incident power density were relaxed from 5 mW/cm2, the value in the 1982 standard for frequencies above 1.5 GHz, to 10 mW/cm2 for frequencies above 3 GHz (exposures in controlled environments) and for frequencies above 15 GHz (exposures in uncontrolled environments). This more realistically reflects biological effects associated with surface heating where the penetration depth is comparable to that of infrared radiation (IR). This is consistent with the corresponding MPEs at IR wavelengths found in the laser safety standards, e.g., ANSI Z136.1 and IEC 60825, for exposure to large area beams (greater than 1,000 cm2) , .

4. Averaging time: More realistic averaging times are incorporated in the 1991 standard in order to address a number of issues including exposure to short high peak power pulses. The averaging time is as follows: In the frequency region where surface heating predominates, the averaging times decrease with increasing frequency. For exposures in controlled environments, the averaging time now decreases from a value of 0.1 h at 15 GHz to 10 s at 300 GHz and decreases from 0.5 h at 15 GHz to 10 s at 300 GHz for exposures in uncontrolled environments. The shorter averaging time mitigates against conditions where skin burns could occur from short but intense exposures to small areas of the skin, which would be permitted with the longer

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averaging times found in the NCRP recommendations and the 1982 ANSI C95.1 standard. For example, an averaging time of 0.1 h at wavelengths where the penetration depth is comparable to that of the far-IR would allow a 0.5 s exposure to small areas of the skin that exceeds the 1.2 - 2.4×104 mW/cm2 skin burn threshold reported by Evans et al. . At 300 GHz, the 10 s averaging time is consistent with the corresponding averaging time at 300 GHz found in the laser safety standards and guidelines.

5. Peak power limits: Peak power limitations have been incorporated to preclude high specific absorption (SA) that could result from exposure to increasingly short, high-amplitude pulses. Specifically, for exposures to pulsed RF fields of pulse durations less than 100 ms and frequencies in the range of 100 kHz to 300 GHz, the MPE in terms of peak power density for a single pulse is limited to the MPE (under normal averaging time conditions) multiplied by the averaging time in seconds, divided by five times the pulse width in seconds, i.e.

If more than five pulses occur during the averaging time, normal time averaging will further reduce the permissible peak power. In addition, a peak E-field limit of 100 kV/m is included and takes precedence over the SA limits above. The peak power limits are based on the literature on the evoked auditory response in humans (microwave hearing) and RF energy induced unconsciousness (stun effect) in rodents. The SA limits are conservative with respect to the stun effect but the peak power density limits are above the threshold for microwave hearing, which while annoying, is not considered harmful. 6. Partial-body exposure: Most situations, particularly in the workplace, exposures are to non-uniform fields

over portions of the body and not to uniform plane-wave fields. It was therefore decided that it is appropriate to address such situations with criteria that would allow relaxation of the MPEs under partial-body exposure conditions. Specifically, the spatial peak mean squared field strengths and the equivalent power density permitted under partial-body exposure conditions are allowed to exceed the spatial average (as averaged over the projected area of the body), as a function of frequency, up to a factor of 20 times. This relaxation is based on animal studies and dosimetric studies which show that under uniform plane-wave exposure conditions, the spatial peak SAR exceeds the whole-body-averaged SAR by a factor of about 20 times. The use of the partial-body relaxation provision is limited because of the accompanying caveat “The following relaxation of power density limits is allowed for exposure of all parts of the body except the eyes and the testes.” This precludes practical implementation in many exposure scenarios. The reasoning behind inclusion of the caveat was concern that at frequencies where the penetration depth was comparable to that in the IR portion of the spectrum, the relaxation would allow exposures to the eye that would exceed the IR MPEs for the eye and skin in the laser safety standards—even with the reduced averaging time.

7. Induced and contact current limits: Induced and contact current limits are incorporated to protect against surface effects (e.g., shocks and burns) associated with electric-field induced currents which predominate at frequencies below a few MHz. For the controlled environment, the maximum contact current and the induced RF current through each foot is limited to 1000 f mA (0.003 < f ≤ 0.1 MHz) and 100 mA (0.1 MHz < f < 100 MHz) and to 450 f mA (0.003 < f ≤ 0.1 MHz) and 45 mA (0.1 MHz < f < 100 MHz) for the uncontrolled environment. The averaging time is 1 s. Guidance is also included on how measurements of foot and contact current should be performed.

8. Minimum measurement distance: In order to minimize the problem of proximity effects, i.e., erroneous measurement results associated with coupling between the sensor (antenna) elements in the instrument and the reactive fields from re-radiating structures, a minimum separation distance of 20 cm from any object is recommended.

9. Low power device exclusion: This exclusion pertains to devices that emit RF energy without control or knowledge of the user. It is generally applied to hand-held devices such as two-way radios. Specifically, at frequencies between 100 kHz and 450 MHz, the MPE may be exceeded if the radiated power is 7 W or less for the controlled environment and less than 1.4 W for the uncontrolled environment. At frequencies between 450 and 1500 MHz, the MPE may be exceeded if the radiated power is 7(450/f ) W or less for the controlled environment and less than 1.4(450 /f ) W for the uncontrolled environment. This exclusion does

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not apply to devices with the radiating structure maintained within 2.5 cm of the body, e.g., personal wireless communication devices such as mobile telephones.

10. SAR Exclusions: As in the 1982 standard, the SAR exclusion allows exposures in excess of the MPEs if it can be shown by reliable means (e.g., laboratory studies) that the whole-body-averaged and peak spatial-average SAR (basic restrictions) are not exceeded. Unlike the SAR exclusions in the 1982 C95.1 standard, which were applicable over the entire frequency range of 300 kHz to 300 GHz, and did not specify a geometric shape for the 1-g averaging volume for localized exposure, the 1991 standard specifies a realistic frequency range of 100 kHz to 6 GHz and an averaging volume in the shape of a cube to eliminate the problem of grossly overestimating the peak spatial-average SAR at frequencies where the depth of penetration is superficial, i.e., at millimeter-wave frequencies. Limiting the frequency range to that where SAR is meaningful and assigning a cubic geometry to the averaging volume more accurately represents the potential for hazard. The recommended whole-body-average SAR exclusion for exposures in controlled environments remains the same as that for the single-tier exclusion in the 1982 C95.1 standard, i.e., 0.4 W/kg (but applicable over the narrower frequency range indicated above). The corresponding value for exposures in uncontrolled environments is 0.08 W/kg. The peak spatial-average SAR is 8 W/kg and 1.6 W/kg for the uncontrolled and uncontrolled environments, respectively (but applicable over a narrower frequency range and averaged over any 1-g of tissue in the shape of a cube). The following additional SAR exclusion is included: for exposure of the extremities, i.e., the hands wrists, feet and ankles, the MPEs can be exceeded provided the peak spatial-average SAR of 20 W/kg (controlled environment) and 4 W/kg (uncontrolled environment) in any 10-g of tissues in the shape of a cube is not exceeded (and the induced current and contact current limits are not exceeded).

Compared with the RFPGs found in the 1982 C95.1 standard and the NCRP recommendations, the 1991 MPEs are far more complex and sophisticated. The complexity in application and measurement is more than offset by having scientifically defensible limits that realistically address known RF hazards by ensuring that the thresholds for adverse effects are not exceeded.

The 1991 standard was approved by the IEEE Standards Board in 1991 and published in 1992. It was also approved for use as an American National Standard by ANSI in 1992. At the time IEEE C95.1-1991 was approved, SC4 had 125 members; approximately 72% from the research community (including university, military and public health service laboratories), the rest from industry (~10%), industry (consulting ~3%), government (administration ~4%), and general public and independent consultants (~11%). At the same time C95.1-1991 was approved, IEEE C95.3-1991 was also approved . C95.3-1991 was developed by SC1 (Techniques, Procedures, and Instrumentation) and replaces ANSI C95.3-1973 and ANSI C95.5-1981 . This recommended practice describes instrumentation, measurement techniques and computational techniques that can be employed to assess compliance with the basic restrictions and MPEs of the C95.1 standards.

In 1997 C95.1-1991 was reaffirmed (without change); in 1999 Supplement 1 was approved to address certain ambiguities in the 1991 standard. A definition of spatial average and recommendations on how spatial average should be measured, i.e., by scanning (with a suitable measurement probe) a planar area equivalent to the area occupied by a standing adult human (projected area), is included in Supplement 1. Also, the averaging time for induced and contact current was increased from 1 s to 6 min for frequencies where heating predominates, i.e., above 100 kHz, and rms ceiling values of 500 and 220 mA for the controlled and uncontrolled environments, respectively, were added as were E-field limits (expressed as a percentage of the MPEs) below which induced current measurements are not required. A detailed description of how induced and contact current should be measured was also added. There were also a number of other changes including the clarification of averaging volume as it applies to average spatial-peak SAR, clarification of the term radiated power as it applies to low-power hand-held devices, clarification of the measurement distance requirements for certain direct radiators (the separation distance for measurements made in proximity to any directly radiating structure or any of its attachments was reduced to 5 cm but remained at 20 cm for indirect radiators and reflectors).

In 2004, a request from IEEE SCC347 led to the development of an amendment (C95.1b-2004) that helped clarify issues relating to the determination of the peak spatial-average SAR associated with the use of hand-held mobile transceivers intended to be operated placed against the side of the head. This amendment, which was approved in

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2004, assigns the same basic restrictions to the pinna as those applicable to the extremities, i.e., peak spatial-average SAR values of 20 and 4 W/kg, for the controlled and uncontrolled environments, respectively, averaged over any 10 g of tissue in the shape of a cubical volume surrounding an evaluation point. For this purpose, the evaluation point is defined as “either the geometric center of the electric field probe sensors at a site used for experimental SAR measurement, or the location of the incremental volume (voxel) in a numerical computation.”

International Commission on Non-Ionizing Radiation Protection (ICNIRP) Guidelines The most commonly used standards throughout the world are based on the IEEE C95 standards, the recommendations of the National Council on Radiation Protection and Measurements (NCRP), and the guidelines of the International Radiation Protection Association’s (IRPA) International Commission on Non-Ionizing Radiation Protection (ICNIRP).8 Like IEEE and NCRP, ICNIRP is an organization with established scientific committees that review the literature and make recommendations regarding exposure to RF/microwave energy. The most recent ICNIRP guidelines were approved in November 1997 and published in 1998 . At the time the guidelines were developed, the Commission included the participation of 17 scientists and 11 external experts from 12 different countries, including Sweden, Australia, Great Britain, Germany, Poland, and the US. Like IEEE and NCRP, ICNIRP carried out an extensive review and interpretation of the literature, from which exposure guidelines were developed. As in the case of the ANSI, IEEE and NCRP committees, the ICNIRP guidelines are based on studies reporting established effects. In agreement with the rationale of C95.1-1991, ICNIRP also found that the established effects that should be used for developing exposure criteria were surface effects at the lower frequencies, e.g., electrostimulation, shocks and burns, and effects associated with tissue heating at the higher frequencies. Although a number of in vitro studies were reviewed, the focus was on in-vivo studies. A number of epidemiological studies of reproductive outcome and cancer were reviewed but because of the lack of adequate exposure assessment and inconsistency of results these studies were found to be of little use for establishing science-based exposure criteria. Studies reporting athermal effects, including “window effects,” e.g., effects associated with ELF amplitude modulated (AM) RF fields, were also considered but ICNIRP concluded “Overall, the literature on athermal effects of AM electromagnetic fields is so complex, the validity of reported effects so poorly established, and the relevance of the effects to human health is so uncertain, that it is impossible to use this body of information as a basis for setting limits on human exposure to these fields” .

Like the ANSI/IEEE and NCRP committees, ICNIRP determined that SAR is the valid dosimetric parameter over the broad whole-body resonance region and also found that the most reliable and sensitive indicator of potential harm was behavioral disruption, with a threshold SAR of 4 W/kg. A safety factor of 10 was incorporated for exposure in the workplace, and an additional factor of 5 for exposure of the general public yielding maximum whole-body-average SAR values of 0.4 and 0.08 W/kg, respectively (called basic restrictions). In addition, basic restrictions in terms of peak spatial-average SAR of 10 and 2 W/kg averaged over any 10 g contiguous tissue are recommended for localized exposure. The somewhat less arbitrary ICNIRP peak spatial-average SAR limits are thought to be based on effects to the eye. Specifically, the threshold associated with the induction of lens opacities in the eyes of rabbits has been shown to be greater than 100 W/kg. The mass of the eye is about 10 g – by incorporating safety factors of 10 and 50 times, the resulting peak spatial-average values are 10 and 2 W/kg averaged over any 10 g of contiguous tissue for occupational exposure and exposure of the public, respectively.

There are also a number of differences between the ICNIRP derived limits (called reference levels) and the MPEs found in the 1991 IEEE standard but these differences are mostly related to engineering issues, e.g., models used to relate the incident fields to the basic restrictions, and differences in philosophy of determining safety factors, and not with any specific biological response or its threshold. Differences between the ICNIRP guidelines and the C95.1-1991 standard include a broader frequency range for the ICNIRP guidelines (0 to 300 kHz9 compared with 3 kHz to 300 GHz for C95.1-1991), different values for the induced and contact current limits, a slightly higher basic restriction for localized exposure, (10 and 2 W/kg for the upper and lower tiers, respectively, compared with 1.6 and 8 W/kg in C95.1-1991), a different averaging volume for the localized exposure basic restriction (“over any 10 g of contiguous tissue” compared with “over any 1 g of tissue in the shape of a cube” in C95.1-1991, a broader resonance region (10 to 400 MHz compared with 30 to 300 MHz in C95.1-1991), a broader frequency range over which SAR applies (100 kHz to 10 GHz compared with 100 kHz to 6 GHz in C95.1-1991), and lower peak-power limits. The ICNIRP peak power limits are based on the evoked auditory response (microwave hearing) whereas the C95.1-1991 limits are based on the stun-effect in small animals (with a suitable margin of safety). That is, while ICNIRP

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considers “microwave hearing” a harmful effect, it is considered a possible annoyance in the C95.1-1991 standard—but not a hazard. There are a number of other minor differences.

IEEE process Compared with other committees that develop recommendations and guidelines for exposure to RF/microwave energy, e.g., ICNIRP and NCRP, C95 committees are by far the largest, most innovative, and had the greatest influence on RF/microwave safety standards worldwide . The subcommittees are open to anyone with a direct material interest and the standards development process has always been open, formal and transparent at every level, i.e., the process is different from that of other committees, such as ICNIRP and NCRP, which tend to be closed, informal and somewhat non-transparent. While the committee operated as an ANSI committee during the development of the 1991 C95.1 standard, then as an ANSI Accredited Standards Committee, then, during the last two years a committee sponsored by the IEEE SASB, in each instance it was subject to the formal rules of the sponsoring organization to ensure due process at every level. In order to understand how the IEEE committees function, the process will be described briefly before discussing the latest revision of the C95.1 standard (IEEE C95.1-2005).

The standards coordinating committees that operate under the sponsorship of the IEEE SASB must rigidly adhere to the policies and procedures of the IEEE, IEEE SASB and the approved polices of the committees. In general, the process begins with the submittal of a Project Authorization Request (PAR) to the New Standards Committee (NesCom), a standing committee of the IEEE SASB. (See Figure 4 for a flowchart that depicts the process.) The PAR outlines the scope and purpose of the proposed standard, the reasons for developing the standard, the number of members of the working group, when the draft will be ready for sponsor ballot, potential conflicts with the scope of other standards or standards projects, plus a number of other questions that must be answered by the submitter. Once deemed complete and accurate by NesCom, a recommendation can be made to the SASB for approval. Following SASB approval, the working group (in this case SC4) can move forward with the development of drafts. In accordance with IEEE SASB and ICES procedures, the membership of SC4 consists of volunteers representing all stakeholders (membership is open to all parties with a direct material interest – IEEE membership is not required). The membership of SC4 consists of volunteers in engineering, physics, statistics, epidemiology, life sciences, medicine, and the public with a balance of representatives from government, industry, academia, and the general public. This wide-ranging participation, including thorough discussions and open decision making, is the hallmark of the process that led to C95.1-2005 .

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When a draft is finally approved by the working group, following the same process mandated for sponsor balloting described below, (except that balloting is carried out by SC4—not the IEEE SA Balloting Center), the draft is submitted to the IEEE SA Balloting Center for sponsor ballot (i.e., by the parent committee – SCC28). Sponsor balloting begins when the IEEE Balloting Center notifies members of the balloting pool that a standard is ready for sponsor ballot and invites members to join the Ballot Group for that standard. The balloting pool consists of the parent committee members (SCC28 – now Technical Committee 95 of the IEEE International Committee on Electromagnetic Safety—ICES) 10 plus all interested parties that may have joined the balloting pool. The balloting pool is open to any IEEE Standards Association (IEEE SA) member, or any non-member who elects to pay a nominal fee to vote and receive drafts. Members of the parent committee who wish to vote, but are not members of the IEEE SA, first have to be approved by the SASB. Requests from the sponsor chair to the SASB secretary outlining why these individuals should be permitted to vote, what they bring to the committee, etc., are usually placed on the consent agenda of the next quarterly SASB meeting and, unless pulled off for discussion, are approved with the agenda. During this time the standard usually undergoes a mandatory editorial review by IEEE Standards Department project editors, a review by SCC10 (Terms and Definitions – to ensure that all terms and definitions are in accord with IEEE definitions where such definitions exist), SCC14 (Quantities, Units, and Letter Symbols – to ensure consistent use of units and letter symbols), and, in some cases, a legal review.

Approval at the sponsor level requires a 75% response from the members of the Ballot Group (including abstentions) and 75% affirmative votes (the ratio of positive to positive plus negative votes) after ballot resolution. Attempts must be made to resolve every negative ballot and every substantive comment that accompanied a ballot, and their resolution (by an ad hoc ballot resolution working group) must be circulated to the Ballot Group to allow each member to confirm, change his or her vote or comment (only on issues raised during the initial ballot or previous recirculation ballot). Once a consensus is achieved the standard, ballot results, copies of the PAR, copies of the recirculation ballots, ballot resolution, and other relevant material are submitted to the SASB Review Committee (RevCom), also a standing committee of the IEEE SASB. RevCom reviews the scope of the standard to ensure that it is in accord with the scope of the PAR, that the draft has gone though legal review (when necessary), editorial review, review by SCC10, and SCC14 and that the Policies and Procedures of the sponsor and those of the IEEE SASB have been meticulously followed to ensure that the process was open, transparent, and due process was afforded at every level. When these conditions are met, RevCom can deem the ballot valid and recommend approval

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by the IEEE SASB. (RevCom deals only with procedural issues—not technical issues.) Once approved, the draft standard becomes an IEEE standard and is forwarded to ANSI for public comment and recognition as an American National Standard. Because of the potential sensitivity of the C95 standards, ICES Policies and Procedures require formal balloting at the working group level adhering strictly to IEEE SASB procedures, i.e., all members of SC4 are invited to join the ballot group, all comments submitted with each ballot are addressed, each revised draft resulting from ballot resolution and all comments and their disposition are circulated to all members of the ballot group to allow them to reaffirm or change their original vote.

IEEE C95.1-2005 C95.1-2005 is far more detailed and inclusive than its predecessor C95.1-1991. The 2005 standard is divided into two major parts—normative and informative. The normative part contains the scope and purpose, the normative references, definitions, recommendations (basic restrictions and MPEs), rules for assessing compliance and the role of an RF safety program. The informative part contains 7 Annexes. The first three explain the revision process, summaries of the literature by biological endpoint, and the rationale for the revision. Examples of practical applications of the standard to typical exposure situations are also included as is a glossary of commonly used terms, the bibliography of seminal papers from the International EMF Project (IEEE/WHO) database that are cited in establishing the basic restrictions and thresholds, and a bibliography of other cited publications.

At the time C95.1-2005 was approved, SC4 had 132 members, 42% from outside the US representing 23 countries. Of the 132 members, 36 were from academia, 56 from laboratories and administrative branches of federal agencies and the Department of Defense, 22 were from industry, 26 were independent consultants, and 2 represented the general public. Of these, 73 participated in the balloting, 57 approved, 5 disapproved with comments and 11 abstained, resulting in 92% approval. During sponsor balloting, the Ballot Group had 59 members, 58 returned ballots, 51 approved, 2 disapproved with comments, 1 disapproved without comments and 4 abstained, resulting in 96% approval. The standard has grown in length from less than one and one-half pages (C95.1-1966) to more than 250 pages—the majority of which addresses the literature reviews and evaluations and the rationale, particularly as it applies to changes.

As with the earlier C95.1 standards, the revision of the 1991 standard began with the identification of relevant papers by the SC4 Literature Surveillance Working Group. The focus was on the identification of reliable studies reporting biological responses – from reversible effects and responses of adaptation to irreversible and biologically harmful effects. At the literature evaluation cutoff date, 31 December 2003, the Literature Surveillance Working Group identified over 2200 papers from a number of databases and inputs from federal agencies and other organizations that were regularly polled. Findings of studies published between 1950 and December 2003 were considered, including a number of studies reviewed for C95.1-1991. Although the literature cutoff date was December 2003, a few papers published in 2004 and 2005 were included. New insights gained from improved experimental and numerical methods and a better understanding of the effects of acute and chronic RF electromagnetic field exposures of animals and humans were considered during the evaluation process. Every attempt was made to include and to evaluate all of the relevant literature in the database, with emphasis on studies carried out under low level exposure conditions where increases in temperature could not be measured or were not expected. SC4 agreed that only peer-reviewed papers and technical reports of original research would constitute the primary database on which any risk analysis would be based. Abstracts and presentations at scientific meetings or technical conferences were expressly excluded. A list of all 1143 papers that were evaluated during the development of C95.1-2005 can be found in Annex E of the standard.11

The literature evaluation was carried out by the Engineering, Epidemiology, In vivo, and In vitro Working Groups. In addition, a Mechanisms Working Group was established to evaluate the technical significance of particular interaction mechanisms with regard to standard-setting. The Engineering WG was tasked with assessing of the exposure systems, field characteristics and measurements, dosimetry, specific absorption rates, induced currents and fields, and temperature/humidity measurements and whether or not the information provided was sufficient to allow a full understanding of how the experiment was performed.

The Epidemiology WG was originally tasked with the evaluation of each paper for study design and population segments, quality of the methods and implementation, merit of data acquisition and analysis for specific endpoints,

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and presence or absence of positive statistical associations. Similarly, the In Vivo and In Vitro WGs examined the technological methodologies employed in each published paper, including the exposure conditions, specific organ systems and/or biological endpoints, the engineering and statistical methodologies employed, and the relevance of each study for standard-setting. The in vitro papers typically emphasized possible effects at the cellular level, including those on cell viability and proliferation, genotoxicity, cell transformation, molecular synthesis, and cell function; the in vivo papers typically examined possible effects of exposure on the whole organism or on specific organ systems, including effects on the embryo/fetus, reproductive ability, immunological system, functional alterations of the metabolic or thermoregulatory system, various histological endpoints, and behavioral changes.

Many of the evaluations went through a formal process beginning with the chair of each WG providing copies of each paper to two independent reviewers, together with specially designed and approved review forms. These forms were in a computer format that required numerical scoring by individual reviewers for entry into a computerized database. When a review was completed, the reviewer gave the paper an overall technical merit rating on a 5-point scale. The rating scale was: Very High = 5; Moderately High = 4; Acceptable = 3; Low = 2; and Very Low = 1. For ratings of 1 or 2, a request was made for justification for the low score in writing by the reviewer. Strong discordance between the two reviews of a given paper required a third independent review. Periodically, the chair of each WG would submit a summary of the completed evaluations to the Chair of the Risk Assessment WG (RAWG) whose charge was to evaluate the implied risk for human beings of exposure to RF electromagnetic fields.

After several years it became clear that the literature evaluation process would not be completed on time following the formal protocol described above. While the engineering WG evaluated nearly all of the papers in the database and the In Vivo WG evaluated more than 90% of their assigned papers, few epidemiology and in vitro papers were evaluated by members of their respective WGs because of a lack of qualified reviewers. Rather than try to evaluate every paper in the database following the protocol described above, certain individuals with considerable expertise in specific areas volunteered or were asked to prepare review papers and summarize their findings in specific topic areas. These included, for example, cancer induction or promotion, teratologic effects, ocular effects, epidemiology, thermoregulation, and animal behavior. In each topic area, one of the goals was to search for definable hazards. The texts and conclusions of the various review papers were made available to the RAWG; the summaries and conclusions from each review paper, which appear in Annex B of C95.1-2005, were further enhanced by 12 review papers published in Supplement 6, 2003 of Bioelectromagnetics . These included reviews of the epidemiology and in vitro literature. The evaluation process took advantage of all completed evaluations in the computerized database plus the review papers.

The overall results of the literature evaluation and review process were used to determine the thresholds of individual responses and dose response functions, i.e., the lowest level at which a potential harmful effect occurs and the function that relates dose rate, e.g., SAR, to response magnitude. The weight of evidence approach was used throughout to develop the thresholds and dose response functions, i.e., the same approach used to develop guidance for assessment of risk from chemical and other physical agents known to be hazardous.12 SC4 agreed that the recommendations (basic restrictions and MPEs) should protect against “established adverse health effects in human beings associated with exposure to electric, magnetic and electromagnetic fields in the frequency range of 3 kHz to 300 GHz” . The term adverse health effect is defined as “A biological effect characterized by a harmful change in health.” Notes to the definition point out that 1) adverse effects do not include biological effects without a harmful health effect, changes in subjective feelings of well-being that are a result of anxiety about RF effects or impacts of RF infrastructure that are not physically related to RF emissions, or indirect effects caused by electromagnetic interference with electronic devices, and 2) sensations (perceptions by human sense organs) per se are not considered adverse effects. Thus a sensation of warmth at millimeter and other wavelengths and the microwave auditory effect under the underlying special conditions are not recognized as effects to be protected against by this standard. Painful or aversive electrostimulation resulting from exposure at frequencies below 0.1 MHz is treated as an adverse effect” . This definition, though somewhat narrower than the WHO definition of adverse effect, i.e., “A biological effect that has a detrimental effect on mental, physical and/or general well being of exposed people either in the short-term, or long term” (cf. ), was chosen to eliminate some of the ambiguity and subjectivity associated with the broader definition.

Once the hazard threshold was identified and enough supporting information was available, a safety factor was applied to the threshold to derive the basic restrictions and MPEs based on the best available scientific information

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using the conservative approach common in standard setting. The safety factor, which is influenced by the uncertainty in the knowledge of the degree of hazard associated with the hazard threshold, is selected to prevent exceeding the hazard threshold value with a sufficiently wide margin. The magnitude of a safety factors in the 2005 standard ranges from unity at low frequencies, where effects associated with electrostimulation occur, e.g., sensation, to significantly greater values at frequencies where heating effects occur, i.e., above 100 kHz. In all cases, however, the selection of the appropriate safety factor was based on informed expert opinion after considering the underlying biological and engineering uncertainties applicable to the exposed population for a broad range of exposure conditions.

The results of the literature evaluation and review process by the SC4 working groups did not provide evidence that would warrant a change in the scientific basis for the adverse effect level for frequencies between 100 kHz and 3 GHz. The threshold value for whole-body average SAR was again found to be 4 W/kg, and again the most reliable reproducible biological endpoint was found to be behavioral disruption of food-motivated behavior in laboratory animals, including non-human primates. Although this conclusion was based on the results of animal studies, it was agreed that whole-body-averaged SARs above 4 W/kg could be potentially harmful in humans. This is the same threshold SAR and endpoint found during the development of C95.1-1982, C95.1-1991, the 1986 NCRP report and the 1998 ICNIRP guidelines. The upper boundary of the frequency range over which whole-body-average SAR is considered the appropriate basic restriction metric was reduced from 6 GHz in the 1991 standard to 3 GHz based on RF penetration depth calculations. Also, peak spatial-average SAR values were changed from 1.6 W/kg and 8 W/kg for the lower and upper tiers to 2 W/kg and 10 W/kg, respectively, and the corresponding tissue averaging mass was changed from 1g to 10 g. This change is based partially on the biologically based rationale of ICNIRP related to exposure of the eyes, the extensive theoretical biophysical research quantifying RF energy penetration in biological tissue, and the desire to harmonize with the ICNIRP guidelines where scientifically justified.

The rationale to set exposure limits for effects associated with electrostimulation at the lower frequencies and temperature-related effects at higher frequencies is explained thoroughly in the standard. Improved numerical and measurement methods in RF dosimetry have increased knowledge about the SAR-temperature relationship following RF energy deposition in human tissue, which is essential when assessing potential biological and health effects of RF exposures. A number of special considerations have been reviewed and are explained in detail in the annexes of the standard.

The 2005 standard incorporates a reasonably large margin of safety and, unlike the earlier standards, an RF safety program is required to provide part of the margin of safety for those exposed above the lower tier, now called an “action level,” rather than exposures in uncontrolled environments. The choice of the term “action level” for the lower tier, rather than limits for the “general public” or “uncontrolled environment,” stems from the fact that the committee concluded that the weight of scientific evidence supports the conclusion that there is no measurable risk associated with RF exposures below the basic restrictions of the upper tier of this standard. The lower tier, with an additional safety factor, recognizes public concerns and also supports the process of harmonization with other recommendations and guidelines, e.g., the NCRP recommendations and the ICNIRP guidelines, and defines the level above which implementation of an RF safety program is recommended. The purpose of the action level is to initiate measures, i.e., implementation of an RF safety program as defined in IEEE C95.7-2005 , to prevent exposures above the upper tier. (The basic restrictions and MPEs of the lower tier can be used for the general population.) The standard is especially conservative, since the safety factors are applied against perception phenomena (electrostimulation and behavioral disruption), which are far less serious effects than any permanent pathology or even reversible tissue damage that could occur at much higher exposure levels than those for perception phenomena .

This revision of IEEE Std C95.1 maintains many of the characteristics of the previous standard but also contains a number of differences from earlier editions that address new dosimetry findings and that simplify the use and application of the standard. These similarities and differences are described in Annex C of C95.1-2005 and are summarized below.

Similarities:

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• All relevant reported biological effects at either low (“non-thermal”) or high (“thermal”) levels were evaluated. Research on the effects of chronic exposure and speculations on the biological significance of low-level interactions have not changed the scientific basis of the adverse effect level.

• Whole-body-average and peak spatial-average SAR remain the basic restrictions over much of the RF spectrum and remain the same as in the earlier standards and guidelines, i.e., 0.4 and 0.08 W/kg.

• The MPE for exposures in controlled environments remain the same as in C95.1-1991.

• The averaging time remains 6 min for frequencies below 3 GHz for effects associated with tissue heating; but the averaging time for effects associated with electrostimulation is now 0.2 s for an rms measurement (not 1 s as in the 1999 Supplement to C95.1-1991 ).

Differences:

• Both C95.1-1991 and C95.1-2005 contain two tiers. While the weight of scientific evidence supports the conclusion that no measurable risk is associated with RF exposures below the limits of the upper tier, it is impossible to scientifically prove absolute safety and, hence, a lower tier has been set with an extra margin of safety. The lower tier recognizes public concerns, takes into account uncertainties in laboratory data and in exposure assessment, and supports the process of harmonization with other standards, e.g., the NCRP recommendations and the ICNIRP guidelines. While the basic restrictions and MPEs of the upper tier in both standards apply to exposures in controlled environments; the lower tier of the 2005 standard is now an action level, rather than specific limits for exposures in uncontrolled environments. This action level, above which an RF safety program shall be implemented to protect against exposures that exceed the upper tier, is tied to C95.7-2005 (RF safety programs) . For practical purposes, however, the lower tier may also be used for the general public.

• The upper frequency boundary over which the whole-body-averaged SAR is deemed to be the basic restriction (i.e., the “resonance” region) has been reduced from 6 GHz to 3 GHz.

• The lower tier MPEs for long-term exposure are different from those in C95.1-1991 and are in general more restrictive between 300 MHz and 300 GHz.

• The peak spatial-average SAR values have been changed from 1.6 W/kg and 8 W/kg for lower and upper tiers to 2 W/kg and 10 W/kg, respectively.

• The averaging mass for determining the peak spatial-average SAR has been changed from 1 g of tissue in the shape of a cube to 10 g of tissue in the shape of a cube.

• Although implicit in previous versions of the C95.1 standard, e.g., as an SAR exclusion, the present standard explicitly relies on “basic restrictions.”

• The C95.1-2005 requires the development and implementation of an RF safety program in controlled environments.

• A more realistic averaging time (based on thermal modeling by Riu and Foster ) for both the upper and lower tiers has been incorporated for frequencies above 3 GHz to take into account penetration depth, which decays rapidly above 5 GHz. (This resolves the need for the caveat “except for the eyes and testes” associated with the partial-body relaxation criteria found in the 1991 standard.)

• The upper frequency at which maximum induced and contact currents are specified is now 110 MHz compared with 100 MHz in the previous standard.

• The frequency at which the upward ramp begins for the relaxation of the power density limits for localized exposure has been changed from 6 GHz to 3 GHz.

In recognition of the differing impact of exposure to particular frequencies, the standard provides sections devoted to three frequency bands: 3 kHz to 5 MHz, 100 kHz to 3 GHz and 3 GHz to 300 GHz. The limits in the first band, which protects against adverse effects associated with electrostimulation, overlaps the second band where the limits also protect against effects associated with heating. The limits in the third band protect against effects associated with heating, particularly superficial heating. Differences between C95.1-1991 and C95.1-2005 within each of these bands are as follows:

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• 3 kHz to 5 MHz: The basic restrictions, based on effects associated with electrostimulation, are now provided in terms of the in situ electric fields for different regions of the body. Magnetic field MPEs are specified for the arms and legs and for the head and torso. The electric field MPE for exposure of the whole body has been increased for exposures in controlled environments; the corresponding magnetic field MPE, with separate requirements for different regions of the body, has been increased for exposures in controlled environments and for the lower tier (action level), and have been made frequency dependent. Formulas for determining maximum permitted peak electric fields for both in situ and environmental conditions are included.

• 100 kHz to 3 GHz: The peak spatial-average SAR criteria for localized exposure of any tissue excluding the hands, wrists, forearms, feet, ankles, lower legs and pinnae, have been relaxed from 8 W/kg and 1.6 W/kg to 10 W/kg and 2 W/kg for the upper and lower tiers, respectively. The corresponding averaging mass has been increased from 1 g to 10 g of tissue in the shape of a cube.13 The peak spatial-average SAR for the hands, wrists, forearms, feet, ankles, lower legs and pinnae, remains 20 W/kg for the upper tier and 4 W/kg for the lower tier. The contact current limits for the frequency range of 100 kHz to 110 MHz have been subdivided into touch and grasping conditions, with the grasping condition confined to the controlled environment. The permissible touch contact current has been reduced for both the controlled environment and the lower tier (action level). In the transition region where effects associated with electrostimulation and tissue-heating occur (100 kHz to 5 MHz), the basic restrictions and MPEs for both must be met. In order to harmonize with ICNIRP, the frequency dependence of the MPEs for frequencies between 300 MHz and 300 GHz has changed and the values made more stringent, but only for the lower tier.

• 3 GHz to 300 GHz: The upper frequency of the SAR region has been reduced from 6 GHz to 3 GHz to better reflect the quasi-optical nature of tissue interactions. As indicated above, the principal change has been in the values and frequency dependence of the MPEs above 300 MHz for the lower tier, but the MPE at 300 GHz and the corresponding averaging time remains the same as in C95.1-1991.

As in the earlier standards, the recommendations are expressed in terms of basic restrictions MPEs, i.e., reference levels, investigation levels. The basic restrictions are limits on the in situ electric field strength for the for the brain, heart, extremities and other tissues (different limits for each) for frequencies between 3 kHz and 100 kHz, whole-body-averaged SAR and peak spatial-average SAR (with relaxed limits for the extremities and the pinna) for frequencies between 100 kHz and 3 GHz, and incident power density for frequencies between 3 GHz and 300 GHz. The MPEs, which are derived from the basic restrictions, are limits on external fields and induced and contact current. In the region where effects associated with electrostimulation predominate, i.e., between 3 kHz and 100 kHz (up to 5 MHz for certain pulsed fields), the MPEs are expressed in terms of the external electric and magnetic field strengths for the head and torso with separate values for the limbs. MPEs for the undisturbed electric field strength (absent a person) are also provided for frequencies between 3 kHz and 100 kHz. In the region where whole-body heating predominates (100 kHz to 3 GHz), the MPEs are expressed in terms of the incident electric and magnetic field strengths for frequencies up to 300 MHz for the upper tier (exposures in controlled environments) and up to 400 MHz for the lower tier (action level – general public) above which the MPEs for both tiers are expressed in terms of the plane-wave equivalent power density. In the transition region of 0.1 to 5 MHz, each of the two sets of MPEs and basic restrictions apply. In this transition region the MPEs and basic restrictions based on heating will be more restrictive for long-term exposures to CW fields, while the MPEs and basic restrictions based on the effects of electrostimulation will be more restrictive for short-term exposure, e.g., short isolated pulses of low duty factor. For frequencies greater than 3 GHz, the MPEs and basic restrictions are expressed in terms of incident power density and are equivalent. Figure 5 and Figure 6 are graphical representations of the C95.1-2005 MPEs for the upper tier (exposures in controlled environments) and the lower tier (action levels), respectively. Figure 7 shows a comparison of the C95.1-2005 MPEs (in terms of the E-field equivalent power density) with the corresponding MPE of the 1998 ICNIRP guidelines. Table 5 is a comparison of features of the ICNIRP guidelines with the corresponding features of IEEE C95.1-2005.

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Figure 5—Graphical representation of the C95.1-2005 MPEs for the upper tier in the frequency region where effects associated with heating predominate (from )

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Figure 6—Graphical representation of the C95.1-2005 MPEs for the lower tier in the frequency region where effects associated with heating predominate )

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Figure 7—Comparison of the C95.1-2005 MPEs (lower tier – expressed in terms of E-field equivalent power density) with the ICNIRP MPEs for the general public. The upper tier MPEs of C95.1-2005 are the same as the C95.1-1991 MPEs.

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Table 5—Comparison of 1998 ICNIRP guidelines and C95.1-2005: Region where the predominant interaction

mechanism is tissue heating

Parameter ICNIRP IEEE C95.1-2005

Frequency range ~ 100 kHz to 300 GHz ~ 100 kHz to 300 GHz

Recognition of whole-body resonance Yes Yes

Incorporation of dosimetry(SAR) Yes Yes

Database of experimental literature Large Large (~1100 citations)

Most significant biological endpoint Behavioral disruption (associated with ~ 1°C temperature rise)

Behavioral disruption (associated with ~ 1°C temperature rise)

Whole-body-averaged SAR associated with behavioral disruption

1-4 W/kg ~ 4W/kg

Limiting whole-body-averaged SAR

– Applicable frequency range

0.4 W/kg (Occupational)

0.08 W/kg (General Public)

100 kHz to 10 GHz

0.4 W/kg (Controlled Environment)

0.08 W/kg (Action Level)

100 kHz to 3 GHz

Peak spatial-average SAR (localized exposure)

–Averaging volume

–Averaging time

10 W/kg (Occupational)

2 W/kg (General Public) 10-g of contiguous tissue

6 min (Occupational)

6 min (General Public)

10 W/kg (Controlled Environment)

2 W/kg (Action Level)

10 g of tissue in the shape of a cube

6 min (Controlled Environments)

30 min (Action Level)

Limits for extremities

–Upper tier

–Lower tier

–Applicable frequency range

20 W/kg (limbs)

4 W/kg (limbs)

100 kHz < f ≤ 10 GHz

20 W/kg (extremities including pinnae)

4 W/kg (extremities including pinnae)

100 kHz < f ≤ 3 GHz

Averaging time (f > 100 kHz)

–Upper tier

–Lower tier

6 min (f ≤ 10 GHz) decreasing to 10 s at 300 GHz

6 min (f ≤ 10 GHz) decreasing to 10 s at

6 min (f ≤ 3 GHz) then decreasing to 10 s at

300 GHz)

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300 GHz 6 min (3 kHz ≤ f ≤ 1.34 MHz). E 2 and H 2

have different averaging times for

1.34 MHz < f ≤100 MHz but both are equal to 30 min at 100 MHz. For 100 MHz < f ≤ 5 GHz the averaging time is 30 min and then decreases to 10 s at 300 GHz.

Induced and contact current limits

–Upper tier

–Lower tier

–Applicable frequency range

40 mA (limb currents)

20 mA (limb current)

100 kHz ≤ f ≤ 110 MHz

90 mA (each foot)

45 mA (each foot)

100 kHz ≤ f ≤ 110 MHz

Special criterion for modulated fields No No

Specific limits for high peak, low average power pulses

Yes—Based on evoked auditory response (“microwave hearing”)

Yes—Based on the stun-effect

RF safety program Not specifically Yes – IEEE C95.7-2005. The BRs and MPEs of the lower tier (action level) are linked to an RF safety program to mitigate against exposures that could exceed the BRs and MPEs of the upper tier.

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Other RF standards

There are a number of other standards available for assessing compliance with the C95.1 standards and the ICNIRP guidelines. These include measurement standards, such as IEEE C95.3-2002 , which describes measurement and computational techniques for assessing human exposure to electric, magnetic and electromagnetic fields, instrument types and limitations, measurement uncertainties, plus a number of other instrument and measurement issues. Although the scope of the standard covers the frequency range of 100 kHz to 300 GHz, its practical range of applicability is closer to 100 kHz to few GHz. There are also a number of product standards available for assessing compliance of specific products, such as hand-held mobile telephones intended to be operated while placed next to the head, with the basic restrictions (peak spatial-average SAR) found in C95.1-2005 and the ICNIRP guidelines. These include IEEE 1528-2003 , IEEE 1528a-2005 and IEC 62209 . These standards describe in detail the procedures for determining the peak spatial-average SAR in an anthropomorphic model of the human head (filled with liquid head-tissue simulant) by means of a robotically-controlled miniature electric field probe. Calibration techniques, measurement uncertainties, recipes for the tissue-equivalent liquids and techniques for measuring their electric properties are just a few of the issues addressed in these standards. These IEEE and IEC standards are in complete harmony. Neither of these standards specifies a specific basic restriction—they can be used for conformity assessment against any commonly used value and averaging mass. Similar standards are now under development to carry out the same assessments using numerical simulations, e.g., FDTD techniques. An IEC project team is now in the process of developing a standard specific to assessing human exposure to mobile telephone base stations.

IEEE C95.7-2005 (RF safety programs) presents guidelines and procedures that can form the basis of a an RF safety program1 for controlling hazards associated with RF sources that operate in the frequency range of 3 kHz to 300 GHz . C95.7 is a general-purpose standard with the goal of preventing potentially hazardous exposures to electromagnetic fields, currents, and/or contact voltages. The standard is modeled somewhat after the laser safety standards, e.g., ANSI Z136.1 and IEC 60825 , where areas in which exposure may be possible are characterized into one of four exposure categories according to the potential risk for exposure in excess of prescribed limits and then specifying the appropriate controls to reduce the likelihood of over-exposure. This standard is designed to complement the IEEE C95 family of standards but may find use in the development of effective programs to ensure conformance with other guidelines, standards, or regulations. Warning signs and labels, which generally are part of any safety program, can be found in IEEE C95.2-1999 .

Summary

Contemporary science-based RF/microwave safety standards and guidelines are based on in-depth evaluations and interpretations of the extant scientific literature. This is an on-going process and, as such, the recommendations in terms of safe limits of exposure evolve as the research becomes more focused and the quality of the research improves. The simple single value frequency-independent limit proposed more than five decades ago has evolved into the sophisticated rather complex standards and guidelines now used throughout the world. The biggest influence on the direction of the standards was the better understanding of dosimetry issues gained through the thermographic studies and numerical modeling that began in the 1970’s. This provided the means for establishing meaningful parameters for relating the external fields to the internal fields and provided the means for readily comparing study results. The concept of SAR, first proposed in 1981 for use as a basic restriction over a limited frequency range, led to the current whole-body-averaged SAR 4 W/kg threshold for adverse effects (behavioral disruption), which has not changed even though the literature database has grown tremendously during that time. Although the MPEs of IEEE C95.1-2005 may differ slightly from those of the 1997 ICNIRP guidelines, the differences are minor and have to do with the engineering aspects of relating the incident fields to the internal fields and the assigned margins of safety—not with philosophical differences in the interpretation of the biology. With each revision, the basic restrictions and derived limits (reference levels, MPEs) of the two most often cited standards and guidelines, i.e., the C95.1 standards and the ICNIRP guidelines, converge.

Bothe the ICNIRP guidelines and C95.1 standards are living documents. If any new adverse effect is established which would require a change in the standard, for example, the standard can be promptly revised by amendments. The IEEE committee continues the literature surveillance and evaluation of the bioeffects research for the next revision. The future replacement of peak SAR by temperature or temperature increase was discussed as a possibility

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during development of the 2005 standard; dosimetry studies are now in progress to identify the relationship between temperature rise and peak spatial average SAR for future consideration.

References

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9. USAS C95.1-1966, Safety Level of Electromagnetic Radiation With Respect to Personnel, United States of America Standards Institute, New York, NY.

10. ANSI C95.1-1974, Safety Level of Electromagnetic Radiation With Respect to Personnel, American National Standards Institute, New York, NY.

11. Durney, C. H., Johnson, C. C., Barber, P. W., Masoudi, H., Iskander, M. F., Lords, J., Ryser, D. K., Allen, S. L. and Mitchell,. J. C. Radiofrequency Dosimetry Handbook, Second Edition. Report SAM-TR-78-22, USAF School of Aerospace Medicine, Brooks Air Force Base, Texas, 1978.

12. Pressman. S., Electromagnetic Fields and Life, (Translation by Sinclair, F. L.,), Plenum Press, New York, 1970.

13. Guy, A. W., “Analyses of electromagnetic fields induced in biological tissues by thermographic studies on equivalent phantom models,” IEEE Trans. Microwave Theory and Tech., vol. MTT-19, pp. 205-214, 1975.

14. Guy, A. W., “Quantitation of induced electromagnetic field patterns in tissue, and associated biological effects,” In Biological Effects and Health Hazards of Microwave Radiation, Czerski, P. (Ed), pp. 203-216, Polish Medical Publishers, Warsaw, 1974.

15. Guy, A. W., Weber, M. D. and Sorensen, C. C., “Determination of power absorption in man exposed to high-frequency electromagnetic fields by thermographic measurements on scale models,” IEEE Trans. on Medical Electronics, vol. 23, pp. 361-371, 1976.

16. Gandhi, O. P., Hunt, D. L. and D’Andrea, J. A., “Deposition of electromagnetic energy in animals and in models of man with and without grounding and reflector effects,” Radio Science, vol. 12, no. 6S, pp. 39-48, 1977.

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17. Gandhi, O. P., Sedigh, K., Beck, G. S. And Hunt, E. L., Distribution of electromagnetic energy deposition in models of man with frequencies near resonance. In Biological Effects of Electromagnetic Waves, Johnson, C. C. and Shore, M. L., (Eds), DHEW Publications (FDA) 77-8011, vol. 2, pp. 44-67, 1976.

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23. Osepchuk, J. M. and Petersen, R. C. “Safety Standards for Exposure to Electromagnetic Fields,” IEEE Microwave Magazine, Vol. 2, No. 2, pp. 57-69, June 2001.

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28. IEC 60825-1, IEC Standard Safety of Laser Products - Part 1: Equipment Classification, Requirements and User's Guide, International Electrotechnical Commission, Geneva, (2001).

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32. IEEE C95.3-1991, IEEE Recommended Practice for the Measurement of Potentially Hazardous Electromagnetic Fields - RF and Microwave. (Replaces ANSI C95.3-1973 and ANSI C95.5-1981.)

33. ANSI C95.3-1973, American National Standard Techniques and Instrumentation for the Measurement of Potentially Hazardous Electromagnetic Radiation at Microwave Frequencies, American National Standards Institute, New York, NY.

34. ANSI C95.5-1981, American National Standard Recommended Practice for the Measurement of Hazardous Electromagnetic Fields - RF and Microwave, American National Standards Institute, New York, NY.

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36. IEEE C95.1b-2004, IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz - Amendment 2: Specific Absorption Rate (SAR) Limits for the Pinna, IEEE New York, NY.

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37. IEEE C95.1-2005, IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz, IEEE New York, NY.

38. Petersen, R.C., “Radiofrequency safety standards-setting in the United States,” in Bersani (ed.), Electricity and Magnetism in Biology and Medicine, Plenum Press, New York, 1999, pp. 761-764.

39. Bioelectromagnetics, Supplement 6, Wiley-Liss, 2003. 40. WHO, “Model legislation for electromagnetic fields protection,” Internet site http://www.who.int/peh-

emf/standards/EMF_model_legislation%5b1%5d.pdf, 2006. 41. IEEE C95.7-2005, IEEE Recommended Practice for Radiofrequency Safety Programs,. IEEE New York,

NY. 42. Riu P. J., Foster K. R., “Heating of tissue by near-field exposure to a dipole: a model analysis,” IEEE

Trans. Biomed Eng., vol. 46, pp. 911 - 917, 1999 43. IEEE C95.3-2002, Recommended Practice for Measurements and Computations of Radio Frequency

Electromagnetic Fields With Respect to Human Exposure to Such Fields, 100 kHz–300 GHz, IEEE New York, NY.

44. IEEE 1528-2003, IEEE Recommended Practice for Determining the Peak Spatial-Average Specific Absorption Rate (SAR) in the Human Head from Wireless Communications Devices: Experimental Techniques, IEEE New York, NY.

45. IEEE 1528a-2005, IEEE Recommended Practice for Determining the Peak Spatial-Average Specific Absorption Rate (SAR) in the Human Head from Wireless Communications Devices: Measurement Techniques Amendment 1: CAD File for Human Head Model (SAM Phantom), IEEE New York, NY.

46. IEC 62209-1, Human Exposure to Radio Frequency Fields from Hand-held and Body-mounted Wireless Communication Devices – Human Models, Instrumentation, and Procedures –Part 1: Procedure to Determine the Specific Absorption Rate (SAR) for Hand-held Devices used in Close Proximity to the Ear (Frequency Range of 300 MHz to 3 GHz), International Electrotechnical Commission, Geneva, (2005).

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1 The American Standards Association later became the American National Standards Institute (ANSI) that now serves as a clearing house for standards developed through an open consensus process.

2 In 1963 the American Institute of Electrical Engineers merged with the Institute of Radio Engineers to form a new professional society, the Institute of Electrical and Electronics Engineers (IEEE).

3 At the time (1974), the C95 Committee consisted of the following seven subcommittees: SC1 (Techniques, Procedures and Instrumentation); SC2/3 Terminology and Units of Measurements); SC4 (Safety Levels and/or Tolerances with Respect to Personnel); SC5 (Safety Levels and/or Tolerances with Respect to Electro-Explosive Devices); SC6 (Safety Levels and/or Tolerances with Respect to Flammable Materials); SC7 (Medical Surveillance).

4 NCRP is a non-profit corporation chartered by the U.S. Congress. The Charter of the NCRP includes as one of its objectives “To collect, analyze, develop and disseminate in the public interest information and recommendations about (a) protection against radiation (referred to herein as radiation protection) and (b) radiation measurements, quantities and units, particularly those concerned with radiation protection.” Although more focused on “ionizing radiation,” e.g., X-rays, gamma-rays, nuclear radiation, NCRP has developed several reports that address radiofrequency issues.

5 During the period the revision of ANSI C95.1-1982 was developed (1982-1990), ANSI ceased sponsoring standards committees and instead became a clearing house for standards developed by committees accredited by ANSI. Although the C95 committee became an ANSI Accredited Standards Committee (ANSI ASC C95), there was consensus of the membership that it would be beneficial to explore the possibility of operating under the sponsorship of the Institute of Electrical and Electrical Engineers (IEEE), which is also an ANSI accredited standards developer. After several meetings with IEEE staff, in 1989 the C95 committee began operating as a Standards Coordinating Committee (SCC28 – now the International Committee on Electromagnetic Safety – ICES) under the sponsorship and subject to the rigid rules, procedures, and oversight of the IEEE Standards Board (now the IEEE Standards Association Standards Board – SASB).

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6 IEEE is a non-profit technical professional society with more than 365,000 members in 150 countries. Within IEEE are 39 societies, including the Consumer Electronics Society, Education Society, Electromagnetic Compatibility Society, Engineering in Medicine and Biology Society, Information Theory Society, Neural Networks Society, Society on Social Implications of Technology. While many IEEE societies sponsor standards committees, when the scope of a proposed standard overlaps the scope of several societies, “Standards Coordinating Committees” (SCC) are established to develop such standards. IEEE membership is not a requirement for participation on an IEEE SCC or on any of its subcommittees.

7 IEEE SCC34 was a standards coordinating committee established in 1995 to develop product standards relative to the safe use of electromagnetic energy. At the time, SCC34 SC2 was developing a standard (IEEE 1528) for measuring the peak spatial-average SAR from RF-emitting devices intended to be operated while placed next to the head (mobile telephones).

8 As indicated on their website “ICNIRP’s beginnings go back to 1973 when, during the 3rd International Congress of the International Radiation Protection Association (IRPA), for the first time, a session on non-ionizing radiation protection was organized.” This was followed in 1974 by the formation of a working group on non-ionizing radiation, in 1975 by a study group to review the field of non-ionizing radiation, in 1977 by the creation of the International Non-Ionizing Radiation Committee (INIRC), and in 1992 ICNIRP was chartered as an independent non-profit scientific body. ICNIRP is also a formally recognized non-governmental organization in non-ionizing radiation for the World Health Organization and the International Labour Office. The work of ICNIRP is carried out by the main Commission, with support of consulting members and four Standing Committees; Epidemiology, Biology, Physics and Engineering and Optical Radiation.

9 Basic restrictions are provided over the frequency range extending from “up to 1 Hz” to 300 GHz.

10 In order to provide a better description of the international aspects of its activities, the name “IEEE International Committee on Electromagnetic Safety” (ICES) was approved for use by the IEEE SASB in 2000. Then, in March 2005, the IEEE SASB approved a new committee, still called ICES, which includes two technical committees: TC34 (formerly SCC34) and TC95 (formerly SCC28). This new committee operates as a standards coordinating committee (SCC39) under the sponsorship of the IEEE SASB. Currently the membership of the two technical committees (not including subcommittee members) stands at approximately 150 professionals, with a balance of disciplines and a balanced representation from the medical, scientific, engineering, industrial, government, and military communities, representing 26 countries. ICES is now international and influence of the C95 standards is now global in scope. Through the World Health Organization’s standards harmonization effort, ICES is working closely with other expert groups toward the development a single science-based global standard.

11 The complete list of papers in the IEEE/WHO database is available online at Internet site http://www10.who.int/peh-emf/emfstudies/IEEEdatabase.cfm

12 For purposes of this standard, the weight of scientific evidence is defined as “the outcome of assessing the published information about the biological and health effects from exposure to RF energy. This process includes evaluation of the quality of test methods, the size and power of the study designs, the consistency of results across studies, and the biological plausibility of dose-response relationships and statistical associations.”

13 The rationale for changing the peak spatial-average SAR and averaging volume was in part the desire to harmonize with the ICNIRP guidelines where scientifically justified and in part based on recent theoretical biophysical research and thermophysiological data showing the inability of RF energy to cause significant local temperature increases in small tissue volumes for inducing adverse health effects .

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The BioElectric Shield Company has been dedicated to helping create a more balanced and peaceful world one person at a time since 1990.

In the 1980’s, when Dr. Charles Brown, DABCN, (Diplomate American College of Chiropractic Neurologists), the inventor of the Shield, became aware that a certain group of his patients exhibited consistent symptoms of stress and a slower rate of healing that the rest of his patient population. This group of patients all worked long hours in front of CRT computer screens for many hours a day, and usually 6 days a week. He began researching the effects of electromagnetic radiation in the literature, and found there were many associated health effects. He wanted to help these patients, and hoped that he could come up with a low-tech, high effect product.

In 1989, he had a series of waking dream that showed him a specific pattern of crystals. Each of 3 dreams clarified the placement of the crystals. He showed the patterns to an individual who can see energy and she confirmed that the pattern produced several positive effects. She explained that the Shield interacts with a person’s energy field (aura) to strengthen and balance it. Effectively it created a cocoon of energy that deflects away energies that are not compatible. In addition, the Shield acts to

balance the physical, mental, emotional and spiritual bodies of the aura. A series of studies was conducted to investigate the possible protection from EMF's wearing this kind of device. Happily the studies were consistent in showing that people remained strong when exposed to these frequencies. Without the shield, most people showed measurable weakening in the presence of both EMF’s and stress. Of interest to us was that these same effects were noted when people IMAGINED stress in their lives. It seems obvious that how we think and what we are exposed to physically both have an energy impact on us. The Shield addresses energy issues-stabilizing a person's energy in adverse conditions. See “How the Shield Works” for more information.

Since that time, we have sold tens of thousands of Shields and had feedback from more people than we could possibly list. Here are just a few of the testimonials we have gotten back from Shield wearers.

Dr. David Getoff was one of the earliest practitioners to begin wearing a Shield and doing his own testing with patients with very good results (video).

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OUR MISSION

Our mission is to make the BioElectric Shield available worldwide. In doing so, we feel we are part of the solution to the health crisis that is, in part, caused by exposure to electromagnetic radiation and well as exposure to massive amounts of stress, from situations and other people’s energy.

We also want to bring more peace, balance and joy to the world - and the Shield offers a vibration of peace, love, and balance in a world filled with fear and uncertainty. Selling a Shield may seem like a small thing in the scheme of things, but each Shield helps one more person find a greater sense of ease, balance and protection, allowing them to focus on living their dreams

To enhance your sense of well-being, (In addition to the Shield, ) we offer other products that provide health and wellness benefits on many levels.

By working together we can, and are, accomplishing miracles.

Charles W. Brown, D.C., D.A.B.C.N.

Dr. Brown graduated in 1979 w ith honors from Palmer College of Chiropractic. He is a Di plomate of the National Board of Chiropractic Examiners and a Di plomate of the A merican B oard o f Chiropractic Neu rologists. He al so i s cer tified i n Ap plied Kinesiology. Dr. Brown has had his own radio show "Health Tips". Additionally, he has taught anatomy at Boston University and the New E ngland Institute of Massage Therapy.

He invented the BioElectric Shield, Conditioning Yourself for Peak Performance (a DVD of series of Peak Performance Postures with Declarations) and Dr. Brown’s Dust and Allergy Air Filters, as well as Dr. Brown’s Dust and Allergy Anti-Microbial, Anti-Viral Spray. He is presently working on other inventions.

Dr. Brown’s experience of the Shield is that it has helped him move deeper into spiritual realms, quantum energy, and creative meditative spaces. It has always been his desire to help others, and he is grateful that the Shield is helping so many people worldwide.

Virginia Bonta Brown, M.S., O.T.R.

As child, I always wanted others feel better. As a teenager, I volunteered as a Candy Striper at the local hospital, wheeling around a cart of gifts to patients’ rooms. The hospital setting didn’t really draw me, so summers were spend teaching tennis to kids at a wonderful camp in Vermont. With the idea of becoming a psychologist, I received a B.S. degree from Hollins College in

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psychology and worked with drug addicts for a year. Called by the practicality of Occupational Therapy, I received an M.S. degree in Occupational Therapy from Boston University in 1974

For the next 16 years, working with ADD, ADHD, autistic and other special needs children was my passion. Because of my specialty in Sensory Integration Dysfunction (a technique based on neurology), I met Anne Shumway Cook, RPT, PhD, a brilliant PT, with a PhD in neurophysiology. We created special therapy techniques for children with vestibular (balance and position in space) dysfunction while she worked with the Vestibular Treatment Center at Good Samaritan, and while I managed the therapy services of the Children's Program at this same hospital in Portland, Oregon. A fun project at that time also included collaborating with a team of other therapists to create a therapy in the public schools manual for OT, PT and Adapted PT procedures. It included goals and treatment plans which has served as a model for nearly every school district in the United States. There was nothing quite so satisfying as seeing a child move from frustration to joy as they began to master their coordination and perceptual skills.

For the next seven years, I shifted my focus. Married to Dr. Charles Brown, we decided that I’d begin to work with him in his Pain and Allergy Clinic, first in Boston and then in Billings, Montana. During this time I began to hear people talk about how thoroughly stressed out they were by their job environment. Their neck and shoulders hurt from sitting in front of computer screens. They were fatigued and overloaded dealing with deadlines and other stressed out people! They wanted to be sheltered from the “storm” of life. Though conversation, myofascial deep tissue and cranio-sacral therapy helped them, the stress never disappeared. It was our patients who really let us know that something that managed their environment and their energy would be a wonderful miracle in their lives.

What could we do to help them? I became an OT so I could help children and adults accomplish whatever it was that they wanted to do. When my husband, Dr. Brown, invented the Shield, initially I felt I was abandoning my patients. Running the company meant I didn’t spend as much time in the clinic. But then I saw what the Shield was accomplishing with people. They got Shields and their lives began to improve. People told me they felt less overwhelmed, didn’t get the headaches in front of the computer, were less affected by other people’s energy and enjoyed life more. I began noticing the same thing!

In 2000, we received a request for a customized shield for a child with ADD/ADHD. After it was designed, our consultant told us that she could create a special shield that would help any person with these symptoms. Read more about the ADD/ADHD Shield.

When we started the company in 1990, I was still seeing patients nearly full time. I was wearing the Shield and began to notice something different about my own life. At the clinic, I noticed my energy was very steady all day. Instead of being exhausted at the end of the day, particularly when I had treated particularly needy patients, I was pleasantly tired and content. I noticed I was more detached from the patient’s problem. In other words, I didn’t allow it to tire me. Instead I became more compassionate and intuitive about what they needed to help them. I was able to hear my Guides more clearly as they helped me help them. As I wore it during meditation, I felt myself go deeper into a space of Unity of all things, from people to mountains to stars.

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Over the years, I’ve spoken with many, many people, from all walks of life. Because they consistently tell me how much it’s helped them, I become more committed each year to offer this to as many people as possible. It is my belief that the Shield is a gift from the Divine, and that those who wear it will be helped on earth to accomplish their own mission, with greater health and greater compassion. For this reason, it is my desire to provide the blessing of the BioElectric Shield to as many people as possible.

Carolyn (Workinger) Nau:

I joined the BioElectric Shield Company in January 1994 when the shipping and order department consisted of one computer and a card table. With my help, the company grew to what it is today. From 1994 to 2000 I traveled and did approximately 100 trade shows, talking to people, muscle testing and really finding out how much difference the Shield makes in people’s lives.

An empath and natural intuitive, I have personally found the Shield to be one of my most important and valued possessions, as it assists me in not taking on everyone else’s stuff. That ability has also been invaluable when I talk to and connect with clients in person, over the phone or even via email. I am frequently able to “tune in” and help advise on the best Shield choice for an individual.

I felt a strong pull to move to California and reluctantly left the company in 2000. While in California I met the love of my life, David Nau. After being married on the pier in Capitola, we relocated to Milwaukee, Wisconsin where he’d accepted a job as design director of an award winning exhibit firm. David is an artist and designer, and has taken all the newest photos of the Shields. They are the most beautiful and accurate images we have ever had! Through the magic of the internet I was able to return to working with the company in January 2008. I love how things have changed to allow me to live where I want and work from home. I am fully involved and even more excited about the Shield’s benefits and the need for people to be strengthened and protected. I am thrilled to be back and loving connecting with old and new customers. It’s great to pick up the phone and have someone say, “Wow, I remember you. You sold me a Shield in Vegas in 1999”

How did I get started making Energy Necklaces? It's not every day that going to a trade show can totally change your life. It did mine. I must have been ready for a drastic change. I just didn't know it. I guess I’ve just always been a natural Quester.

Quite by chance, I went to the Bead and Button Show in Milwaukee. The show is an entire convention center filled with beads, baubles and semi-precious stones. I looked over my purchases at the end of the first day and realized I didn't have enough of some for earrings. So I went back with a friend who normally is the voice of reason. I thought if I got carried away she’d help me stop. Joke was on me.

I was unable to resist all those incredible goodies. My friend turned out to be a very bad influence, she’d find fabulous things and hold semi-precious and even precious stones in front of me saying "Have you seen this?". How can a woman resist all that beauty? I can’t! I couldn't. I walked out with a suitcase full of beads and stones. The only problem was, I didn’t even know how to make jewelry.

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I spent the summer taking classes, reading books, practicing jewelry making. Immediately people were stopping me in the street asking about the jewelry I was wearing. It finally dawned on me that just maybe I was meant to design and share my creations. Thus Bold Bodacious Jewelry was born.

I still laugh about this whole process. Obviously the Universe or someone was guiding me. Looking back it should have been obvious that I was buying enough to start a business. But at the time, it just felt like the right thing to do. Not a conscious plan. Sometimes following your gut can change your life.

In the fall of 2008, I felt a pull to examine how various gemstones could enhance the protective and healing effects of the BioElectric Shield. I also wanted to wear great jewelry and my gold and diamond Shied at the same time, so I created something new so I could do that. After making a few “Shield energy necklaces”, I was convinced that not only was my jewelry beautiful and fun to wear, it had additional healing qualities as well. Since then I’ve been immersed in studying stones and their properties, paying particular attention to the magical transformation that happens when stones are combined. Much like the Shield, the combined properties of the stones in my jewelry are more powerful than the same combination of stones loose in your hand. To view gem properties and styles to complement your shield, please visit Shield Energy necklaces .

David Nau:

We’re pleased to have added David to our team. David is an award winning creative designer who readily calls on the wide variety of experience he has gained in a design career spanning over thirty years. His familiarity with the business allows him to create a stunning design, but also one that works for the needs of the client. The design has impact, and functions as needed for a successful event. Having

owned his own business, David maintains awareness of cost as he designs, assuring the most value achieved within a budget.

A Graduate of Pratt Institute, Brooklyn, NY, David’s career has included positions as Senior Exhibit Designer, Owner of an exhibit design company, Design Director, and Salesman. This variety of positions has provided experience in all phases of the exhibit business; designing, quoting, selling, directly working with clients, interfacing with builders and manufacturers, staging and supervising set-up.

David has worked closely with many key clients in the branding of their products and themselves in all phases of marketing, both within and outside the tradeshow realm. He has designed tradeshow exhibits, museum environments and showrooms for many large accounts including Kodak, Commerce One, Candela Laser, The Holmes Group, Kendell Hospital Products, Enterasys, Stratus, Pfizer, Ligand Medical, Polaroid, Welch Allyn, and Nortel. He has also designed museum and visitor centers for Charlottesville, NASA Goddard, Hartford and Boston children’s museums.

David’s artistic eye has added to other aspects of our BioElectric Shield site and we appreciate his ongoing contributions. David is currently unemployed and so has started going to trade shows with Carolyn. For someone who has been designing trade shows for 35 years actually being in the booth he designed is a whole new experience for him.

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Sam Sokol

Sam is our Internet consultant, bringing expertise and wisdom to this area of communication for our company. Sam works with a wide variety of companies in many industries to build, market and maintain their online presence. He has helped both small and big companies to increase their online sales and build their businesses. He has helped us to grow BioElectric Shield by giving us direct access to great tools to make changes to our web site.

Dedicated to helping create a more balanced and peaceful world one person at a time Let's change our lives and our worlds one thought, one action at a time.