Linear no-threshold model
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The linear no-threshold model (LNT) is a model of the damage caused by ionizing radiation which presupposes that the response is linear (i.e., directly proportional to the dose) at all dose levels. Thus LNT asserts that there is no threshold of exposure below which the response ceases to be linear. LNT, or at least "no threshold", is sometimes applied to other cancer hazards such as polychlorinated biphenyls in drinking water.[1]
Some evidence indicates that, below a certain level, radiation exposure is in reality harmless (see background radiation). Another alternative model is radiation hormesis, which asserts that radiation is beneficial in low doses, while recognizing that it is harmful in high doses.
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[edit] Applications
If a particular dose of radiation is found to produce one extra case of a type of cancer in every thousand people exposed, the LNTM predicts that one thousandth of this dose will produce one extra case in every million people so exposed, and that one millionth of this dose will produce one extra case in every billion people exposed.
A linear model has long been used in health physics to set maximum acceptable radiation exposures.
The United States based National Council on Radiation Protection and Measurements (NCRP), a body commissioned by U.S. Congress, recently released report written by the national experts in the field which states that, for the sake of caution, radiation's effects should be considered to be proportional to the dose an individual receives, regardless of how small that dose is.
[edit] Controversy
Some regard the LNTM as conservative or even completely wrong for predicting the effect of low doses of radiation. They claim that there is no evidence supporting the assumption that there is no threshold, and that recent studies suggest changes in this assumption:
"When conducting risk assessments, the US Environmental Protection Agency (EPA) does not currently consider the beneficial effects from exposure to concentrations of agents below the no observed adverse effect level (NOAEL). If such benefits were observed, and if the beneficial and toxicological mechanisms of action were identical, this would probably be represented as a ‘j–shaped’ hormetic dose–response curve. If such data are available, they should be considered when assigning uncertainty factors for safe exposure calculations.
wrote Dr John DeSesso, academic expert in teratology.[2] Scientists simply guessed that if high-level radiation was dangerous then lower levels would also be hazardous – they made "an assumption", observes Dr Michael Repacholi of the World Health Organisation.[3]
Professor Wade Allison of Oxford University (a lecturer in medical physics and particle physics) gave a talk on ionising radiation on 2006-11-24 in which he demonstrated how incorrect assumptions concerning low levels of exposure are widely accepted. In a closely reasoned argument using statistics from therapeutic radiation, exposure to elevated natural radiation (the presence of radon gas in homes) and the diseases of Hiroshima and Nagasaki survivors he demonstrated that the linear no-threshold model should not be applied to low-level exposure in humans, as it ignores the well-known natural repair mechanisms of the body.[4] Professor Bernard Cohen of the University of Pittsburgh arrived at the same conclusion in his comparison of the effects from differing levels of environmental radon in 1601 U.S. counties.[5]
[edit] References
- ^ Consumer Factsheet on: polychlorinated biphenyls US Environment Protection Agency.
- ^ The case for integrating low dose, beneficial responses into US EPA risk assessments Human & Experimental Toxicology journal.
- ^ Chernobyl's 'nuclear nightmares' British Broadcasting Corporation.
- ^ Allison, Wade (2006-11-24). How dangerous is ionising radiation?.
- ^ Cohen, Bernard L. Test of the linear-no threshold model theory of radiation carcinogenesis for inhaled radiation decay products "Health Physics" February 1995, pp 157-174.
