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Federal Register / Vol. 86, No. 156 / Tuesday, August 17, 2021 / Proposed Rules
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a DDREF in its dose calculations aligns with the LNT model.
Comment: Several commenters observed that mammals evolved in an environment with a constant low dose of radiation. One commenter noted that humans developed DNA repair mechanisms to compensate. This commenter further stated that we experience far more DNA double strand breaks during mitotic cell division than we do from exposure to background radiation. As the biological mechanisms deployed to repair DNA damage caused by mitotic cell division are well documented, the commenter concludes that the rate of DNA damage that we can accommodate is also documented. This commenter reasons that because the rate of damage is substantially greater than zero, the LNT model cannot be correct.
Response: The NRC disagrees with this comment. There is substantial scientific uncertainty regarding the ability of the human bodys immune system, or other forms of adaptive response, to repair cells damaged by ionizing radiation. According to the NCI
comments, the available data does not show that any immune or other adaptive response offsets the carcinogenic damage caused by a given dose of ionizing radiation.109 NCI, in its comments, states that the repair of DNA double strand breaks DSBs relies on a number of pathways, and that these pathways are prone to errors, which may result in cell mutations, a fraction of which may lead to cancer.110 NCI further notes that the petitioners, and by extension, the commenter, do not reference data which shows that various cohorts subjected to protracted radiation exposures develop an increase in stable chromosome aberrations and other markers of biological damage in the peripheral blood lymphocytes. 111 NCI
states that such chromosome aberrations may increase the risk of cancer, and concluded that there is little data to suggest a threshold in dose, or possible hormetic beneficial effects of low-dose radiation exposure. 112
Comments Supporting the Petitions Assertions That There Are No Observable Adverse Effects From Background Radiation Comment: Several commenters remarked that background levels of ionizing radiation, which vary significantly around the world, have never been demonstrated to be a health
These studies did not find higher disease rates in geographic areas with high background levels of radiation exposure compared to areas with lower background levels. However, these studies were ecologic in design and utilized population-based measures of exposure rather than individual estimates of radiation dose. Thus, they cannot provide any quantitative estimates of disease risk associated with the exposure levels found in the areas studied.115

Also, the United Nations Scientific Committee on the Effects of Atomic Radiation UNSCEAR has recently
109 NCI
110 Id.

2015, at 3.
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hazard to humans. Some commenters also noted that in regions of the world such as Brazil or India where background radiation levels are higher than normal, epidemiological studies of large cohorts of subjects living in these areas did not reveal excess cancers or diseases linked to radiation exposure.
On this basis, these commenters conclude that the LNT model is based on a premise that is not supported by evidence.
Response: The NRC disagrees with these comments. The NRC notes that, in general, the inability to observe an effect does not mean that the effect has not occurred. These high background exposure studies are epidemiological in nature. They cannot be used as quantitative estimates of disease risk associated with the radiation exposure levels found in the areas studied, because the studies lack sufficient quantifiable evidence of the absence of cancer risk. As explained by NCI there are limitations associated with reliance on epidemiological studies in any effort to invalidate the LNT model. NCI noted that cancer risks predicted by the LNT model are likely to be small at low doses; so small as to be difficult to detect in the presence of large numbers of cancers resulting from other causes. 113 In this regard, NCI further stated that because epidemiologic studies are observational in nature and not controlled experiments, differences in risks in exposed and unexposed populations may reflect differences in life style factors such as smoking and may not necessarily result from radiation exposure. 114
In addition, the BEIR VII report prepared by NAS indicates that studies of populations exposed to natural background radiation are limited in their ability to define risk of disease in relation to radiation dose. In discussing four studies of populations exposed to natural background radiation, the BEIR
VII Phase 2 report states:

113 Id.,
at 1.

111 Id.

114 Id.

112 Id.

115 NAS

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published a review of cancer risk due to low dose rate radiation from environmental sources.116 UNSCEAR
concluded that the results of the studies of cancer risk due to radiation exposure at low dose rates from environmental radiation do not provide strong evidence for materially lower risks per unit exposure than in studies of high radiation doses and dose rates. 117 In this regard, UNSCEAR
noted that methodological improvements in environmental studies are needed to overcome low statistical power, dosimetric uncertainties, imperfections in control of confounding, and any other biases to include under-ascertainment of cases deaths or diagnoses, inaccurate cancer diagnosis, imprecise dose assessment, and residual confounding. 118
Therefore, no direct inferences about radiation effects can be drawn from studies where background radiation levels are higher than normal.
Comments Supporting the Petitions Objections to ALARA
Comment: One commenter asserted that current regulations are too restrictive and focus too heavily on radiation protection, thus creating a system that emphasizes compliance with ALARA at the expense of basic lab safety, such as somebody falling and hitting their head. The commenter posits that such accidents are far more likely than receiving a fatal radiation dose.
Response: The NRC disagrees with this comment. The NRC interprets the commenters use of the phrase basic lab safety as meaning compliance with non-radiologic safety requirements.
Non-radiologic safety issues are the oversight responsibility of the Occupational Safety and Health Administration OSHA and appropriate State and local government agencies.
Licensees are required and expected to comply with both applicable NRC
requirements as well as those of OSHA
and the pertinent State and local authorities. Moreover, licensees demonstrate compliance with ALARA
by such actions as establishing appropriate procedures and engineering controls, providing the proper training 116 UNSCEAR, Sources, Effects and Risks of Ionizing Radiation, Annex B: Epidemiological studies of cancer risk due to low-dose-rate radiation from environmental sources, Report to the General Assembly with Scientific Annexes 2017
UNSCEAR 2017 Report, Ann. B.
117 UNSCEAR 2017 Report, Ann. B, at 153.
118 Id., at 155.

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