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Linear no-threshold model
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.
Additional recommended knowledge
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."
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.
In the scientific community, expert panels are often convened to consider and write reports on the most important and controversial topics of the day. Several of these expert panels have been convened on the topic of the Linear no-threshold model.
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,
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,
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,
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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Linear_no-threshold_model". A list of authors is available in Wikipedia.|