Power-f req uency and people

fields

by J. Swanson and D. C. Renew For as long as human beings have been wing electrin’ty, there have been worries about its safety.

The last decade has seengrowing public concern about possible health ejects ofelectric and magneticjelds produced by electricity generation, transmission and use. This concern is

inneasingly afecting electricity utilities; engineers cannot afford to ignore it. However, muck of the concern is based on misunderstandings. This paper lookrjrst at the various sources ofpower-

fiequencyfields, then suweys the various ejects suchfields may have.

What are fields?

t ra&o or microwave fiequencies, people are used to tallang about radation, where the electric and magnetic fields are coupled A together. However, at the fiequency of the

electric power system in the UK, SO Hz, the wave- length is 6000 km. The &stance to the source of the fields is always very much less than one wavelength, meaning that rahation is neghgible. It is necessary to consider the separate electric and magnetic fields.

This paper is concerned wirh SO Hz electric and magnetic fields, and concentrates on magnetic fields, as these are where most concern has focused. Electric fields (measured in Vm-’) arise whenever there is a voltage, regardless of whether there is any current. Magnetic fields (measured in tesla, T, or A niP) arise wherever there is a current, regardless of the voltage. These are elementary points, but are often confused. Electric fields are easily perturbed and screened (by sheaths of cables, by the ground and vegetation, by buildmgs and even by people). Magnetic fields, in contrast, are relatively unaffected by buildings or by people, being significantly perturbed only by large amounts of metal, particularly ferrous metal.

For a single conductor with a distant rehirn path, the electric field is radial and the magnetic field is circumferential, and their magnitude falls as the reciprocal of bstancc. With multiphase arrays of conductors as found in practice, there is partial cancellation ofthe fields from the various conductors. The resultant field is &polar or quadrupolar, and falls

more rapidly with &stance. Examples of magnetic field patterns are shown in Fig. 1.

Sources of field

Xansmission lines In England and Wales there are 7000 kni of overhead

transmission lines at 275 kV and 400 kV Virtually all of these lines carry two three-phase circuits, one each side of the towers. They produce a magnetic field which peaks underneath the conductors and falls rapidly with distance either side, as illustrated in Fig. 2a.

The actual field produced depends on several factors’ :

0 It depends on the currents. The largest lines in use have a rating of over 4 kA per circuit, but the average current in a typical circuit is more like 700 A.

0 It depends on the clearance of the line. The minimum ground clearance of a 400 kV line is 7.6 m, but it is rare for lines to be this low, and the ground-level field falls rapidly with the height of the line above ground.

0 The field also depends on the relative phasing of the two circuits (Figs. IC and i d ) . A few lines have ‘untransposed’ phasing, with the phases in the same order from top to bottom on the two sides of the towers. This produces a field which falls off as the inverse square of distance from the line. However most lines have ‘transposed’ phasing, with the opposite order ofthe phases on one side to the other. This introduces an extra degree of symmetry and

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Fig. 1 (b) a three-phase array (typical of low-voltage distribution) produces a dipolar pattern; (c) a double-circuit transmission circuit with the same order of phases on the two sides also produces a dipolar pattern; ( d ) the same transmission line but with transposed phasing produces a quadrupolar pattern, distorted because the three phases are not quite in a vertical line

Examples of magnetic-field-line patterns: (a) a single, isolated current produces a monopolar pattern;

extra cancellation between the fields from equal currents on the two sides; the resultant ticld falls niore nearly as the inverse cube of distance, producing a much lower field at large distarices &om the line. This is illustrated in Fig. 2u.

When all these factors are taken irito account, the steady-state maximum ground-level field beneath a transmission line is 100 pl; but in practice fields are oken below 10 pT. Sindar considerations apply to electric fields (Fig. 2). The maximum ground-level electric field beneath a 400 kV line i h 11.2 k V d .

4'% of the high-voltage network in England and Wales is underground, rilainly in urban areas or areas of great scenic beauty. With undergroiind cables the uidividud conductors, being insulated, can be closei- together, leading to greatrr cancellation and lower fields. However, unless they are buried very deeply. they caii also be approached niore closely, leading to higher fields. Overall, ground-lrvel riiagnctic fields from cablo fall much more rapidly with distance than those fimi a correspondnig overhead line, but can actually be higher at small distances from the cahlr. This is illustrated in Fig. 3 which compares the magnetic fields produced by one particular rablc dcsigi used in

London and an overhead line carrying the same currents.

t.ower-voltagc lines generally produce lower fields, both because the currents carried are lower and because thc conductors are closer together.

Although high-voltage trarisrrllssiori lines arc v~sually quite prominent and have accordingly attracted most attention from the public, only 0.1%) of homes are within 30 111 of one. Moct people receive thr bulk of their exposure to power-frequency fields from other courcrs

415 I'dixtditition Fig. 1 shows the distribution of niagirtic field over

the ground floor ofa typical house. The peaks from the various appliances arc supcrin~poscd on ;I low back- ground, which varies only slightly acrocs the house. I n the vast majority of homes, this background field conies fi-om the 1 1 i V dictribution systrm.

A minority of homes have 415 V distribution with ceparatrd-phase overhead wiring. With separated phases, magnetic field5 caii arise from the load currentc on the conciuctors. just as with transmission lines.

However, nioct liomrs have irridrrground distribu- tion, where the individual conductors are very much

10 ~

. clearance Ern - clearance am - ~

20 251

-100 -50 0 50 100 -100 -50 0 so 100

horizontal distance from centre of line m horizontal distance from centre of line rn a b

Fig. 2 (a) magnetic (30% maximum current: 1035 A); (b) electric (operating voltage: 400 kV)

Fields produced by double-circuit transmission line 1 m above ground level at typical operating conditions:

closer together within a single sheath. Fig. 5, represents a simple circuit of this type, where the load current drawn hy a house passes out along the phase conductor and back along the neutral conductor. The currents are rxactly balanced, and because the conducton are extremely close together, thc nuhmetic fields cancel, and there is negligible external field.

In practice the situation is more coniplicated. Fig. 5b represents the typical arrarigerricrit 111 urban areas, where circuits from adjacent substations meet at a link hox. Their tic~itrals are interconnected but their phase conductors are generally not. The load current for a house on one circuit has to pass along the phase conductor of the sarric circuit, but can return along the neutral conductor ofeither circuit. Thr currents are no longer balanced. and both cables have a ‘net current’. The magnetic field frotii net current\, varying as the inverse first power of distance, fornir the hackground field in the majority of homes.

Fig. 5c shows, in addition to intrrcoii~icctcd ricutrals. protective multiple earthing (PME). PME has become increasingly conmion on 41 5 V distribution circuits since it was first introduced with pilot 1930s, and is now applied to about 85 circuits, 65% of undrrground circuits arid 30’% of supplies to individual consumers in England and Wales. PME involves carthing the neutral conductor.at points along its length and bonding i t to othrr service\. Evcn where homes do not officially have PME, up to 20‘% may have accidental nrutral-to-cartti corinectioix.

With PME, wnie fraction of the neutral current in a circuit can divcrt out of the neutral conductor a n d return to the substation through watrr pipes, gcih pipes, sewers, or the ground itself. This producer net cui-rents not only i n the distribution circuits but also in any conducting utilities, all of which conti-ibutr to the bnckground riiagnctic field in homes.

Virtually evrrv distribution ~ i r c u i t in thc couritry ha\

a net current, hut it5 magnitude depends on the inipedances of individual Ph4E links and inter- connections between circuits, making it difficult to predict. Recent studes undertaken by the National Grid Conipmy found the average net current in a sample of underground 41 5 V distribution circuitr in urban areas to be 3.6 A, which on average was 15% of the neutral current. Thr geometric-rncan background field in a sample of homes throughout the country (causcd predominantly by net currents) was 36 nT. Background fields typically vary behvern honirs from below 10 nT to above 100 nT (even in the absence of higher-voltage lines). In any given home they also vary -

120,

100 1 n cable

60 I

C l

20 overhead line

-30 -20 -10 0 10 20 30 distance from centre line. rn

Flg. 3 Magnetic fields produced by a 400 kV cable and overhead line at 1 m above ground level. The current is 2038 A, the rating of this particular cable

Fig. 4 The magnetic field over the ground floor of a typical house, measured on a 1 m grid at 1 m above floor level

with time, broadly following the daily and annual variations ofload on the relevant circuit (Fig. 6).

Appliances Main-powered appliances produce magnetic fields

whenever they draw current. Such fields generally fall 3s the inverse cube of distance, and thus are significant only within a metre or two of-the appliance. Although the peak field can be h i g h i c r t a i n electric razors produce over 1 mT at their surfacepeople do not ucually spend much time close to appliances, so the appliance contribution to time-average exposure is limited. In typical homes, people probably receive ahout a third of thcir time-averaged exposure from appliances and the remainder from net ctirrerits in the distribution system.

Fig. 4 shows that the highest fields come h m some of the smallest appliances, such as the transforrner for the front-door bcU. This is because motors or transformers designed for lightweight appliances often havc a minimum of iron in their magnetic circuit, allowing more of the magnctic field to escape.

Soirrces ( f j e l d at work Many workers receive more exposure a t work than

at home, despite the lower time spent there. Fields i n workplaces tend to be higher than in homes, partly because ofthe greater concentration ofappliaricrs. 'I'hc mean Geld experienced at work by a recent sample of office workers was 180 nT.

Certain industries have particular items of equipment which involve high currents and produce

high fields. I n the Electricity Supply Industry (ESI), examples are generator busbars in power stations and some reactive-conipeIisatiori plant 111 substations. In other industries, certain welding, heating and electrolytic processes can produce high fields. In general these high fields affect only specific workers and riot the general public. An increasing number of industries (including the ESI) h u t the exposure of their workers according to the guidance of the National I<adiological Protection Board (NRPB), discussed below.

Fig. 7 summarises these various sources of exposure, showing clearly the wide range of fields produced. When the time-weighted average exposure of the population as a whole is considered, transmission, distribution. domestic appliances, arid work-rclatcd sources all make some contribution.

Effects of fields

Having surveyed the sot~rces ofpower-frcquericy fields, this paper now considers the effects that fields have, first on cipipnicnt and then on people.

kfecrcts on equknient There are scvcrd types of equipment that can be

affected by fields. However, the fieldi required are usually rather higher than those conlnionly encountered (see Fig. 7).

Credit cardi, railway tickets etc. have information encoded on a magnetic strip. This can be corrupted

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by magnetic fields above about 10 mT. Such field$ almost never occur at 50 Hz, but a problem can arise with static fields such as those from magnetic catchei 011 handbags.

0 Some cars with electronic control systems have been found to be susceptible to interference from power- frequency magnetic fields above about 2 mT. Again, such fields are rare at 50 Hz. This tends to be mom of a problem at higher frequencies.

0 Quartz watches with analogue dials use a small stepper motor to drive the hands. This stepper motor can be driven by a suitably oriented external power- frequency magnetic field of about 1 inT or greater, causing the hands to rotate 100 or more times faster than normal. The effect is spectacular and has led to one of the authors leaving work embarrassingly early, but has not been found to cause any damage to the watch. Interference has been reported in certain models of implanted cardiac pacemaker with electric fields’ above about 2 kV m-’ and with magnetic fields’ as low as 20 FT at 50 Hz. This affects only certain

models of pacemaker and is gcricrally not regarded as a health hazard. There is no known instance of anyone suffering injury as a result, but nonetheless it would be wise to avoid the possibility of this effect occurring.

0 I n cathode-ray tubes, the electron beam can be deflected by an external magnetic field. ‘This effcct, colloquially known as ‘VDU wobble’, results in the picture on the screen wobbling at the beat frequency of the field (50 Hz) and the screen refresh rate (usually 60 Hz). It g e n e d y becomes noticeable at about 1 pT and a serious problem at about 10 pT. Limited amelioration can be effected by careful orientation of the VDU and by screening. Screening magnetic fields is, however, difficult; even using high-permeability alloys, worthwhile screening factors still require large amounts of the screening material.

Ejects dfields on people The energy of a quantum of 50 Hz electromagnetic

radation, given by Planck’t conctant tiiiie~ the

frequency. is IxlO-'.' eV As the energy required to c'iiise ionisation by breakmg a chemical bond is typically 1 cV, it is clear that power-frequency radiation does not cause ionisation. Instead, the main known \ v ~ y 50 Hz fields interact with peoplc is by inducing currents.

Any alternating magnetic Geld will induce an electric field, which in turn produces a current in a conducting mediuni. The human body is conducting and will therefore have a current induced in it. In power-frequency calculations, it is coliinion to assunie the human body has a radius of 0.2 ni and a conductivity of0.2 S n P . Uting this niodel, a magnetic field of 0.16 mT induces a peripheral current dentity of-1 nA nr'. More accurate nunicrical calculations can be done \vhich take account of the actual shape of the body and the varying conductivities of dfferent tissuec.

Alternating electric fields also induce currents in the body. The calculation has to take account of the

"1 week

''1 year

Aug SepOdl Nov Re% Jsn FebMar Apr Mey Jun Jul

Flg. 6 Variation of background magnetic field in a given home with time: (a) 5-minute average field over 24 hours; (6) 30-minute-average field over one week; (c) 24-hour- average field over one year

perturbation to the field caused by the body itself. For a typical person standing in a vertical field, a current of 1 mA through the body is induced by 70 kV m-'.

Within the body, currents induced by fields have the sanie range of effects as currents injected via electrodes, e.g. in an electric shock. Thus current densities of about 0.1 Am-' can stimulate excitable tissue and current densities above about 1 A m ' can interfere with the action of the heart by causing ventricular fibrillation, as well as producing heating. However these current densities correspond to fields far larger than are cver encountered at 50 Hz.

At lower fields a range of possiblc effects have been reported'. The effect observed in humans at the lowest magnetic field is the magnetophosphene effect, where a flickering sensation is produced in peripheral vision by 50 Hz niapetic fields above about 10 mT. Magnetophosphenes are probably caused by induced current densities in the retina; the threshold at 20 Hz (the most sensitive frequency) is about 20 mA m-'.

Talung account of all these effects, the National Radiological Protection Board (NRPB) recommends' that people should not be exposed to current densities in the head, neck and trunk of greater than 10 tnA m ' (the 'basic restriction'). To ensure that this basic restriction is not exceeded, it suggests that the fields pcople are exposed to at 50 Hz should not exceed 1.6 niT or 12 kV 111.' (the 'investigation levels').

As well as inducing currents, electric fieldc can produce nucroschocks. These most commonly occur when a person touches a large conducting object, such as a lorry or fence, which is insulated from earth. A charge is induced on such an object in an electric field, and if the person is reasonably well earthed through their shoes, they provide a discharage path. This feels sirmlar to the familiar static-electricity shock.

Microshocks can be avoided by suitable earthing. Most tyres are in fact sufficiently conductive to prevent microshocks from vehicles, and recommendations are made to farmers on earthng fences underneath transmission lines.

Possible +cts at lowerfields The NRPB guidance is designed to protect against

the known effects of induced currents. Other bodies, such as the International Radiation Protection Association, have issued guidance on a similar basis. As can be seen from Fig. 7, the investigation level for magnetic fields (1.6 niT) ic above any power-frequency Gelds the general public are likely to encounter. The increating concerns in this and other countries are about the possibility that much lower magnetic fields may be affecting health.

These concerns are based on a number of cpidcmiological studles which have linked certain cancer?, particularly childhoold leukaemia, with exposure to power-frequency fields". The first such ctudy was conducted in Denver, Colorado and published in 1979. Since then there have been several rnorc studies in America, the UK, Scandinavian

Fig. 7 Typical power-frequency magnetic fields produced by various sources. Parallel bars indicate common fields, tapers indicate rarer fields

countries, and elscwhere. The studies have generally focused on exposure to time-weighted-average magnetic fields, though other possibilities include exposure to electric fields, peak magnetic fields, ‘resonant’ combinations of fields, and transients, and it is not clear which of these exposures is the most relevant. The magnetic field iniplicated is of the ordcr of 0.2 pT, four orders of magnitude lower than the NRPD investigation level and well within the range commonly encountered in the environment.

These epidemiological studies are usually ‘case- control’ studies. In these, two groups of people are identified the cases, who have the disease being studied, and the controls, who are selected from the same population as the cases, sirrdar to them in every respect except that the controls do not have the dsease. The exposures of these two groups to the factor being studied (in this instance magnetic fields) are measured, and each of the cases and the controls classified as ‘exposed’ or ‘non-exposed’. The result of the study is expressed as an odds ratio OR:

= exposed cases non-cxposrd rotztrols non-exposed cases exposed coritrols

If the odds ratio is 1, no difference has been found between the exposure of people with the disease and people without the dsease. If the odds ratio is greater than 1, however, the cases were more likely to have been exposed than the controls, and thcrc is an

association benveen the disease and exposure. The odds ratio deduced from a case-control study is

an estimate o f the relative risk in the population. It has confidence limits associated with it, conventionally the 95% confidence linits: it is 95% ccrtain that the relative risk in the population lies within the confidence limitt ofthe study. A simplistic but common way of assessing the significance of an epidemiological study is to ask whether the confidence limits include an odds ratio of 1. If they do include 1, it is riot ccrtain that the study has found an association, as the elevated odds ratio could just be due to random chance. If the confidence limits do not include 1, the elevated odds ratio is 93% certain not to be due to random chance, and the study is said to have found a statistically significant association.

Fig. 8 compares retults from 11 case-control studies. Any such comparison involves arbitrary decisions, as no two studies have exactly coniparable results. This comparison concerns childhood leukaemia and residential exposurc, and givcs just one result for each different method of exposure acsessnient used in each study. Studies typically give results for several different thresholds between ‘exposed’ and ‘non-exposed’. In Fig. 8, thresholds are selected to allow as consistent a comparison as possible behveen studies.

The picture which emerges is of odds ratios which are generally elevated above 1. However, few odds ratios are elevated much (most are less than 2) and few of the studies are statistically significant (most of the confidence limits include 1). For comparison, the

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Fig. 8 could indicate magnetic-field exposure. Left-hand column gives first author and date of release of results. Main figure gives odds ratio (vertical red line) and 95% confidence limits (coloured bars). Right-hand column gives method of assessment of exposure. For further discussion see Reference 6

Summaty of resultsfrom epidemiological case-control studies looking at childhood leukaemia and factors which

ground-breaking ttudy on sniolung and cancer by Doll and Hill’ found an odds ratio of 27.6 and confidence h i t s of 11.3 to 67.6 for the highest exposure category.

When assessing these results, it is useful to understand some of the difficulties encountered in performing epidemiological studies. The first hf l i cu l ty is in mounting a large enough study. The larger the study, the narrower the confidence linlits are likely to be. However, tragic though individual m e r of chddhood leukaemia are, it is in fact a rare disease, and obtaining adequate study sizes is riot easy.

A second difficulty is the assessment of exposure. Until recently, personal monitoring was not feasible. Suitable monitors are now availablc, hut can give information about exposure only when the study is conducted, rathcr than exposures at the time in the past when the disease was contracted. Studies have used other methods, such as ‘wire codes’ (a method of estimating the rriagrictic field in a homc horn the type of distribution wiring outside it), distances to electrical

facilities, calculated fiel&, or single measurements of the field in the homes. None of these gives a satisfactory estimate of total exposure. Much research effort worldwidc is devoted to dcvclopirlg better methods of assessing exposure.

A third difficulty lies in the selection of controls. It is important that they are representative of the same population as the cases. Suppose, for example, that the controls were, on average, of a higher socioeconomic status than the cases. They would then tend to live in bigger houses with bigger gardens. Such houses terd to he further from the distribution wiring (which is also more likely to be underground) so the magnetic fields wdl tcrid to be lower. Thus the study would have a n odds ratio greater than 1 not because ofany association between cancer and exposure, but purely because of the way the controls were selected.

A related dfficulty is that of confounding factors. To give an example, in America, distribution wirlrlg tends to follow roads, starting on main roads and branching

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out on to quieter roads. Thus magnetic fields tend to be hgher on main roads. However main roads also have higher traffic densities. Thus a seeming a~sociation between magnetic fields and cancer might in fact be caused by pollution from cars and have nothing to do with magnetic fields.

Recently, the NRPB asked Sir Richard Doll, one of Britain’s foremost epidemiologists, to chair a committee to examine, amongst other things, all the epidemiological evidence concerning magnetic fields. T h g account of the results of the studies and the various difticulties discussed above, they concluded6: ‘. . . the epidemiological findings that have been reviewed provide no firm evidence of the existence of a carcinogenic hazard from exposure of paternal gonads, the fetus, children, or adults to the extremely low frequency electromagnetic fields that might be associated with residence near major sources of electricity supply, the use of electrical appliances, or work in the electrical, clectronic, and tele- communications industries’. Of the residential studes they also commented: ‘. . . the methodological short- comings of the studies are such that the evidence is insufficient to allow conclusions to be drawn.’

The need for better studles is widely recognised. In this country, the United Kingdom Coordmating Committee for Cancer Kesearch has recently launched a major study, which aims to include every case of childhood cancer in England and Wales over a period of about four years, arid will use the best available methods of assessing exposure.

The needfor a plaiuible mechanism Even if an epidemiological study demonstrated a

statistical association between magnetic fields and a health risk, that would not prove that fields cause dsease. Epidemiology can only describe associations and can never prove cause and effect. To establish a causal link, it would be helpful to have a plausible mechanism by which the fields implicated by the epidemiology can interact with biological systems. However, no such plausible mechanism exists.

The d15culty is that the field levels implicated are so small8, typically 0.2 pT. Such fields induce a current density of 1.25 pA tn ’ in the body, vastly smaller than the typical ambient current densities arising from neuromuscular activity in the body of 1-10 mAm-’. The currents induced by the field have an energy density orders of magnitude lower than the thermal energy density of tissue, and will produce a voltage over typical cells and cell membranes orders of magnitude lower than the noise voltage.

Given these dscrepancies it ic hard to see how any mechanism can dependjust on the induced current. In recent years attention has focused instead on various resonance mechanisms, possibly involving the earth‘s static field in combination with the 50 Hz field, but the evidence for such mechanisms is slim. Much research proceeds to try to establish reproducible biological

effects at these low field levels’, but so far without unambiguous results.

Conclusions

Electricity has become an integral part of our society, and power-frequency electric and magnetic fields are widespread and inescapable. There are numerous well- established effects of these fields, but generally at levels well above those encountered in the environment. The worries that much lower fields might &ect health are based on wcak epidemiological results, and are not supported by biological evidence or hy a plausible mechanism.

Nevertheless, the worries are growing. No engineer can &ord to ignore them, and it is important that research continues. However the responsible position at the moment seems to be that of the NRPB”’: ‘The evidence is not sufficient to justify excessive concern about magnetic-field levels in the UK.. . hut neither, on the other hand, is there any justification for complacency.’

References

1 MADDOCK, B. J.: ‘Overhead line design in relation to electric and magnetic Geld limits’, Power 0 z g . J . September 1992, 6, (5) . pp.217-224

2 BUTROUS, G. S., MALE, J. C., WEBBER, R. S., BARTON, V. G., MELDKUM, S. J., BONNELL, J. A., and CAMM, A. J.: ‘The effect of power frequency high intensity electric fields on implanted cardiac pacemakers’, PACE, 1983,6, pp.1282-1292

3 IRNICH, W.: ‘Interference i n pacemakers’, PACE, 1984, 7, pp.1021-1048

4 SIENKIEWICZ, Z. J.. SAUNDERS, R . ‘D., and KOWALCZUK, C. I . : ‘Biological effects of exposure to non-ionising electromagnetic fields arid radiation: 11. Extremely low frequency electric and magnetic fields’, NRPB-R239, 1991

5 ‘Board sutement on restrictions on human exposure to static and time varying electromagnetic Gelds and radiation’, Documents ofthe NKIJB, 1993, 4, (5)

6 ‘Electromagnetic Gelds and the risk of cancer. Report of an advisory group on non-ionising mdiation’, chairman DOLL, R., Donrmrnts of the N-RI‘B, 1992, 3, (1)

7 DOLL, R., and HILL, A. B.: ‘A study of the aetiology of carcinoma of the lung’, Bc Medicti/ 4, 1952, 2,

8 ADAIR, R. K: ‘Constraints on biological effects of wrak extremely-low-frequency electronlagnetic Gelds’, Phys. Rev.

9 ‘The possible biological effects of low-frequency electro- magnetic fields’. IEE Public Affairs Board Report No. 10, 199 1

10 VENNIS, J. A., MUIKHEAL), C. R., and ENNIS, J. R.: ‘Epidenuological studies of exposures to electromagnetic fields: 11. Cancer’.J Radiof. Prot., 1991, 11, pp.1.>25

pp. 1271-1 286

A, 1991.43, pp.1039-1048

0 IEE: 1994

The authors are with the National Grid Technology and Science Division, Kelvin Avenue, Leatherhead. Surrcy KT22 7ST, UK

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