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Illinois Institute of Technology
Physics 561
Radiation Biophysics
Lecture 13:
The Linear Non-threshold Hypothesis
15 July 2014
Andrew Howard
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Lecture 16 Plans
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The LNT hypothesis
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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
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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
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Dose-response curves:
LNT hypothesis
Risk
Experimental or
Epidemiological Data
LNT
Risk with
No Exposure
Average dose to population, mSv
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Dose-Response Curves:
Weak-Form Hormesis
Risk
Experimental or
Epidemiological Data
LNT
Risk with
No Exposure
Hormesis:
Weak Form
Average dose to population, mSv
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Dose-Response Curves:
Strong-Form Hormesis
Risk
Experimental or
Epidemiological Data
Risk with
No Exposure
LNT
Hormesis:
Weak Form
Hormesis:
Strong Form
Average dose to population, mSv
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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:
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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
Experimental or
Epidemiological Data
LNT
Acceptable
Risk
Risk with
No Exposure
LNT-based dose limit
Average dose to population, mSv
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Acceptable Risk:
LNT vs Hormetic Models
Experimental or
Epidemiological Data
LNT
Acceptable
Risk
Risk with
No Exposure
LNT-based
dose limit
Hormesis-based
dose limit
Average dose to population, mSv
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