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Radiation hormesis
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== Studies of low-level radiation == === Cancer rates and very high natural background gamma radiation at Kerala, India === Kerala's [[monazite]] sand (containing a third of the world's economically recoverable [[List of countries by thorium resources|reserves]] of radioactive [[thorium]]) emits about 8 micro[[sievert]]s 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 β0.13 per Gy (95% CI: β0.58, 0.46)", indicating no statistically significant positive or negative relationship between background radiation levels and cancer risk in this sample.<ref name=Nair2009>{{cite journal |doi=10.1097/01.HP.0000327646.54923.11 |title=Background Radiation and Cancer Incidence in Kerala, India β Karanagappally Cohort Study |year=2009 |last1=Nair |first1=Raghu Ram K. |last2=Rajan |first2=Balakrishnan |last3=Akiba |first3=Suminori |last4=Jayalekshmi |first4=P. |last5=Nair |first5=M. Krishnan |last6=Gangadharan |first6=P. |last7=Koga |first7=Taeko |last8=Morishima |first8=Hiroshige |last9=Nakamura |first9=Seiichi |last10=Sugahara |first10=Tsutomu |journal=Health Physics |volume=96 |pages=55β66 |pmid=19066487 |issue=1|s2cid=24657628 |display-authors=8 }}</ref> === 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.<ref>{{cite journal| author=Azzam, E. I. |journal=Radiation Research |year=1994 |volume=138 |pmid=8146320 |pages=S28βS31 |doi=10.2307/3578755 |title=Radiation-Induced Adaptive Response for Protection against Micronucleus Formation and Neoplastic Transformation in C3H 10T1/2 Mouse Embryo Cells |last2=Raaphorst |first2=G. P. |last3=Mitchel |first3=R. E. J. |issue=1 |jstor=3578755 |bibcode=1994RadR..138S..28A}}</ref> 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.<ref>{{cite journal |doi=10.1667/RR0640.1 |title=Adaptive Responses to Low-Dose/Low-Dose-Rate Ξ³ Rays in Normal Human Fibroblasts: The Role of Growth Architecture and Oxidative Metabolism |year=2006 |last1=De Toledo |first1=Sonia M. |last2=Asaad |first2=Nesrin |last3=Venkatachalam |first3=Perumal |last4=Li |first4=Ling |last5=Howell |first5=Roger W. |last6=Spitz |first6=Douglas R. |last7=Azzam |first7=Edouard I. |journal=Radiation Research |volume=166 |issue=6 |pages=849β857 |pmid=17149977|bibcode=2006RadR..166..849D |s2cid=31148344 }}</ref> 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/[[Gray (unit)|Gy]] at 2 Gy, and 64 RIF/Gy at 0.1 Gy.<ref name="Neumaier2011" /> These results suggest that low dose levels of [[ionizing radiation]] may not increase cancer risk directly proportional to dose and thus contradict the linear-no-threshold standard model.<ref name=HC24865 >{{Cite web |url=http://www.healthcanal.com/public-health-safety/24865-New-Take-Impacts-Low-Dose-Radiation.html |title = New Take on Impacts of Low Dose Radiation |date = 20 December 2011}}</ref> [[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."<ref name=HC24865/> === Animals === An early study on mice exposed to low dose of radiation daily (0.11 [[roentgen (unit)|R]] per day) suggest that they may outlive control animals.<ref>{{cite journal |title=Long-Term Effects of Acute and Chronic Irradiation in Mice. I. Survival and Tumor Incidence Following Chronic Irradiation of 0.11 R Per Day |author1=Egon Lorenz |author2=Joanne Weikel Hollcroft |author3=Eliza Miller |author4=Charles C. Congdon |author5=Robert Schweisthal |journal=Journal of the National Cancer Institute |volume=15 |issue=4 |date=1 February 1955 |pages=1049β1058 |doi=10.1093/jnci/15.4.1049 |pmid=13233949}}</ref> A study by Otsuka and collaborators found hormesis in animals.<ref>{{cite journal |doi=10.1667/RR0561.1 |title=Activation of Antioxidative Enzymes Induced by Low-Dose-Rate Whole-Body Ξ³ Irradiation: Adaptive Response in Terms of Initial DNA Damage |year=2006 |last1=Otsuka |first1=Kensuke |last2=Koana |first2=Takao |last3=Tauchi |first3=Hiroshi |last4=Sakai |first4=Kazuo |journal=Radiation Research |volume=166 |issue=3 |pages=474β78 |pmid=16953665|bibcode=2006RadR..166..474O |s2cid=44742877 }}</ref> 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.<ref>{{cite journal |pmid=10817047 |year=2000 |last1=Miyachi |first1=Y |title=Acute mild hypothermia caused by a low dose of X-irradiation induces a protective effect against mid-lethal doses of X-rays, and a low level concentration of ozone may act as a radiomimetic |volume=73 |issue=867 |pages=298β304 |journal=[[The British Journal of Radiology]] |doi=10.1259/bjr.73.867.10817047}}</ref> In another rodent study, Sakai and collaborators found that (1 mGy/[[hour|h]]) gamma irradiation prevents the development of cancer (induced by chemical means, injection of [[methylcholanthrene]]).<ref>{{cite journal |doi=10.1016/S0531-5131(01)00861-5 |title=Suppressive effect of long-term low-dose rate gamma-irradiation on chemical carcinogenesis in mice |year=2002 |last1=Sakai |first1=Kazuo |last2=Iwasaki |first2=Toshiyasu |last3=Hoshi |first3=Yuko |last4=Nomura |first4=Takaharu |last5=Oda |first5=Takeshi |last6=Fujita |first6=Kazuko |last7=Yamada |first7=Takeshi |last8=Tanooka |first8=Hiroshi |journal=International Congress Series |volume=1236 |pages=487β490}} </ref> In a 2006 paper,<ref>{{cite journal |doi=10.1667/RR0682.1 |title=The Effect of Dose Rate on Radiation-Induced Neoplastic TransformationIn Vitroby Low Doses of Low-LET Radiation |year=2006 |last1=Elmore |first1=E. |last2=Lao |first2=X.-Y. |last3=Kapadia |first3=R. |last4=Redpath |first4=J. L. |journal=[[Radiation Research]] |volume=166 |issue=6 |pages=832β838 |pmid=17149982 |bibcode=2006RadR..166..832E |s2cid=24775008 }}</ref> 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.<ref>{{cite journal |doi=10.1093/carcin/5.2.193 |title=Multifractionation of <sup>60</sup>Co gamma-rays reduces neoplastic transformation in vitro |year=1984 |last1=Hill |first1=C. K. |last2=Han |first2=A. |last3=Buonaguro |first3=F. |last4=Elkind |first4=M. M. |journal=Carcinogenesis |volume=5 |issue=2 |pages=193β197 |pmid=6697436}}</ref> 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.<ref>{{cite journal |doi=10.1080/09553009214552011 |title=Enhanced Sensitivity to Neoplastic Transformation by <sup>137</sup>Cs Ξ³-rays of Cells in the G2-/M-phase Age Interval |year=1992 |last1=Cao |first1=J. |last2=Wells |first2=R. L. |last3=Elkind |first3=M. M. |journal=[[International Journal of Radiation Biology]] |volume=62 |issue=2 |pages=191β199 |pmid=1355513}}</ref> Caution must be used in interpreting these results, as it noted in the BEIR VII report, these pre-doses can also increase cancer risk:<ref name=BEIR_VII/> {{blockquote|In chronic low-dose experiments with dogs (75 mGy/d for the duration of life), vital hematopoietic progenitors showed increased radioresistance along with renewed proliferative capacity (Seed and Kaspar 1992). Under the same conditions, a subset of animals showed an increased repair capacity as judged by the unscheduled DNA synthesis assay (Seed and Meyers 1993). Although one might interpret these observations as an adaptive effect at the cellular level, the exposed animal population experienced a high incidence of myeloid leukemia and related myeloproliferative disorders. The authors concluded that "the acquisition of radioresistance and associated repair functions under the strong selective and mutagenic pressure of chronic radiation is tied temporally and causally to leukemogenic transformation by the radiation exposure" (Seed and Kaspar 1992).}} However, 75 mGy/d cannot be accurately described as a low dose rate β it is equivalent to over 27 sieverts per year. The same study on dogs showed no increase in cancer nor reduction in life expectancy for dogs irradiated at 3 mGy/d.<ref>{{Cite web |url=http://www.nuclearsafety.gc.ca/eng/pdfs/Presentations/Guest-Speakers/2013/20130625-Cuttler-CNSC-Fukushima-and-beneficial-effects-low-radiation.pdf |author=Jerry M. Cuttler Cuttler & Associates Inc. |work=Canadian Nuclear Safety Commission Ottawa, Ontario 2013 June 25 |title=Fukushima and Beneficial Health Effects of Low Radiation |access-date=2014-08-24 |archive-date=2014-07-10 |archive-url=https://web.archive.org/web/20140710235841/http://nuclearsafety.gc.ca/eng/pdfs/Presentations/Guest-Speakers/2013/20130625-Cuttler-CNSC-Fukushima-and-beneficial-effects-low-radiation.pdf }}</ref> === Humans === ==== Effects of slightly increased radiation level ==== In long-term study of Chernobyl disaster liquidators<ref>{{cite web |url=https://www.rsu.lv/sites/default/files/dissertations/JReste_Promocijas_darba_kopsavilkums_EN.pdf |author=JeΔΌena Reste |title=THE AGING ASPECTS OF HUMANS PROTRACTEDLY EXPOSED TO IONIZING RADIATION |work=Summary of Doctoral Thesis Speciality. Occupational and Environmental Medicine |publisher=RΔ«ga StradiΕΕ‘ University |date=2013 |location=Riga}}</ref> 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 [[Linear no-threshold model|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 [[Atomic bombings of Hiroshima and Nagasaki|Hirsohima atomic bomb explosion]] yielded similar results.<ref>{{cite journal |last1=Sutou |first1=Shizuyo |title=Black rain in Hiroshima: a critique to the Life Span Study of A-bomb survivors, basis of the linear no-threshold model |journal=Genes and Environment |pages=1 |language=en |doi=10.1186/s41021-019-0141-8 |date=1 January 2020|volume=42 |issue=1 |doi-access=free |pmid=31908690 |bibcode=2020GeneE..42....1S |pmc=6937943 }}</ref> ====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.<ref>{{cite journal |doi=10.1093/mutage/ges026 |title=Sunlight and vitamin D affect DNA damage, cell division and cell death in human lymphocytes: A cross-sectional study in South Australia |year=2012 |last1=Nair-Shalliker |first1=V. |last2=Fenech |first2=M. |last3=Forder |first3=P. M. |last4=Clements |first4=M. S. |last5=Armstrong |first5=B. K. |journal=Mutagenesis |volume=27 |issue=5 |pages=609β614 |pmid=22547344 |doi-access= }}</ref> ====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 and exposed thousands of Taiwanese to gamma radiation levels of up to >1000 times background (average 47.7 mSv, maximum 2360 mSv excess cumulative dose). The radioactive contamination was discovered in 1992. A seriously flawed 2004 study compared the building's younger residents with the much older general population of Taiwan and determined that the younger residents were less likely to have been diagnosed with cancer than older people; this was touted as evidence of a radiation hormesis effect.<ref name="Hwang1">{{cite journal |doi=10.1080/09553000601085980 |title=Cancer risks in a population with prolonged low dose-rate Ξ³-radiation exposure in radiocontaminated buildings, 1983β2002 |year=2006 |last1=Hwang |first1=S.-L. |last2=Guo |first2=H.-R. |last3=Hsieh |first3=W.-A. |last4=Hwang |first4=J.-S. |last5=Lee |first5=S.-D. |last6=Tang |first6=J.-L. |last7=Chen |first7=C.-C. |last8=Chang |first8=T.-C. |last9=Wang |first9=J.-D. |last10=Chang |first10=W. P. |journal=International Journal of Radiation Biology |volume=82 |issue=12 |pages=849β858 |pmid=17178625 |s2cid=20545464 }}</ref><ref name="Chen-Age">{{Cite conference | last = Chen | first = C. Y. | author2=Y. J. Chen | title = The Social Migration Effect Toward Population Aging-The Application of Perston's Rate of Change of a Population's Mean Age Improvement Model in Taiwan | conference = The 23rd Conference of the European Network for Housing Research | access-date = 2012-05-09 | year = 2011 | url = http://140.116.240.46/ISAD/files/P28941029-a.pdf }}</ref> (Older people have much higher cancer rates even in the absence of excess radiation exposure.) In the years shortly after exposure, the total number cancer cases have been reported to be either lower than the society-wide average or slightly elevated.<ref name=":0">{{Cite book |title=Casarett & Doull's essentials of toxicology |date=2021 |author1=Curtis D. Klaassen |author2=John B. Watkins, III |isbn=978-1-260-45229-7 |edition=4th |publisher=McGraw-Hill Education |location=New York |page=459 |oclc=1159605376}}</ref><ref>{{Cite book |title=Environmental pollutant exposures and public health |date=2021 |publisher=CPI Group |isbn=978-1-83916-043-1 |editor-last=Harrison |editor-first=Roy M. |location=London |page=50 |oclc=1204222461 |quote=Cancer incidence in a cohort of ~6250 people has been studied, and marginally raised levels of cancer in relation to assessed doses have been reported, although there are a number of uncertainties in the student, including lack of control for confounding factors such as smoking}}</ref> Leukaemia and thyroid cancer were substantially elevated.<ref name="Hwang1" /><ref name=":0" /> When a lower rate of "all cancers" was found, it was thought to be due to the exposed residents having a higher [[socioeconomic status]], and thus overall healthier lifestyle.<ref name="Hwang1" /><ref name=":0" /> Additionally, Hwang, et al. cautioned in 2006 that [[leukaemia]] was the first cancer type found to be elevated amongst the survivors of the Hiroshima and Nagasaki bombings, so it could be decades before any increase in more common cancer types is seen.<ref name="Hwang1" /> Besides the excess risks of leukaemia and thyroid cancer, a later publication notes various DNA anomalies and other health effects among the exposed population:<ref name="Hwang2008">{{cite journal |doi=10.1667/RR0732.1 |title=Estimates of Relative Risks for Cancers in a Population after Prolonged Low-Dose-Rate Radiation Exposure: A Follow-up Assessment from 1983 to 2005 |year=2008 |last1=Hwang |first1=Su-Lun |last2=Hwang |first2=Jing-Shiang |last3=Yang |first3=Yi-Ta |last4=Hsieh |first4=Wanhua A. |last5=Chang |first5=Tien-Chun |last6=Guo |first6=How-Ran |last7=Tsai |first7=Mong-Hsun |last8=Tang |first8=Jih-Luh |last9=Lin |first9=I-Feng |last10=Chang |first10=Wushou Peter |journal=Radiation Research |volume=170 |issue=2 |pages=143β148 |pmid=18666807 |bibcode=2008RadR..170..143H |s2cid=41512364 |url=http://ntur.lib.ntu.edu.tw/bitstream/246246/246866/-1/index.html }}</ref> <blockquote> There have been several reports concerning the radiation effects on the exposed population, including cytogenetic analysis that showed increased micronucleus frequencies in peripheral lymphocytes in the exposed population, increases in acentromeric and single or multiple centromeric cytogenetic damages, and higher frequencies of chromosomal translocations, rings and dicentrics. Other analyses have shown persistent depression of peripheral leucocytes and neutrophils, increased eosinophils, altered distributions of lymphocyte subpopulations, increased frequencies of lens opacities, delays in physical development among exposed children, increased risk of thyroid abnormalities, and late consequences in hematopoietic adaptation in children. </blockquote> People living in these buildings also experienced infertility.<ref>{{Cite journal |last1=Lin |first1=C.-M. |last2=Chang |first2=W. P. |last3=Doyle |first3=P. |last4=Wang |first4=J.-D. |last5=Lee |first5=L.-T. |last6=Lee |first6=C. L. |last7=Chen |first7=P.-C. |date=March 2010 |title=Prolonged time to pregnancy in residents exposed to ionising radiation in cobalt-60-contaminated buildings |journal=Occupational and Environmental Medicine |volume=67 |issue=3 |pages=187β195 |doi=10.1136/oem.2008.045260 |issn=1470-7926 |pmid=19773284|s2cid=40448903 }}</ref> ==== Radon therapy ==== {{Further|Health effects of radon#Bathing}} 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.
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