Linear no-threshold model

Overview
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.

The LNT Model stands in contrast to theories in which below a certain level, radiation exposure is harmless - in other words that there is threshold for radiation damage. Another alternative model is radiation hormesis, which asserts that radiation is beneficial in low doses, while recognizing that it is harmful in high doses. Other alternatives include those in which response to radiation increase more than linearly at low doses or that the LNT model underestimates risk at low radiation exposure. In the later hypothetical case, below a certain threshold the subtle damage caused maybe missed and not repaired by the body, leading to a greater risk of disease then indicated by the LNT model. The LNT model and each of these alternatives have plausible mechanisms that could bring them about, but definitive conclusions are hard to make given the difficulty of doing longitudinal studies involving large cohorts over long periods.

It is possible that some cancers respond linearly while others do not.

A review of the various studies published in the authoritative Proceedings of the National Academy of Sciences concludes that "given our current state of knowledge, the most reasonable assumption is that the cancer risks from low doses of x- or gamma-rays decrease linearly with decreasing dose."

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 the United States Congress, recently released report written by the national experts in the field which states that, radiation's effects should be considered to be proportional to the dose an individual receives, regardless of how small that dose is.

Controversy
In the scientific community, expert panels are often convened to consider and write reports on the most important are controversial topics of the day. Several of these expert panels have been convened on the topic of the Linear no-threshold model.


 * United States National Research Council (part of the National Academy of Sciences) supported the linear no threshold model.


 * the National Council on Radiation Protection and Measurements (a body commissioned by the United States Congress). endorsed the LNT model in a 2001 report that attempted to survey existing literature critical of the model.

"Until the [...] uncertainties on low-dose response are resolved, the Committee believes that an increase in the risk of tumour induction proportionate to the radiation dose is consistent with developing knowledge and that it remains, accordingly, the most scientifically defensible approximation of low-dose response. However, a strictly linear dose response should not be expected in all circumstances."
 * the United Nations Scientific Committee on the Effects of Ionizing Radiation (UNSCEAR) wrote in its most recent report

However, some organisations disagree with using the Linear No Threshold Hypothesis to estimate risk from environmental and occupational low-level radiation. They include the 11,000-member American Nuclear Society and the 6000-member Health Physics Society. American Nuclear Society position statement regarding the health effects of low-level radiation released in June 2001, states, "'It is the position of the American Nuclear Society that there is insufficient scientific evidence to support the use of the Linear No Threshold Hypothesis (LNTH) in the projection of the health effects of low-level radiation.”" And the Health Physics Society's position statement adopted in January 1996 and approved following revision in August 2004 by the societies' Health Physics Society and issued by the Scientific and Public Issues Committee, reads, "'In accordance with the current knowledge of radiation health risks, the Health Physics Society recommends against quantitative estimation of health risks below an individual dose of 5 rem in one year or a lifetime dose of 10 rem in addition to background radiation. Risk estimation in this dose range should be strictly qualitative accentuating a range of hypothetical health outcomes with an emphasis on the likely possibility of zero adverse health effects. The current philosophy of radiation protection is based on the assumption that any radiation dose, no matter how small, may result in human effects, such as cancer and hereditary genetic damage. There is substantial and convincing scientific evidence for health risks at high dose. Below 10 rem (which includes occupational and environmental exposures) risks of health effects are either too small to be observed or are non-existent.”"

Several scientists also disagree with the Linear No Threshold Hypothesis. In the extreme case, some authors promote Radiation hormesis, the idea that some radiation is good for people. Others simply regard the LNTM as conservative or even completely wrong for predicting the effect of low doses of radiation. As an example, Dr John DeSesso, academic expert in teratology writes, "'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." And Dr Michael Repacholi of the World Health Organisation claims that scientists simply guessed that if high-level radiation was dangerous then lower levels would also be hazardous – they made "an assumption".

A paper from Professor Wade Allison of Oxford University (a lecturer in medical physics and particle physics) argues that incorrect assumptions concerning low levels of exposure are widely accepted. He used statistics from therapeutic radiation, exposure to elevated natural radiation (the presence of radon gas in homes) and the diseases of Hiroshima and Nagasaki survivors to show 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. 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.

Resources

 * Report from the European Committee on Radiation Risk broadly supporting the Linear No Threshold model


 * ECRR report on Chernobyl (April 2006) claiming deliberate suppression of the LNTM in public health studies
 * BBC article discussing doubts over LNTM


 * How dangerous is ionising radiation? Reprinted "Powerpoint" notes from a colloquium at the Physics Department, Oxford University, 24 November 2006

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