Year : 2021 | Volume
: 44 | Issue : 1 | Page : 1--2
Is as low as reasonably achievable (ALARA) concept relevant to low-dose exposures?
Former Head, Radiation Safety Systems Division, BARC, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
M R Iyer
Former Head, Radiation Safety Systems Division, BARC, Bhabha Atomic Research Centre, Mumbai, Maharashtra
|How to cite this article:|
Iyer M R. Is as low as reasonably achievable (ALARA) concept relevant to low-dose exposures?.Radiat Prot Environ 2021;44:1-2
|How to cite this URL:|
Iyer M R. Is as low as reasonably achievable (ALARA) concept relevant to low-dose exposures?. Radiat Prot Environ [serial online] 2021 [cited 2022 Oct 6 ];44:1-2
Available from: https://www.rpe.org.in/text.asp?2021/44/1/1/317943
The ICRP prescribes the radiation safety norms and limits of exposure which is followed by all national regulatory agencies.
Radiation dose is quantified by measuring the energy absorbed in matter. Gray is the deposition of 1 J/kg of matter. Sievert takes into account the quality factor of the radiation also. The limit for occupational exposure is 20 mSv/year and for the public, it is 1 mSv. This is qualified to be “over and above the 2.4 mSv” on the average, everybody receives from natural sources which vary from place to place. This seems to imply an arbitrary distinction between a dose of 1 mSv from “nuclear operations” and that from “natural background”. The limit of 1 mSv is only a fraction of even the normal variation in natural radiation levels. In high background radiation areas (HBRAs), the population is exposed to a much higher radiation even by a factor of 50 or more.
Why the Public Suspicion of Radiation?
While safe limits of exposure are clearly recommended by ICRP, they also introduced certain controls which are more like radiation protection philosophy that are not quantifiable and likely to be misinterpreted! Public suspicion of nuclear power is attributed to these exaggerated notions on radiation safety which has become deep rooted during the last 60 years of its evolution. The way in which nuclear energy was demonstrated first through destructive means also contributed to these misgivings.
Radiation Effects at Low Doses
Ionizing radiations (IRs) of short wavelength knock out electrons from atoms and can alter the function of DNA. Hence, exposure to radiation can manifest as carcinogenic and genetic effects in man. Deterministic effects occur at high doses due to massive cell killing, manifest within a short duration, and have a threshold dose – LD-50 dose. This is relevant essentially for severe accident or medical therapy exposures.
Stochastic effects arise from mutagenic action of the radiation, presumed to manifest over years or even generations, and assumed to occur at all dose levels without any threshold involving a probability. Its manifestation is also epidemiological. The subject of discussion of low-dose radiation concerns these stochastic effects. At low levels of radiation, further confusion occurs due to what are known as “confounding factors.” Radiation is not the only carcinogenic agent! There are innumerable, often overwhelming other factors natural and man made to which man is exposed. For the minuscule effects of low levels of radiation, they form a large background and make it cofounding.
Traditionally, biologists carried out microbiological experiments on living organisms on the effects of radiation at moderate and high levels where these effects can easily be quantified and tried intuitively to extrapolate the effects linearly to low dose levels where these could not be easily quantified. At lower doses typically <100 mSv, these are not easily discernible due to its low probability, intrinsic cell repair mechanisms, and the confounding factors. If one linearly extrapolates without taking into consideration such hidden factors, it will lead to a nonzero risk even at fraction of the natural background radiation. This gives rise to a wrong notion that any amount of radiation is harmful and has given rise to the linear no-threshold (LNT) hypothesis and led to the philosophical concept as low as reasonably achievable (ALARA) concept in radiation protection. These concepts have been stumbling blocks on making nuclear energy acceptable to the public. 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.”
In principle, a single particle of radiation hitting a single DNA molecule in a single-cell nucleus of a human body can initiate a cancer. The probability of a cancer initiation is therefore taken proportional to the number of such hits, which is proportional to the dose. Thus, the risk is assumed linearly dependent on the dose; this is the LNT. This presupposes that all radiation is deadly and there is no dose below which harmful effects will not occur. This is a drastic conclusion based on a wrong interpretation of the findings at the cellular level to a biological system. LNT does not take into account the organism's immune system, biological recovery time between doses, or other relevant mechanisms that operate at low doses on an actual organism versus cells in a Petri dish. Biological systems have a large redundancy and hence highly resilient. Cancer is a multifactorial condition that needs more than just a few mutations.
Consider the Effect of Elevated Natural Radiation to Fix Limits
The HBRAs serve as a natural laboratory for the study of effect of low-to-moderate dose of radiations on population living there for several generations. We have one such area in the West Coast of India in Kerala. The population there is exposed to radiation doses even higher than the limit of annual exposure for occupational exposures and orders of magnitude higher than the public exposure limit. The Radiation Biology Division of BARC has carried out some of the outstanding work on the microbiological investigations ever done of the exposed population at transcriptome level and presented invaluable input to the UNSCEAR. The findings are astounding and did not show any significant increase in the frequency of any type of congenital malformations in HBRAs compared to control areas. The epidemiological investigations there and elsewhere in the world on HBRAs also show no significant effects.
Investigations involving children of atomic bomb survivors after 62 years of follow-up were carried out by Grant and Furukawa published in Lancet Oncology. No deleterious effects of radiation exposure have been reported from assessed mortality in children of the atomic bomb survivors after 62 years of follow-up. This is contrary to the results of calculations using models of the transgenerational effects of radiation based on LNT which predict more genetic disease in these children.
In fact, a new line of research is being suggested to establish the effect of low-level radiation in preventing cancer based on the hormesis effects of radiation. The reserve repair mechanisms are hypothesized to be sufficiently effective when stimulated as to not only cancel the detrimental effects of IR at low levels but also inhibit disease not related to radiation exposure. This hypothesis has captured the attention of scientists and public alike in recent years. This is a natural corollary that man has been evolving in a radiation environment which would have been much higher eons back and the body mechanism has evolved and tuned to repair the damage.
T. D. Luckey, 2006, examined over 3000 scientific research papers and showed that low-dose irradiation is stimulatory and/or beneficial. Advances in cell and molecular biology have also contributed new information on the mechanisms through which cells respond to radiation-induced damage. In 2019, Ricci from Italy University of Bologna on the basis of his investigations came to the conclusion that “Ionizing radiations epidemiology does not support the LNT model.” Modeling of the effect of IR exposures based on LNT at low doses has generally not included confounding factors. To justify application of the LNT to low doses, Hiroshima–Nagasaki data are linearly extrapolated to background incidence. It would imply that the biological mechanisms leading to cancer are exactly the same as at low doses. Ricci shows that extrapolations are incorrect because either the biological and epidemiological evidence for thresholds, or other nonlinearities, are more than substantial.
As Low As Reasonably Achievable Is Error Of Judgement On Low-Dose Radiation
In the light of evolving knowledge base on microbiological and epidemiological studies, such a risk predicted by LNT at the limit of 1 mSv/year is a few million times less than the risk from even congenital carcinogenicity. This needs to be considered in fixing up the limits. Over and above this, what is the logic of further recommending ALARA at such levels? It is high time that radiation protection standards are redefined to take into account the existence of natural background radiation that has not shown any deleterious effects. Some doubters ask if there is a proof for a threshold, but the question they should consider is if there is any positive proof for the effects on low level of radiation typically 1–100 mSv. The limits of public radiation exposure should take into account the existence of background radiation and there is no scope for introducing further imponderables like ALARA into this.