|Year : 2015 | Volume
| Issue : 3 | Page : 78-82
Low-level radiation exposures: Time to revisit linear no-threshold concept
MR Iyer1, Pushparaja2
1 Ex-Bhabha Atomic Research Centre (BARC), Ex-International Atomic Energy Agency, Vienna, Austria
2 Ex-Bhabha Atomic Research Centre, Mumbai 400085, India
|Date of Web Publication||10-Nov-2015|
M R Iyer
Bungalow D 4, Raj Kunj Society, Chembur, Mumbai - 400 074
Source of Support: None, Conflict of Interest: None
The concepts of LNT (Linear No-Threshold) and the resulting ALARA (As Low As Reasonably Achievable) used for radiological protection have been stumbling blocks for public acceptability of nuclear power. Often, public get confused and easily get exploited by interested people. The application of this concept has perhaps resulted in a more harmful phenomenon now known as "radiophobia." Over the years, LNT has become the corner stone of radiation protection philosophy for the international organizations like ICRP, UNSCEAR etc. which is followed by all national regulators. The genesis of these theories is the cellular level findings of half a century back. Most of these are findings at high dose levels in macro systems and extrapolated to low dose. It is time that international radiation safety organizations revisit the assumptions and have a more pragmatic approach towards these abstract concepts in the light of new findings. The article reviews the evolution of LNT hypothesis and the basis for LNT, examines the possibility that there might be a threshold dose, below which there would be no radiation-related cancer risk. Evidences against LNT and the possible existence of a threshold dose are reviewed. The article concludes that this is definitely time to have a re-look of the corner stone concepts in radiation protection philosophy.
Keywords: As low as reasonably achievable, linear no-threshold concept, radiation protection philosophy
|How to cite this article:|
Iyer M R, Pushparaja. Low-level radiation exposures: Time to revisit linear no-threshold concept. Radiat Prot Environ 2015;38:78-82
|How to cite this URL:|
Iyer M R, Pushparaja. Low-level radiation exposures: Time to revisit linear no-threshold concept. Radiat Prot Environ [serial online] 2015 [cited 2020 Jul 5];38:78-82. Available from: http://www.rpe.org.in/text.asp?2015/38/3/78/169375
| Introduction|| |
The concepts of linear no-threshold (LNT) and the resulting As Low As Reasonably Achievable (ALARA) principle used for radiological protection have been the stumbling block for public acceptability of nuclear power. To the common man, the terminology smacks of helplessness on the part of nuclear operators in firmly putting it across "what is safe and what is not.0" If this is not done, it is difficult to convince even unbiased people. Imagine, how disastrous it would be to say everything is safe but "be careful to keep it as low as reasonable achievable" despite its doubtful microbiological basis and much less epidemiological support.
Quibbling scientific exactitudes will not sell in public! They get confused, and this can easily be exploited by interested people. The application of this concept has perhaps resulted in a more harmful phenomenon now known as "radiophobia."
Over the years, LNT has become the cornerstone of radiation protection philosophy for the international organizations such as the International Commission of Radiological Protection (ICRP) and United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), which is followed by all national regulators. The genesis of these theories is the cellular level findings of half a century back. Most of these are findings at high dose levels in macro systems and extrapolated to low dose. It is time that the International Radiation Safety Organizations revisit assumptions and have a more pragmatic approach toward these abstract concepts.
| The initial stages|| |
American geneticist Hermann J. Mullar, who was awarded the Noble Prize in Medicine in his Noble Prize Lecture, argued that the dose-response for radiation-induced germ cell mutation is linear, and there is no threshold. This judgment was made, in spite of the reported findings of the experiments on fruit fly germ cell mutations carried out by Prof. Mullar and Prof. Curt Stern at the University of Rochester, which failed to support the linear dose-response model at low exposure levels.
Within a year, the National Academy of Sciences (NAS) accepted the linear model for gonadal mutations; the practice was extrapolated to somatic cells and cancer. Twenty years later, NAS adopted the linear approach for chemicals. Soon thereafter, the U.S. Environmental Protection Agency (US-EPA) announced that it would use the linear model for risk assessment. The ICRP followed suit and assumed the LNT approach as the basis for setting standards for radiological protection!
Edward J. Calabrese, the Professor of Toxicology at the University of Massachusetts, School of Public Health, Amherst, United States, uncovered the correspondence between Muller and Stern. He said, Muller's decision not to mention the key scientific evidence against his position has had a far-reaching impact on our approach to regulating radiation and chemical exposure. In fact, Calabrese's career research shows that low doses of some chemicals and radiation are benign or even helpful.
If the truth was to be told, the standards for radiation exposure would have been drastically different today.
| Basis for Linear No-Threshold|| |
The principal basis for the LNT is theoretical and very simple. A single particle of radiation hitting a single DNA molecule in a single cell nucleus of a human body can initiate cancer. The probability of a cancer initiation is, therefore, proportional to the number of such hits, which is proportional to the number of particles of radiation, which in turn is proportional to the dose.
Carcinogenesis, however, is a highly complex multi-step process involving one or a combination of chemical, physical, biological, and genetical damage to the cell. It is assumed that the damage thus caused escapes the powerful DNA damage repair mechanism existing in the body.
In the absence of convincing hard facts, the available data at high exposures (Hiroshima and Nagasaki data) were fitted to get a dose response graph, the graph was extrapolated to zero assuming linearity in low-level exposures to low LET radiation. It is known that biological responses, such as radiation-induced cancer to low dose and low dose rate responses differ from a high dose and high dose rate responses qualitatively and quantitatively.
To recap LNT, the LNT dose hypothesis is a supposition that all exposures to radiation are dangerous, and there is no dose below which harmful effects will not occur. ICRP assumes that the site-specific cancer risk from low LET radiation is proportional to dose received which is consistent with the LNT model.
This is a drastic judgment based on wrong or misplaced interpretation of the findings to a practical protection system.
| Challenges|| |
The ICRP's view on the possibility of threshold dose for radiation-induced cancer, is given in ICRP Publication 103 (2007),  Para 178 entitled dose thresholds as: "Possibility that there might be a threshold dose, below which there would be no radiation-related cancer risk, has been ignored. The LNT model is not universally accepted as biological truth, but rather, because we do not actually know what level of risk is associated with very-low-dose exposure, it is considered to be prudent judgement for public policy aimed at avoiding unnecessary risk from exposure.0"
Recently, the ICRP has taken the cognizance of the effect of repairs of such damaged/mutated cells by an existing DNA repair mechanism in the body, post-exposures. In addition to this, the emerging results with respect to adaptive responses, hormesis, and bystander effects add to the uncertainty of quantifying the risks at low level exposures by extrapolation of the well-known risks at high level exposures. LNT concept is slowly being challenged!
Now comes the hint that the UNSCEAR has finally admitted that we cannot use the LNT hypothesis to predict cancer from low doses of radiation. A recent study concluded what was suspected for decades - exposures to radiation doses less than about 100 mSv are not a big deal. The LNT dose hypothesis does not apply to exposures to low doses, encountered in occupational radiation protection settings. However, the question is who will take the lead?
| Evidences Against Linear No-Threshold|| |
The UNSCEAR (UNSCEAR 2012) submitted the report that, among other things, states that uncertainties at low doses are such that UNSCEAR "does not recommend multiplying low doses by large numbers of individuals to estimate numbers of radiation-induced health effects within a population exposed to incremental doses at levels equivalent to or below natural background levels." However, the concepts followed by those organizations precisely lead to those extrapolations to be faced by nuclear operators. This leads to declaring "virtual deaths" rather than real deaths in all nuclear accident cases. Moreover, the process leads to including people some of whom would not have received any radiation dose at all! As Dr. Brenner told at the ANS 2012 meeting, the idea of virtual deaths is detesting.
Late B. L. Cohen , Professor Emeritus of Physics, University of Pittsburgh of United States, after extensive research and review of the scientific data, challenged the very foundation of the LNT hypothesis and concluded that "the LNT theory fails very badly in the low-dose region, grossly overestimating the risk from low-level radiation. This means that the cancer risk from the vast majority of normally encountered radiation exposures is much lower than given by usual estimates, and may well be zero or even negative."
Epidemiological studies indicated that there are no observable effects in any population group around the planet that suggest LNT is true below 100 mSv/y even in areas of the Middle East, Brazil, and France where natural background doses exceed a few mSv/y. Detailed studies of the genetic and carcinogenic effects in the population in the high background areas in the West Coast of India have clearly shown that there had been no deleterious effects on the population groups staying there for generations in a radiation field often 10-50 times higher than the natural radiation background elsewhere. Further, these background doses are higher than most of the exposures of occupational workers in the nuclear industry and certainly much more than the trivial public exposures around nuclear power plants (NPPs).
The large volume of findings of long-term intense research on the Hiroshima and Nagasaki bomb survivors, on the population of high background radiation areas, and even in the Chernobyl follow-up cases have also not indicated the existence of such hypothesis. In fact, the longest follow-up study, over 60 years (published recently),  of children born to the parents of exposed to significant dose of ionizing radiation during the bombings of Hiroshima and Nagasaki, did not indicate any increased risk of cancer, non-cancer diseases or genetic effects.
Radiobiology has taken a big stride in a better understanding of the mechanisms resulting in health effects at low doses using new techniques. Analysis of the main epidemiological studies reported on radiation-induced carcinogenesis leads us to conclude that there is no convincing evidence of a carcinogenic effect in humans or experimental animals for doses of < 100 mGy of low linear energy transfer (LET) radiation.
| Evidences for threshold dose|| |
The possibility of a dose threshold is also discussed in other documents dealing with radiation protection, i.e., in "Low-dose extrapolation of radiation-related cancer risk," ICRP-99, 2005,  and "Biological mechanisms of radiation actions at low doses," UNSCEAR Report, NY-2012.  An extensive overview of the issues related with the LNT and its inconsistencies with the experimental data are given by Tubiana et al.,  Morgan and Bair,  and Cohen.  Further, interpretation of dose-response relationships for solid cancer data indicated a threshold at <100 mSv and somewhere between 40 and 60 mSv. The estimated cancer risks are such that they are infinitesimally lower than the risks from various other agents subjected to the public that it is not observable at all and thus in practice non-existent.
There is evidence for the existence of adaptive response provided by normal levels of background radiation. The organizations such as the UNCEAR now agrees to define low doses as those of 100 mSv or less, which is consistent with that used by ICRP and Biologic Effects of Ionizing Radiation VII reports. 
There is no direct evidence of heritable effects of radiation on human populations. Consensus is emerging that low-LET irradiation below 0.5 Gy dose, particularly by the doses of <0.1-0.2 Gy, does not cause transmissible instability. This is important since it strongly suggests the radiation-induced transmissible instability does not contribute to the development of health effects resulting from low doses of low LET radiation.
| Implications of Linear No-Threshold|| |
This LNT concept will give a fatality probability from any amount of exposure to radiation-though it may be good cell biology, but will not in any way help in improving public health. The cost of radiation protection, particularly in the public domain, is increasing day by day due to regulators forcing the industry to adopt these stringent unproductive controls. People often go behind the nonexistent scare of radiation effects around NPP where the cause to effect aspect is often forgotten.
As per the observations made in the recent UNSCEAR , " As a matter of general practice, the Committee does not use the risks inferred from studies of populations following radiation exposure at moderate and high doses to project absolute numbers of radiation-induced cancers following exposure at low and very low doses."
On the other hand, use of LNT concept has given rise to a new phenomenon which results in more damage and fatalities due to the psycho radiation effects as noticed in Fukushima. Hence, the time has come if the mission of ensuring safety from radiation is served by clinging to these concepts, or we need to have a more balanced approach. The statistical uncertainties and futility of proving or disproving have further added to the confusion.
It has also been pointed out that man has been evolving in a radiation environment which would have been much higher eons back, and the body mechanism are tuned to repair the damage caused by the exposure to ionizing radiation. In fact, there are many other man-made chemical and physical carcinogens existing in the environment whose effects are several times that from low-level radiation. Singling out a weak carcinogen-like ionizing radiation which is part of the natural environment to which all human beings are exposed, for risk assessment is not justifiable.  These aberrations are repaired by the existing defense mechanisms of the damaged cell repair system in the body or get eliminated by apoptosis and cell senescence. These responses for the aberrations are known to work better at low doses and dose rates (chronic exposures) as compared to high doses and dose rates (acute exposures).
| The Linear No-Threshold Model is not Universally Accepted|| |
The major organizations which have the strong reservations of accepting LNT model are: The French Academy of Sciences and the National Academy of Medicine, the Health Physics Society (HPS), and the American Nuclear Society. The opinion is loud and clear that the lack of concrete scientific evidence for health effects below 100 mSv means one should not try and make models about risk below this level. In fact, the French Academies support a threshold model indicating a dose threshold of 10 mSv below which there is no increased risk of cancer.
The American Nuclear Society  conducted a special meeting in 2012 to highlight the problems. In the opinion of American Nuclear Society, further research is required as convincing scientific evidence for acceptance of LNT hypothesis. The LNT is still a hypothesis and needs to be validated scientifically.
While, HPS, the principal organization for radiation protection scientists, has stated its position as "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 are either too small or are nonexistent." The HPS does not support LNT which implies that any level of radiation is harmful. The society is against the estimation of health risks below an individual dose of 50 mSv in 1 year or a lifetime dose of 100 mSv above the exposures received from natural radiation sources.
The authorities in India are also seized of the problem and intensifying the research to generate and consolidate further proof against LNT. We are in a better position to challenge the LNT concept using results of our extensive on-going research on the effect of exposures in the high radiation background areas in our country. The authorities should keep in mind the UNSCEAR Scientific Committee recommendation of not "multiplying very low doses by large numbers of individuals to estimate numbers of radiation-induced health effects within a population exposed to incremental doses at levels equivalent to or lower than normal natural background levels."
Let us not be eager to show Indian standards are much better than the Western standards by still following dose limits lower than the ICRP values, and limiting radioactivity levels in food items which are not even measurable with any accuracy.
It is believed that humans are continuously being exposed background radiation fields; they are adapted to that level of radiation, and some level of radiation may even serve a useful protective function (hormesis). According to some, there is considerable evidence supporting the use of low-dose radiation to prevent cancers and other major diseases such as Alzheimer's. According to a conservative estimate, about 10% of the current deaths from cancer can be prevented using low-dose radiation.
| Present and future of radiation protection standards|| |
In spite of the dissenting views on LNT, radiological protection standards today are based on the LNT concept which allows radiation doses to be averaged within an organ or tissue, added and added over time. The radiation protection quantities as of now are all based on the LNT assumption. It is generally applicable for the risk assessment at high doses. However, it is not justifiable to use this controversial concept to quantitative risk estimation of cancer deaths from low-level radiation exposures.
For the time being, the organizations such as ICRP, NCRP which provide guidelines, recommendations for radiological protection, and NRC and US-EPA opted for assuming LNT hypothesis for carrying out regulatory work until there is an adequate scientific evidence for threshold. There is an important nonscientific reason, i.e., ease of application to practical situations.
| If there exists a Dose Threshold?|| |
If there exists a dose threshold what about the regulation of occupational radiation exposures and public exposures which are typically in the range of low-level radiation exposures, i.e. below 100 mSv, without the assessment of risk? It is time that the concerned authorities and organizations prepare themselves for developing protection standards and to respond to regulatory issues when the radiobiologists come out with a concrete scientific basis for a dose threshold, say of the order of 50 mSv for causing stochastic health effects, in low level radiation exposure situations. This has support from the ICRP-103 document, Para A91, which touches upon the capacity of the cells to sustain and repair DNA damages occurring spontaneously and states that an additional approximately 2 DNA double-strand lesions, or 1 complex cluster per cell at 50 mGy low LET, will be of little or no consequence for cancer risk.
| Conclusions|| |
This is definitely the time to have a re-look of the cornerstone concepts in radiation protection philosophy. In fact, this concept has led to more deaths than preventing "virtual" deaths. It is a good augury that in recent times, opinion is building up around the globe by ANS, by the authorities in India and elsewhere. In fact, we in India, have a much higher responsibility in prevailing upon the international organizations to go away from LNT concept in view of the excellent epidemiological data from Indian high background areas against it.
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Conflicts of interest
There are no conflicts of interest.
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