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Internalized Α-Particle Emitting Radionuclides

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Internalized Α-Particle Emitting Radionuclides INTERNALIZED α-PARTICLE EMITTING RADIONUCLIDES Internalized radionuclides that emit α-particles were considered by a previous IARC Working Group in 2000 (IARC, 2001). Since that time, new data have become available, these have been incorporated into the Monograph, and taken into consideration in the present evaluation. 1. Exposure Data The epidemiological evidence on the cancer risks from radon is derived largely from cohort See Section 1 of the Monograph on X-radiation studies of underground miners that had been and γ-radiation in this volume. exposed to high levels of radon in the past. More recently, a series of case–control studies of lower exposures to residential radon have also been 2. Cancer in Humans conducted. The previous IARC Monograph on radon IARC (1988) states that radon is a cause of lung 2.1 Radon cancer in humans, based on clear excess lung Radon is a natural radioactive gas produced cancer rates consistently observed in under- by the decay of uranium and thorium, which ground miners, and elevated lung cancer risks are present in all rocks and soils in small quan- seen in experimental animals exposed to radon. tities. There are several isotopes of radon, the In a subsequent evaluation by IARC (2001), addi- most important of which are 222Rn (produced tional epidemiological evidence of an increased from 238U) and 220Rn (produced from thorium). lung cancer was also seen in case–control studies 220Rn is also known as thoron because of its of residential radon. Although results from the parent radionuclide. In the United Kingdom, it 13 case–control studies available at that time has been shown that 220Rn delivers much smaller were not conclusive, the Working Group noted doses to the public in indoor environments than that the risk estimates from a meta-analyses of 222Rn (The Independent Advisory Group on eight such studies were consistent with estimates Ionising Radiation, 2009). Unlike 222Rn, 220Rn is based on the underground miner data (Lubin & not formed during the radioactive decay of 238U, Boice, 1997). and is hence not present at appreciable levels in In a detailed evaluation of the health risks uranium mines. of radon by the Committee on the Biological Effects of Ionizing Radiation (BEIR) within the 241 IARC MONOGRAPHS – 100D US National Research Council (BEIR IV, 1988), 2.1.1 Occupational studies of underground it was also reported that radon is a cause of lung miners cancer in humans. An important aspect of this (a) Early observations of lung disease in miners work was the development of risk projection models for radon-related lung cancer, which Underground mining was the first occupa- provides estimates of the lung cancer risk asso- tion associated with an increased risk of lung ciated with residential radon, depending on age, cancer. Metal ores were mined in the Erz moun- time since exposure, and either concentration or tains (a range between Bohemia and Saxony), in duration of exposure. Schneeberg from the 1400s and in Joachimsthal In an effort to synthesize the main epidemio- (Jachymov) from the 1500s. As early as the 16th logical findings and assist in the evaluation of the century, Georg Agricola, in his treatise ‘De re lung cancer risks associated with occupational Metallica’, described exceptionally high mortality and environmental exposure to radon, several rates from respiratory diseases among miners combined analyses of the primary raw data in the Erz mountains. The disease in miners from studies of radon and lung cancer have been was recognized as cancer in 1879 by Harting & conducted. Several combined analyses of epide- Hesse (1879). This report provided clinical and miological data from 11 cohorts of underground autopsy descriptions of intrathoracic neoplasms miners have been conducted (BEIR IV, 1988; in miners, which were classified as lymphosar- Lubin et al., 1994; Lubin & Boice, 1997; BEIR coma. During the early 20th century, histopatho- VI, 1999). Howe (2006) conducted a combined logical review of a series of cases established that analysis of data from three cohorts of uranium the malignancy prevalent among miners in the miners from Canada, and Tomášek et al. (2008) Erz mountains was primary cancer of the lung conducted a combined analysis of Czech and (Arnstein, 1913; Rostocki, 1926). Many authors French uranium miners. Combined analyses offered explanations for this excess including of epidemiological data from seven North exposures to dusts or metals in the ore (particu- American case–control studies of residential larly arsenic). In 1932, Pirchan and Sikl suggested radon and lung cancer (Krewski et al., 2005), 13 that radioactivity was the most probable cause European studies (Darby et al., 2005, 2006), and of the cancers observed in Jachymov (Pirchan & two studies from the People’s Republic of China Sikl, 1932). (Lubin et al., 2004) have also been conducted. Cancers other than lung cancer, notably (b) Cohort studies haematopoietic lesions, have been investigated The first epidemiological evidence of an in some of the cohort studies of miners. Case– increased lung cancer risk among underground control studies of residential radon and child- miners exposed to radon in the Colorado Plateau hood cancers, including leukaemia, have also was given by Archer et al. (1962). Subsequent anal- been conducted. Ecological studies of environ- yses of this cohort were conducted by Wagoner mental radon and the risk of lung and other et al. (1964, 1965) as additional lung cancer cases cancers have been reported, but these are less accrued; the latter analysis was the first to relate informative than the cohort and case–control lung cancer risk to cumulative exposure to radon studies discussed previously (IARC, 2001). progeny in terms of working-level months (WLM). Stram et al. (1999) conducted detailed analyses of the effects of uncertainties in radon exposures within this cohort on radon-related lung cancer risk estimates. Another early study reported 242 α-Particle emitters lung cancer risk in Canadian fluorspar mines in exposure concentration or duration of exposure Newfoundland, where substantial amounts of (see Table 2.2 available at http://monographs. water seeping through the mines contain radon iarc.fr/ENG/Monographs/vol100D/100D-04- gas (de Villiers, 1966). The first statistical study Table2.2.pdf). The previous risk model developed on the incidence of lung cancer among uranium by the BEIR IV committee did not consider expo- miners from former Czechoslovakia (the Czech sure concentration or duration. The BEIR VI risk Republic) was published in 1966 by (Rericha models indicated that lung cancer risk decreased et al., 1966), followed by results on autopsy-veri- with time since exposure and age; for a fixed fied lung cancer cases Horacek,( 1968). The first cumulative exposure, the risk decreased with epidemiological study in uranium miners from increasing exposure concentration (reflecting an former Czechoslovakia (the Czech Republic) inverse exposure–rate effect), and increased with was initiated in the late 1960s, with first results duration of exposure. reported shortly thereafter Sevc( et al., 1971). In Another pooled analysis was conducted in contrast to other epidemiological studies, there a joint cohort of Czech and French uranium were hundreds of radon measurements per year miners, including a total of 10100 miners and in every mine. As of now, cancer risks in 19 574 lung cancers (Tomášek et al., 2008). Cohort cohorts of underground miners exposed to radon members were subject to relatively low levels of have been investigated (see Table 2.1 available at radon exposure (mostly below 4 working-level http://monographs.iarc.fr/ENG/Monographs/ (WL)); exposure measurements were available for vol100D/100D-04-Table2.1.pdf). In each of these over 96% of the total exposure time experienced cohorts, occupational exposure to radon decay by individuals in this joint cohort. The effect products was associated with increased lung of the quality of the exposure data in this joint cancer risk. study was analysed by distinguishing exposures To increase statistical power, particularly based on measurements from those that were in quantifying the modifying effect of different estimated or extrapolated. If exposure quality is factors related to time or age, attempts were made not accounted for, the estimated ERR/WLM is to pool individual data from related studies for substantially underestimated by a factor of 3.4 in the joint estimation of risk and the evaluation of the French study; however, effect modification by modifying factors. The first such analyses were exposure quality was not observed in this study conducted by the BEIR IV committee (BEIR with relatively low annual exposures, for which IV, 1988), and included a combined analysis of measurements were almost always available. The three studies of uranium miners in the Colorado term ERR/WLM quantifies the increased in risk Plateau, USA, the Eldorado mine in Ontario, per exposure in working-level months. More Canada, and Swedish iron miners in Malmberget. specifically, WLM is a time-integrated expo- By building on initial work by Lubin et al. sure measure, and it is the product of the time (1994, 1995) and Lubin & Boice (1997), a subse- in working months (170 hours) and working- quent report by the US National Research level. One WL equals any combination of radon Council (Lubin, 2003) extended the combined progeny in 1 litre of air that gives the ultimate analysis to encompass 11 cohorts of underground emission of 130000 MeV of energy of α-particles. miners (see Table 2.1 on-line). An important Consequently, 1 WLM corresponds to 2.08 x 10−5 aspect of this analysis was the development of J/m3 x 170 hours = 3.5 x 10−3 J-hours/m3. a comprehensive risk model for radon-induced Predictions of lung cancer risk were not lung cancer in underground miners taking into substantially different from those based on the account age, time since exposure, and either BEIR VI risk models (Table 2.2 on-line).
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