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.
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 such as the threshold model. The radiation hormesis model, also in contrast to the LNT model asserts that radiation is beneficial in very low doses, while still recognizing that it is harmful in large doses.
LNT, or at least "no threshold", is sometimes applied to other cancer hazards such as polychlorinated biphenyls in drinking water.[1]
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The linear-no-threshold model was first expressed by John Gofman, and rejected by the Department of Energy, according to Gofman, because it was "inconvenient".[2] The National Academy of Sciences Biological Effects of Ionizing Radiation report, NAS BEIR VII was the first to clearly state that there is no safe level of radiation, although earlier reports had presented equivocal and inconclusive discussions on the issue.[3]
Other researchers with an interest in the linear no-threshold model and related low-level radiation topics include: Ernest Sternglass, Alice Stewart, John Gofman, Christopher Busby, and Edward B. Lewis.
If a particular dose of radiation is found to produce one extra case of a type of cancer in every thousand people exposed, LNT 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. This means that any given quantity of radiation will produce the same number of cancers, no matter how thinly it is spread. The model's virtue is its simplicity: a quantity of radiation can be translated into a number of deaths without any adjustment for the distribution.
The linear no-threshold model is used to calculate the expected number of extra deaths caused by exposure to environmental radiation, and it therefore has a great impact on public policy. The model allows any radiation release, like that from a dirty bomb, to be translated into a number of lives lost, while any reduction in radiation exposure, for example as a consequence of radon detection, can be immediately translated into a number of lives saved. When the doses are low, the model predicts new cancers only in a very small fraction of the population, but for a large population, the number of lives can easily reach hundreds or thousands, and this can sway public policy.
A linear model has long been used in health physics to set maximum acceptable radiation exposures. It was accepted for pragmatic reasons--- it is simple, plausible and predictive. The United States based National Council on Radiation Protection and Measurements (NCRP), a body commissioned by the United States Congress, recently released a 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 the dose is.
The LNT model and the alternatives to it each 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.
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."[4]
The LNT model for radiation damage may be too conservative according to recent work showing that there was a larger than expected reduction in IQ at very low doses from the fallout from Chernobyl, in children who were then fetuses of between 8 and 25 weeks gestation.[5] Neurological damage has a different biology than cancer, and for cancer rates there are conflicting studies.
In recent years, the accuracy of the LNT model at low dosage has been questioned. Many believe that when radiation is distributed thinly enough, so that the levels are comparable to the natural levels, it has no harmful health effects.
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.
The assumption that any stimulatory hormetic effects from low doses of ionizing radiation will have a significant health benefit to humans that exceeds potential detrimental effects from the radiation exposure is unwarranted.
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.
However, other organisations disagree with using the Linear no-threshold model to estimate risk from environmental and occupational low-level radiation exposure. The French Academy of Sciences (Académie des Sciences) and the National Academy of Medicine (Académie nationale de Médecine) published a report in 2005 (at the same time as BEIR VII report in the United States) that rejected the Linear no-threshold model in favor of a threshold dose response and a significantly reduced risk at low radiation exposure, they wrote:[10][11]
In conclusion, this report raises doubts on the validity of using LNT for evaluating the carcinogenic risk of low doses (< 100 mSv) and even more for very low doses (< 10 mSv). TheLNT concept can be a useful pragmatic tool for assessing rules in radioprotection for doses above 10 mSv; however since it is not based on biological concepts of our current knowledge, it should not be used without precaution for assessing by extrapolation the risks associated with low and even more so, with very low doses (< 10 mSv), especially for benefit-risk assessments imposed on radiologists by the European directive 97-43.
The American Nuclear Society position statement regarding the health effects of low-level radiation released in June 2001, states:[12]
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 first adopted in January 1996 and approved following revision in August 2004 by the societies' Health Physics Society, states:[13]
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 LNT 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,[14]
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.
A book Radiation and Reason by Professor Wade Allison argues that linearity is not appropriate in biology, or in any scientific context where correction or feedback is significant. He shows that data on therapeutic radiation, exposure to elevated natural radiation (the presence of radon gas in homes), the diseases of Hiroshima and Nagasaki survivors and other data are consistent with the expected non-linearity. Consequently he suggests that new radiation safety levels be established that reflect a more reasonable balance of risk between personal medical and public environmental radiation exposures. Such a change would have major effects on the acceptability and costs of nuclear technology.[15] Professor Bernard Cohen of the University of Pittsburgh arrived at a similar conclusion in his comparison of the effects from differing levels of environmental radon in 1601 U.S. counties.[16]
   | BBC article discussing doubts over LNTM - BBC NEWS | Science/Nature | Chernobyl's 'nuclear nightmares' |
| | Report critical of LNTM - 2003 Recommendations of the European Committee on Radiation Risk |
| | The case for integrating low dose, beneficial responses into US EPA risk assessments - The case for integrating low dose, beneficial responses into US EPA risk assessments -- DeSesso and Watson 25 (1): 7 -- Human & Experimental Toxicology |
| | ''Consumer Factsheet on: polychlorinated biphenyls'' - EPA Ground Water & Drinking Water > breadcrumb? > Consumer Factsheet on: POLYCHLORINATED BIPHENYLS |
| | ''How dangerous is ionising radiation?'' - Oxford Physics - Page Not Found |
| | The safety of nuclear radiation; a careful re-examination for a world facing climate change - Oxford Physics - Page Not Found |
| | NCRP report - Report No. 136 - Evaluation of the Linear-Nonthreshold Dose-Response Model for Ionizing Radiation |
| | Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2 - Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2 |
| | NCRP, National Council on Radiation Protection and Measurements, USA - National Council on Radiation Protection & Measurements (NCRP) - Radiation Exposure Data, Radiation Protection Guidelines |
| | UNSCEAR, United Nations Scientific Committee on the effects of Ionizing Radiations - United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) |
| | Gofman on the health effects of radiation: "There is no safe threshold" - 1/20/94 Gofman interview in UCSF publication, "synapse", Vol. 38, Number 16 |
| | HPA (ex NCRP), Health Protection Agency, UK - HPA - Health Protection Agency Homepage - Protecting people, Preventing harm, Preparing for threats |
| | ''Authors' reply'' - Authors' reply -- Heyes et al. 79 (946): 855 -- The British Journal of Radiology |
| | IAEA, International Atomic Agency Enegy Agency - International Atomic Energy Agency (IAEA) |
| | IARC, International Agency for Research on Cancer - IARC - INTERNATIONAL AGENCY FOR RESEARCH ON CANCER |
| | ICRP, International Commission on Radiation Protection - ICRP - International Commission on Radiological Protection |
| | ICRU, International Commission on Radiation Units - International Commission on Radiation Units and Measurements (ICRU) - Home |
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