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Illinois Institute of Technology

Physics 561 Radiation Biophysics

Lecture 13:The Linear Non-threshold Hypothesis

15 July 2014Andrew Howard

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Lecture 16 Plans

The LNT hypothesis– What it says– Evidence for it– Evidence against it– Scientific Consequences of it– Regulatory Consequences

What do we do with it?

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Linear non-threshold hypothesis Asserts that the population dose-response to a

toxicant at low doses is linear from the origin up to the lowest value for which reliable data can be derived

On the causal side, this hypothesis can be applied to chemical toxicants as well as ionizing radiation

On the effect side, this is usually applied to cancer risk but it can apply to other (usually chronic) conditions

Dose-response curves:LNT hypothesis

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Risk

Average dose to population, mSv

Experimental or Epidemiological Data

Risk withNo Exposure

LNT

Dose-Response Curves: Weak-Form Hormesis

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Risk

Average dose to population, mSv

Experimental or Epidemiological Data

Risk withNo Exposure

LNT

Hormesis:Weak Form

Dose-Response Curves:Strong-Form Hormesis

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Risk

Average dose to population, mSv

Experimental or Epidemiological Data

Risk withNo Exposure

LNTHormesis:Weak Form

Hormesis: Strong Form

Is this a new idea?

Of course not… It underlies much of the regulatory

machinery in this country and elsewhere for ionizing radiation and other risks

Goes back to before WWII In recent years it’s been called

into question

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American Nuclear Society 2001 report

Emphasizes lack of evidence for any deleterious effects of ionizing radiation below 1 Sv

Brings forward numerous studies that suggest beneficial effects at low doses and harm from artificial lowering of background

Authors assert that LNT is unlikely for cancer given cancer’s multi-step, complex causality

Numerous references

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ICRP Publication 99: a 2005 report Committee includes many researchers whose

work we have cited in this course: Little, Kellerer, Ullrich

Considers epidemiological, mechanistic, and animal studies

Explicitly takes error analysis into account Concludes that there is no universal threshold

but is willing to concede that there might be thresholds for cancers of specific tissues, maybe all of them

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The ALARA concept Advocated by ICRP itself as well as by

NRC, DOE As low as reasonably achievable: But who decides what is reasonable WNS points out that one should take

economic and social factors into account

Is that what really happens, though?

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Rutherford’s discussion A patient pedagogical approach Already organized into a PDF that is

PowerPoint-like, so I will simply show it to you directly

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World Nuclear Society 2013 report

This is a trade organization, not a governmental body Report looks at risks associated with radiation in the

context of other risks Specifically examines Fukushima and argues that the

aggressive measures taken to protect the public from radiation at Fukushima caused considerable suffering (physical and psychic) and several deaths

Discusses Chernobyl and suggests that most of the health effects arose from fear and dislocation

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Regulatory consequences of LNT

Clearly the prospect of a threshold would relax the exposure limits, i.e. they’d be higher than they are now

As it is:– Nuclear power plants have difficulties operating– Researchers have to jump through hoops to use

radionuclides or X-ray generators– Hospitals have to keep extremely careful records of the

nuclides they use and the X-ray sources they employ Clearly there are costs associated with this!

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Acceptable Risk: LNT

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Average dose to population, mSv

Experimental or Epidemiological Data

LNT

Risk withNo Exposure

Acceptable Risk

LNT-based dose limit

Acceptable Risk:LNT vs Hormetic Models

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Average dose to population, mSv

Experimental or Epidemiological Data

LNT

Risk withNo Exposure

Acceptable Risk

LNT-based dose limit

Hormesis-based dose limit