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Illinois Institute of Technology

Illinois Institute of Technology. Physics 561 Radiation Biophysics Lecture 13: The Linear Non-threshold Hypothesis 15 July 2014 Andrew Howard. Lecture 16 Plans. The LNT hypothesis What it says Evidence for it Evidence against it Scientific Consequences of it Regulatory Consequences

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Illinois Institute of Technology

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  1. Illinois Institute of Technology Physics 561 Radiation Biophysics Lecture 13: The Linear Non-threshold Hypothesis15 July 2014 Andrew Howard Linear Non-Threshold

  2. 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? Linear Non-Threshold

  3. 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 Linear Non-Threshold

  4. Dose-response curves:LNT hypothesis Risk Experimental or Epidemiological Data LNT Risk withNo Exposure Average dose to population, mSv Linear Non-Threshold

  5. Dose-Response Curves: Weak-Form Hormesis Risk Experimental or Epidemiological Data LNT Risk withNo Exposure Hormesis:Weak Form Average dose to population, mSv Linear Non-Threshold

  6. Dose-Response Curves:Strong-Form Hormesis Risk Experimental or Epidemiological Data Risk withNo Exposure LNT Hormesis:Weak Form Hormesis: Strong Form Average dose to population, mSv Linear Non-Threshold

  7. 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 Linear Non-Threshold

  8. 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 Linear Non-Threshold

  9. 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 Linear Non-Threshold

  10. 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? Linear Non-Threshold

  11. 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 Linear Non-Threshold

  12. 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 Linear Non-Threshold

  13. 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! Linear Non-Threshold

  14. Acceptable Risk: LNT Experimental or Epidemiological Data LNT Acceptable Risk Risk withNo Exposure LNT-based dose limit Average dose to population, mSv Linear Non-Threshold

  15. Acceptable Risk:LNT vsHormetic Models Experimental or Epidemiological Data LNT Acceptable Risk Risk withNo Exposure Hormesis-based dose limit LNT-based dose limit Average dose to population, mSv Linear Non-Threshold

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