Linear no-threshold model
Encyclopedia
The linear no-threshold model (LNT) is a method for predicting the long term, biological damage caused by ionizing radiation
and is based on the assumption that the risk is directly proportional to the dose at all dose levels.
In other words, the sum of several very small exposures have the same effect as one larger exposure. The LNT model therefore predicts higher risks than either the threshold model
, which assumes that very small exposures are negligible, or the radiation hormesis
model, which predicts the least risk by assuming that radiation at very small doses can be beneficial. Because the current data are inconclusive, scientists disagree on which method should be used.
LNT, or at least "no threshold", is sometimes applied to other cancer
hazards such as polychlorinated biphenyl
s in drinking water.
, and rejected by the Department of Energy, according to Gofman, because it was "inconvenient".
The National Academy of Sciences (NAS) Biological Effects of Ionizing Radiation (BEIR) report, NAS BEIR VII was an expert panel who reviewed available peer reviewed literature and writes, "the committee concludes that the preponderance of information indicates that there will be some risk, even at low doses".
embedded in an organ (such as lung) results in a very high dose in the cells directly adjacent to the hot particle
, but a much lower whole-organ and whole-body dose. Thus, even if a safe low dose threshold was found to exist at cellular level for radiation induced mutagenesis
, the threshold would not exist for environmental pollution with hot particles, and could not be safely assumed to exist when the distribution of dose is unknown.
The linear no-threshold model is used to calculate by extrapolation, 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 very low, at natural background levels, in the absence of evidence, the model predicts via extrapolation, 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. 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.
Ramsar, located in Iran
, is often quoted as being a counter example to LNT. Based on preliminary results, it was considered as having the highest natural background radiation levels on Earth, several times higher than the ICRP-recommended radiation dose limits for radiation workers, whilst the local population did not seem to suffer any ill effects. Actually, the population of the high-radiation districts is small (about 1800 inhabitants) and only receive an average of 10 millisieverts per year, so that cancer epidemiology data are too imprecise to draw any conclusions. On the other hand, there may be non-cancer effects of the background radiation such as
chromosomal abberations or female infertility.
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."
A 2007 study of Swedish children exposed to fallout from Chernobyl
while they were fetuses between 8 and 25 weeks gestation has found that the reduction in IQ
at very low doses was greater than expected, given a simple LNT model for radiation damage, indicating that the LNT model may be too conservative when it comes to neurological damage. Neurological damage has a different biology than cancer, and for cancer rates there are conflicting studies.
Several expert scientific panels have been convened on the topic of the Linear no-threshold model.
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:
The Health Physics Society's position statement first adopted in January 1996, as revised in July 2010, states:
The American Nuclear Society
recommended further research on the Linear No Threshold Hypothesis before making adjustments to current radiation protection guidelines, concurring with the Health Physics Society's position that :
Dr John DeSesso, academic expert in teratology
writes,
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
and is based on the assumption that the risk is directly proportional to the dose at all dose levels.
In other words, the sum of several very small exposures have the same effect as one larger exposure. The LNT model therefore predicts higher risks than either the threshold model
Threshold model
In mathematical or statistical modelling a threshold model is any model where a threshold value, or set of threshold values, is used to distinguish ranges of values where the behaviour predicted by the model differs in some important way...
, which assumes that very small exposures are negligible, or the radiation hormesis
Radiation hormesis
Radiation hormesis is the hypothesis that low doses of ionizing radiation are beneficial, stimulating the activation of repair mechanisms that protect against disease, that are not activated in absence of ionizing radiation...
model, which predicts the least risk by assuming that radiation at very small doses can be beneficial. Because the current data are inconclusive, scientists disagree on which method should be used.
LNT, or at least "no threshold", is sometimes applied to other cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
hazards such as polychlorinated biphenyl
Polychlorinated biphenyl
Polychlorinated biphenyls are a class of organic compounds with 2 to 10 chlorine atoms attached to biphenyl, which is a molecule composed of two benzene rings. The chemical formula for PCBs is C12H10-xClx...
s in drinking water.
History
The linear-no-threshold model was first expressed by John GofmanJohn Gofman
John William Gofman was an American scientist and advocate. He was Professor Emeritus of Molecular and Cell Biology at University of California at Berkeley. Some of his early work was on the Manhattan Project, and he shares patents on the fissionability of uranium-233 as well as on early processes...
, and rejected by the Department of Energy, according to Gofman, because it was "inconvenient".
The National Academy of Sciences (NAS) Biological Effects of Ionizing Radiation (BEIR) report, NAS BEIR VII was an expert panel who reviewed available peer reviewed literature and writes, "the committee concludes that the preponderance of information indicates that there will be some risk, even at low doses".
Radiation precautions and public policy
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 is simple to apply: a quantity of radiation can be translated into a number of deaths without any adjustment for the distribution of exposure, including the distribution of exposure within a single exposed individual. For example, a hot particleHot particle
A hot particle is a small, highly radioactive object, with significant content of radionuclides. Because radioactivity can be inherent to a substance or induced, and there are many initial sources of radioactivity, hot particles can originate from a multitude of sources.- Attributes :Hot particles...
embedded in an organ (such as lung) results in a very high dose in the cells directly adjacent to the hot particle
Hot particle
A hot particle is a small, highly radioactive object, with significant content of radionuclides. Because radioactivity can be inherent to a substance or induced, and there are many initial sources of radioactivity, hot particles can originate from a multitude of sources.- Attributes :Hot particles...
, but a much lower whole-organ and whole-body dose. Thus, even if a safe low dose threshold was found to exist at cellular level for radiation induced mutagenesis
Mutagenesis
Mutagenesis is a process by which the genetic information of an organism is changed in a stable manner, resulting in a mutation. It may occur spontaneously in nature, or as a result of exposure to mutagens. It can also be achieved experimentally using laboratory procedures...
, the threshold would not exist for environmental pollution with hot particles, and could not be safely assumed to exist when the distribution of dose is unknown.
The linear no-threshold model is used to calculate by extrapolation, the expected number of extra deaths caused by exposure to environmental radiation, and it therefore has a great impact on public policy
Public policy
Public policy as government action is generally the principled guide to action taken by the administrative or executive branches of the state with regard to a class of issues in a manner consistent with law and institutional customs. In general, the foundation is the pertinent national and...
. The model allows any radiation release, like that from a dirty bomb
Dirty bomb
A dirty bomb is a speculative radiological weapon that combines radioactive material with conventional explosives. The purpose of the weapon is to contaminate the area around the explosion with radioactive material, hence the attribute "dirty"....
, to be translated into a number of lives lost, while any reduction in radiation exposure
Radiation exposure
The term radiation exposure commonly has several uses:* Absorption of high-energy ionizing radiation by an object. In living beings a high absorbed dose can lead to radiation poisoning.* Absorption by an object of non-ionizing radiation...
, for example as a consequence of radon
Radon
Radon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as the decay product of uranium or thorium. Its most stable isotope, 222Rn, has a half-life of 3.8 days...
detection, can be immediately translated into a number of lives saved. When the doses are very low, at natural background levels, in the absence of evidence, the model predicts via extrapolation, 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
Health physics
Health physics is a field of science concerned with radiation physics and radiation biology with the goal of providing technical information and proper techniques regarding the safe use of ionizing radiation...
to set maximum acceptable radiation exposures. The United States based National Council on Radiation Protection and Measurements
National Council on Radiation Protection and Measurements
The National Council on Radiation Protection and Measurements is a U.S. organization. It has a congressional charter under Title 36 of the United States Code, but this does not imply it has any sort of oversight or supervision from Congress; it is not a government entity.This text appears on the...
(NCRP), a body commissioned by the United States Congress
United States Congress
The United States Congress is the bicameral legislature of the federal government of the United States, consisting of the Senate and the House of Representatives. The Congress meets in the United States Capitol in Washington, D.C....
, 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.
Ramsar, located in Iran
Iran
Iran , officially the Islamic Republic of Iran , is a country in Southern and Western Asia. The name "Iran" has been in use natively since the Sassanian era and came into use internationally in 1935, before which the country was known to the Western world as Persia...
, is often quoted as being a counter example to LNT. Based on preliminary results, it was considered as having the highest natural background radiation levels on Earth, several times higher than the ICRP-recommended radiation dose limits for radiation workers, whilst the local population did not seem to suffer any ill effects. Actually, the population of the high-radiation districts is small (about 1800 inhabitants) and only receive an average of 10 millisieverts per year, so that cancer epidemiology data are too imprecise to draw any conclusions. On the other hand, there may be non-cancer effects of the background radiation such as
chromosomal abberations or female infertility.
Fieldwork
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 longitudinalLongitudinal study
A longitudinal study is a correlational research study that involves repeated observations of the same variables over long periods of time — often many decades. It is a type of observational study. Longitudinal studies are often used in psychology to study developmental trends across the...
studies involving large cohorts
Cohort (statistics)
In statistics and demography, a cohort is a group of subjects who have shared a particular time together during a particular time span . Cohorts may be tracked over extended periods in a cohort study. The cohort can be modified by censoring, i.e...
over long periods.
A review of the various studies published in the authoritative Proceedings of the National Academy of Sciences
Proceedings of the National Academy of Sciences
The Proceedings of the National Academy of Sciences of the United States of America, usually referred to as PNAS, is the official journal of the United States 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."
A 2007 study of Swedish children exposed to fallout from Chernobyl
Chernobyl disaster
The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine , which was under the direct jurisdiction of the central authorities in Moscow...
while they were fetuses between 8 and 25 weeks gestation has found that the reduction in IQ
Intelligence quotient
An intelligence quotient, or IQ, is a score derived from one of several different standardized tests designed to assess intelligence. When modern IQ tests are constructed, the mean score within an age group is set to 100 and the standard deviation to 15...
at very low doses was greater than expected, given a simple LNT model for radiation damage, indicating that the LNT model may be too conservative when it comes to neurological damage. Neurological damage has a different biology than cancer, and for cancer rates there are conflicting studies.
Controversy
In recent years, the accuracy of the LNT model at low dosage has been questioned. Some believe that if radiation is distributed evenly enough, so that the levels are comparable to the natural levels, it has no harmful health effects.Several expert scientific panels have been convened on the topic of the Linear no-threshold model.
- In 2004 the United States National Research CouncilUnited States National Research CouncilThe National Research Council of the USA is the working arm of the United States National Academies, carrying out most of the studies done in their names.The National Academies include:* National Academy of Sciences...
(part of the National Academy of SciencesUnited States National Academy of SciencesThe National Academy of Sciences is a corporation in the United States whose members serve pro bono as "advisers to the nation on science, engineering, and medicine." As a national academy, new members of the organization are elected annually by current members, based on their distinguished and...
) supported the linear no threshold model and stated regarding Radiation hormesisRadiation hormesisRadiation hormesis is the hypothesis that low doses of ionizing radiation are beneficial, stimulating the activation of repair mechanisms that protect against disease, that are not activated in absence of ionizing radiation...
: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 at this time.
- the National Council on Radiation Protection and MeasurementsNational Council on Radiation Protection and MeasurementsThe National Council on Radiation Protection and Measurements is a U.S. organization. It has a congressional charter under Title 36 of the United States Code, but this does not imply it has any sort of oversight or supervision from Congress; it is not a government entity.This text appears on the...
(a body commissioned by the United States CongressUnited States CongressThe United States Congress is the bicameral legislature of the federal government of the United States, consisting of the Senate and the House of Representatives. The Congress meets in the United States Capitol in Washington, D.C....
). endorsed the LNT model in a 2001 report that attempted to survey existing literature critical of the model.
- the United Nations Scientific Committee on the Effects of Atomic RadiationUnited Nations Scientific Committee on the Effects of Atomic RadiationThe United Nations Scientific Committee on the Effects of Atomic Radiation was set up by resolution of the United Nations General Assembly in 1955. 21 countries are designated to provide scientists to serve as members of the committee which holds formal meetings annually and submits a report to...
(UNSCEAR) wrote in its 2000 reportUntil 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 States Environmental Protection AgencyUnited States Environmental Protection AgencyThe U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...
also endorses the LNT model in its 2011 report on radiogenic cancer risk:"Underlying the risk models is a large body of epidemiological and radiobiological data. In general, results from both lines of research are consistent with a linear, no-threshold dose (LNT) response model in which the risk of inducing a cancer in an irradiated tissue by low doses of radiation is proportional to the dose to that tissue."
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
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
) that rejected the Linear no-threshold model in favor of a threshold dose response and a significantly reduced risk at low radiation exposure:
The Health Physics Society's position statement first adopted in January 1996, as revised in July 2010, states:
The American Nuclear Society
American Nuclear Society
The American Nuclear Society is an international, not-for-profit 501 scientific and educational organization with a membership of approximately 11,000 scientists, engineers, educators, students, and other associate members. Approximately 900 members live outside the United States in 40 countries....
recommended further research on the Linear No Threshold Hypothesis before making adjustments to current radiation protection guidelines, concurring with the Health Physics Society's position that :
Dr John DeSesso, academic expert in teratology
Teratology
Teratology is the study of abnormalities of physiological development. It is often thought of as the study of human birth defects, but it is much broader than that, taking in other non-birth developmental stages, including puberty; and other non-human life forms, including plants.- Etymology :The...
writes,
See also
- Nuclear powerNuclear powerNuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...
- Nuclear power debate#Health effects on population near nuclear power plants and workers
- Radiation hormesisRadiation hormesisRadiation hormesis is the hypothesis that low doses of ionizing radiation are beneficial, stimulating the activation of repair mechanisms that protect against disease, that are not activated in absence of ionizing radiation...
- RadiologyRadiologyRadiology is a medical specialty that employs the use of imaging to both diagnose and treat disease visualized within the human body. Radiologists use an array of imaging technologies to diagnose or treat diseases...
- Radiotherapy
External links
- ICRP, International Commission on Radiation Protection
- ICRU, International Commission on Radiation Units
- IAEA, International Atomic Agency Enegy Agency
- UNSCEAR, United Nations Scientific Committee on the effects of Ionizing Radiations
- IARC, International Agency for Research on Cancer
- HPA (ex NCRP), Health Protection Agency, UK
- IRPA, International Radiation Protection Association
- NCRP, National Council on Radiation Protection and Measurements, USA
- IRSN, Institute for Radioprotection and Nuclear Safety, France
- 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 LNT in public health studies
- BBC article discussing doubts over LNT
- How dangerous is ionising radiation? Reprinted "Powerpoint" notes from a colloquium at the Physics Department, Oxford University, 24 November 2006
- International Dose-Response Society - dedicated to the enhancement, exchange, and dissemination of ongoing global research in hormesis, a dose-response phenomenon characterized by low-dose stimulation and high-dose inhibition.