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Executive Summary The Environmental Protection Agency (EPA) Office of Radiation Programs (now the Office of Radiation and Indoor Air) asked the National Research Council to consider recently published data on the risks associated with human exposure to radon. EPA has given high priority to re-evaluation of risks to ensure that new data, including data on exposure in homes and schools, can be expeditiously incorporated into risk evaluation. This report was prepared by the Committee on Health Risks of Exposure to Radon: BEIR VI in the Board on Radiation Effects Research (BRER) of the Research Council's Commission on Life Sciences, which was charged, in Phase I, with gathering evidence that had accumulated on radon and lung cancer since two prior reviews by BRER committees and determining whether the new information justified a comprehensive study which would constitute Phase II. In the committee's judgment, information that has become available since publication of the 1988 Health Risks of Radon and Other Internally Deposited Alpha-Emitters: BEIR IV and the
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1991 Comparative Dosimetry of Radon in Mines and Homes makes it desirable and feasible to proceed with Phase II-a comprehensive reanalysis of health risks associated with radon. The committee based this judgment on The completion of a joint analysis by Jay Lubin and his colleagues at the National Cancer Institute of data from 11 cohorts of underground miners (including 68,000 miners who experienced over 2,780 lung cancer cases in comparison with the 22,190 miners who experienced 360 lung cancer deaths in the four cohort studies of BEIR IV) that offers a new risk model and strengthens the basis for quantifying indoor (residential) radon as a public-health problem. New experimental and epidemiologic evidence of an effect of exposure rate on -particle carcinogenesis, such as from the laboratories of Richard Miller, David Brenner, Eric Hall, Helen Evans, and Mortimer Elkind. Reports of several completed studies, including the recent report of a large study in Sweden by Goran Pershagen and colleagues and the projected completion and publication of additional case-control studies of residential exposure and lung cancer in Europe and the United States during the next 2 years. New evidence from the study of miners that the interaction of smoking and radon might be less than multiplicative. Further information relevant to the dosimetry of radon in mines and homes. This information emerges from new instrumentation for field measurements of the concentration and activity-size distributions of radon progeny developed in the laboratory of Philip Hopke and the application of noninvasive methods for monitoring ventilation in the field by Jonathan Samet and others. New evidence of the potential importance of other factors in
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mine atmospheres, such as the presence of arsenic in Chinese tin mines and silica in underground uranium mines. The committee concluded that new evidence could lead to the development of a risk model for radon and lung cancer that would be substantially different from that developed by the BEIR IV committee. An opportunity now exists for the development of multidisciplinary models of radon carcinogenesis that incorporate new concepts from cellular and molecular studies. Evaluation of new information concerning the molecular changes occurring in radon-induced cancer and the correlation of these changes to the nature of radon-induced chromosomal aberrations, mutations, and DNA lesions should be informative as to the mechanisms involved in the induction of lung cancer and should be useful in the development of a risk model. Further key uncertainties in the previous estimates and models might be reduced and should be considered formally in a Phase II study. The finding of an inverse dose-rate effect in the miner data establishes a need for extrapolating this effect to lower dose rates. The committee recognizes that much epidemiologic and experimental work is going on and that not all of it is likely to be completed during the time frame for Phase II. Nevertheless, the findings since the earlier BRER-committee reports and those to come soon are sufficient to warrant evaluation and synthesis. With respect to radiation biology and carcinogenesis, the committee recommends that a Phase II study Evaluate experimental evidence on inverse dose-rate relations and the implications of this evidence for domestic radon risks. Attempt to correlate risks of radon-induced lung tumors with
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those of tumors induced by external exposures to low-LET radiations. Examine in more detail the induction and repair of molecular changes after exposure to particles. With respect to exposure-dose relations, a Phase II study could Gain access to additional information that is relevant to radon dosimetry from the use of noninvasive methods for monitoring ventilation and from an improved model. Use activity-weighted size-distribution data that have become available on residences. Use the recent data on growth of particles from various sources typically found in homes. Use biologic dosimetry to reduce the uncertainties associated with the exposure-dose relationship and apply the reduced uncertainties in various aspects of lung dosimetry to risk calculations. With respect to studies of miners, a Phase II study could Critique the recently completed pooled analyses of 11 cohorts and whatever new risk models emerge from them and use the database to suggest additional analyses. Model the pooled data with emphasis placed on biologically driven modeling. Formally evaluate sources of exposure error in the miner cohorts and the consequences of these errors for risk estimation and risk assessment. With respect to studies of lung cancer in the general population, a Phase II study could
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Evaluate and interpret the results of completed case-control and ecologic studies, including evaluation of their limitations and uncertainties. Determine the appropriate role of case-control studies in developing a risk model of residential exposure to radon. Make general recommendations regarding the potential role of case-control studies that are not yet complete. Estimate the statistical power for various end points of the completed studies as a group and identify the expected upper and lower confidence limits according to the assumptions of both the recommended model and alternative models. Estimate the potential of the completed studies for providing information on the modifying effects of smoking and other factors, taking into account uncertainties in exposure estimates. Make recommendations on the desirability of initiating new case-control or ecologic studies of residential radon exposure. The previously cited Phase II committee tasks include assignments that were included in the original project, as well as new tasks identified by the committee. The committee also modified some of the original projected charges and now recommends that, with respect to data from studies of miners, a Phase II study Assess the validity and scientific reliability of analytic approaches used by various investigators. Examine in detail the interaction between radon exposure and cigarette-smoking on the basis of data on U.S. and other miners. Re-examine the effect of the rate of exposure to radon on the incidence of lung cancer. Critically examine exposure estimates for miner cohorts and reassess the consequences of the exposure rates.
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Propose, in light of the Phase I findings, a mathematical model of risk based on more complete and up-to-date U.S. and international miner data. Assess the role of arsenic, silica, and other contaminants in mines on the consequences of exposure. Examine the uncertainties associated with the miner studies. With respect to the analysis of data from studies of residential exposure, the committee modified certain proposed tasks so that a Phase II study could also Critically review studies and comment on their strengths and weaknesses and their current and future roles in risk assessment. If EPA and BRER have agreed that it is feasible, update the assumptions and estimates in the Research Council report that compared miner and home dosimetry, focusing on recent data on such physical and biological factors as aerosol size distribution, ultrafine fraction, equilibrium fraction, and hygroscopicity and deposition of radon daughters in the respiratory tract. If EPA and BRER have agreed that it is feasible, examine models of carcinogenesis resulting from the combined effects of radon exposure and cigarette-smoking. Test and possibly revise models in light of available residential data. Consider the contribution of radon-220 to risk in mines and homes. Examine the effects of age, sex, and smoking on radon-associated risk. Incorporate concepts from cellular and molecular biology into models for risk assessment.
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The committee is concerned that the original time of 24 months for a Phase II study is too short to complete all tasks suggested here. It recommends that the Phase II study be extended by 10 months for a total of 34 months to end December 31, 1996. This recommendation is not driven by the expectation that additional epidemiologic studies in homes will be completed, but rather by the belief that it would take more time to identify and obtain data and complete the analysis required. Finally, the present committee recommends that a Phase II committee include at least 12 members. The expertise that should be represented includes radiation biology, biophysics, aerosol science, cellular and molecular biology, epidemiology, biostatistics, carcinogenesis, lung pathology, risk assessment (including uncertainty analysis), and animal studies.
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HEALTH EFFECTS OF EXPOSURE TO RADON: Time for Reassessment?
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Representative terms from entire chapter: