Radiation hormesis

the opposite response to a very high dose. Radiation hormesis proposes that radiation exposure comparable to and just above the natural background level of radiation is not harmful but beneficial, while accepting that much higher levels of radiation are hazardous. Proponents of radiation hormesis typically claim that radio-protective responses in cells and the immune system not only counter the harmful effects of radiation but additionally act to inhibit spontaneous cancer not related to radiation exposure. Radiation hormesis stands in stark contrast to the more generally accepted linear no-threshold model (LNT), which states that the radiation dose-risk relationship is linear across all doses, so that small doses are still damaging, albeit less so than higher ones. Opinion pieces on chemical and radiobiological hormesis appeared in the journals Nature While epidemiological studies on populations of people exposed to an acute dose of high level radiation such as Japanese atomic bomb survivors () have robustly upheld the LNT (mean dose ~210 mSv), studies involving low doses and low dose rates have failed to detect any increased cancer rate. obscuring the subtle effects of low level radiation. Epidemiological studies may be capable of detecting elevated cancer rates as low as 1.2 to 1.3 i.e. 20% to 30% increase. But for low doses (1–100 mSv) the predicted elevated risks are only 1.001 to 1.04 and excess cancer cases, if present, cannot be detected due to confounding factors, errors and biases. In particular, variations in smoking prevalence or even accuracy in reporting smoking cause wide variation in excess cancer and measurement error bias. Thus, even a large study of many thousands of subjects with imperfect smoking prevalence information will fail to detect the effects of low level radiation than a smaller study that properly compensates for smoking prevalence. Given the absence of direct epidemiological evidence, there is considerable debate as to whether the dose-response relationship −3 and warned that smoking greatly increases the risk posed by radon exposure i.e. reducing the prevalence of smoking would decrease deaths caused by radon. However, the discussion about the opposite experimental results is still going on, especially the popular US and German studies have found some hormetic effects. Furthermore, particle microbeam studies show that passage of even a single alpha particle (e.g. from radon and its progeny) through cell nuclei is highly mutagenic, and that alpha radiation may have a higher mutagenic effect at low doses (even if a small fraction of cells are hit by alpha particles) than predicted by linear no-threshold model, a phenomenon attributed to bystander effect. However, there is currently insufficient evidence at hand to suggest that the bystander effect promotes carcinogenesis in humans at low doses. ==Statements by leading nuclear bodies==

Radiation hormesis has not been accepted by either the United States National Research Council, In May 2018, the NCRP published the report of an interdisciplinary group of radiation experts who critically reviewed 29 high-quality epidemiologic studies of populations exposed to radiation in the low dose and low dose-rate range, mostly published within the last 10 years. The group of experts concluded: The recent epidemiologic studies support the continued use of the LNT model for radiation protection. This is in accord with judgments by other national and international scientific committees, based on somewhat older data, that no alternative dose-response relationship appears more pragmatic or prudent for radiation protection purposes than the LNT model. In addition, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) wrote in its 2000 report: 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. This is a reference to the fact that very low doses of radiation have only marginal impacts on individual health outcomes. It is therefore difficult to detect the 'signal' of decreased or increased morbidity and mortality due to low-level radiation exposure in the 'noise' of other effects. The notion of radiation hormesis has been rejected by the National Research Council's (part of the National Academy of Sciences) 16-year-long study on the Biological Effects of Ionizing Radiation. "The scientific research base shows that there is no threshold of exposure below which low levels of ionizing radiation can be demonstrated to be harmless or beneficial. The health risks – particularly the development of solid cancers in organs – rise proportionally with exposure" says Richard R. Monson, associate dean for professional education and professor of epidemiology, Harvard School of Public Health, Boston. == Studies of low-level radiation ==

Cancer rates and very high natural background gamma radiation at Kerala, India The monazite sand in the Indian state of Kerala (which contains a third of the world's economically recoverable reserves of radioactive thorium) emits about 8 microsieverts per hour of gamma radiation, 80 times the dose rate equivalent in London, but a decade-long study of 69,985 residents published in Health Physics in 2009 "showed no excess cancer risk from exposure to terrestrial gamma radiation. The excess relative risk of cancer excluding leukemia was estimated to be −0.13 per Gy (95% CI: −0.58, 0.46)", indicating no statistically significant positive or negative relationship between background radiation levels and cancer risk in this sample. Cultures Studies in cell cultures can be useful for finding mechanisms for biological processes, but they also can be criticized for not effectively capturing the whole of the living organism. A study by E. I. Azzam suggested that pre-exposure to radiation causes cells to turn on protection mechanisms. A different study by de Toledo and collaborators has shown that irradiation with gamma rays increases the concentration of glutathione, an antioxidant found in cells. In 2011, an in vitro study led by S. V. Costes showed in time-lapse images a strongly non-linear response of certain cellular repair mechanisms called radiation-induced foci (RIF). The study found that low doses of radiation prompted higher rates of RIF formation than high doses, and that after low-dose exposure RIF continued to form after the radiation had ended. Measured rates of RIF formation were 15 RIF/Gy at 2 Gy, and 64 RIF/Gy at 0.1 Gy. Mina Bissell, a world-renowned breast-cancer researcher and collaborator in this study stated: "Our data show that at lower doses of ionizing radiation, DNA repair mechanisms work much better than at higher doses. This non-linear DNA damage response casts doubt on the general assumption that any amount of ionizing radiation is harmful and additive." A study by Otsuka and collaborators found hormesis in animals. Miyachi conducted a study on mice and found that a 200 mGy X-ray dose protects mice against both further X-ray exposure and ozone gas. In another rodent study, Sakai and collaborators found that (1 mGy/h) gamma irradiation prevents the development of cancer (induced by chemical means, injection of methylcholanthrene). In a 2006 paper, a dose of 1 Gy was delivered to the cells (at constant rate from a radioactive source) over a series of lengths of time. These were between 8.77 and 87.7 hours, the abstract states for a dose delivered over 35 hours or more (low dose rate) no transformation of the cells occurred. Also for the 1 Gy dose delivered over 8.77 to 18.3 hours that the biological effect (neoplastic transformation) was about "1.5 times less than that measured at high dose rate in previous studies with a similar quality of [X-ray] radiation". Likewise it has been reported that fractionation of gamma irradiation reduces the likelihood of a neoplastic transformation. Pre-exposure to fast neutrons and gamma rays from Cs-137 is reported to increase the ability of a second dose to induce a neoplastic transformation. Caution must be used in interpreting these results, as it noted in the BEIR VII report, these pre-doses can also increase cancer risk: Humans Effects of slightly increased radiation level In long-term study of Chernobyl disaster liquidators was found that: "During current research paradoxically longer telomeres were found among persons, who have received heavier long-term irradiation." and "Mortality due to oncologic diseases was lower than in general population in all age groups that may reflect efficient health care of this group." Though in conclusion interim results were ignored and conclusion followed LNT hypothesis: "The signs of premature aging were found in Chernobyl disaster clean-up workers; moreover, aging process developed in heavier form and at younger age in humans, who underwent greater exposure to ionizing radiation." A study of survivors of the Hirsohima atomic bomb explosion yielded similar results. Effects of sunlight exposure In an Australian study which analyzed the association between solar UV exposure and DNA damage, the results indicated that although the frequency of cells with chromosome breakage increased with increasing sun exposure, the misrepair of DNA strand breaks decreased as sun exposure was heightened. Effects of cobalt-60 exposure The health of the inhabitants of radioactive apartment buildings in Taiwan has received prominent attention. In 1982, more than 20,000 tons of steel was accidentally contaminated with cobalt-60, and much of this radioactive steel was used to build apartments, exposing thousands of Taiwanese to gamma-radiation levels up to over 1,000 times background (average 47.7 mSv; maximum 2,360 mSv excess cumulative dose). The radioactive contamination was discovered in 1992. In the years shortly after exposure, the total number of cancer cases was reported to be either lower than the society-wide average or slightly elevated. A 2004 paper by Chen et al. claimed that residents of Taiwanese buildings constructed with cobalt-60-contaminated steel had lower cancer mortality than the general population and interpreted this as possible evidence of a radiation hormesis effect. However, the study was widely criticized for methodological flaws, including comparing a predominantly young exposed cohort with the much older general population of Taiwan and failing to adjust for age and other confounders. The later, peer-reviewed study by Hwang et al. (2006), based on a complete registry of 7,271 residents and age- and sex-adjusted incidence analyses, found instead that prolonged low-dose-rate gamma exposure in the cobalt-60-contaminated apartments increased the risk of certain cancers—particularly leukemia, lymphoma, thyroid, and breast cancers—among those first exposed before age 30. Hwang et al. (2006) additionally cautioned that, based on the experience of the Hiroshima and Nagasaki bombings, it could be decades before an increase in other cancer types is observed. Besides the excess risks of leukemia, lymphoma, thyroid, and breast cancer, later publications noted elevated DNA anomalies and other health effects among the exposed population: There have been several reports concerning the radiation effects on the exposed population, including cytogenetic analyses showing increased micronucleus frequencies in peripheral lymphocytes, increases in acentromeric and centromeric chromosomal damage, and higher frequencies of chromosomal translocations, rings, and dicentrics. Other analyses have reported persistent depression of peripheral leucocytes and neutrophils, increased eosinophils, altered distributions of lymphocyte subpopulations, increased frequencies of lens opacities, delayed physical development among exposed children, increased risk of thyroid abnormalities, and late consequences in haematopoietic adaptation in children. Subsequent peer-reviewed studies have continued to follow the Taiwanese population exposed to cobalt-60-contaminated buildings. A 30-year follow-up (1983–2012) of 6,242 individuals found dose-dependent increases in the incidence of leukemia (excluding chronic lymphocytic leukemia), breast cancer, and all cancers combined, with the strongest effects among those first exposed before age 20, a similar pattern previously reported by Hwang et al. (2008). Additional studies have examined non-cancer outcomes: residents exposed during childhood showed higher frequencies of lens opacities (radiation-induced cataracts) and reduced fecundability (longer time-to-pregnancy) associated with higher dose rates. Together, these investigations indicate that prolonged low-dose-rate gamma exposure can increase certain cancer risks and produce measurable non-cancer biological effects, particularly in individuals exposed early in life. Radon therapy Intentional exposure to water and air containing increased amounts of radon is perceived as therapeutic, and "radon spas" can be found in United States, Czechia, Poland, Germany, Austria and other countries. ==Effects of no radiation==

Given the uncertain effects of low-level and very-low-level radiation, there is a pressing need for quality research in this area. An expert panel convened at the 2006 Ultra-Low-Level Radiation Effects Summit at Carlsbad, New Mexico, proposed the construction of an Ultra-Low-Level Radiation laboratory. The proposed laboratory would investigate the effects of almost no radiation on laboratory animals and cell cultures, and compare these groups to control groups exposed to natural radiation levels. Precautions would be made, for example, to remove potassium-40 from the food of laboratory animals. The expert panel concluded that the Ultra-Low-Level Radiation Laboratory could uniquely explore with authority and confidence the effects of low-level radiation, and potentially confirm or discard competing radiobiological models such as the LNT, threshold, and radiation hormesis hypotheses. Since the 2006 Carlsbad summit, several deep-underground and ultra-low-background radiation biology experiments have been established to investigate whether shielding organisms from natural ionising radiation produces measurable biological effects. One of the main research sites, the Low Background Radiation Experiment (LBRE) at the U.S. Waste Isolation Pilot Plant in New Mexico, provides a radiation environment roughly 70–80 times lower than surface levels and has been used to culture bacteria, protozoa, and mammalian cells under near-zero radiation conditions. Studies at WIPP and in European underground laboratories such as the Laboratori Nazionali del Gran Sasso (LNGS) in Italy have shown that microorganisms and cell lines shielded from natural background radiation can exhibit changes in growth rate, gene expression, and stress responses compared to controls kept at surface levels. A 2022 review of reduced-background studies concluded that, while measurable physiological and genetic changes occur in simple organisms and cultured cells, there is still no consistent evidence for either harmful or beneficial health effects in mammals or humans exposed to very-low-level or sub-background radiation. Recent experimental platforms at LNGS have refined dosimetry and environmental control to allow long-term mammalian cell-exposure studies, but the biological mechanisms and implications for radiation protection policy remain under active investigation. ==See also==