directly measured, to the low end where risks must be extrapolated; the degree of uncertainty at the low end of the range has contributed to persistent questioning of the appropriateness of directing risk-management strategies uniformly across the full range of population exposures. In the United States, for example, the Environmental Protection Agency (EPA) has called for the voluntary measurement of indoor radon concentration in single-family residences and mitigation of radon in homes with average annual concentrations above 148 Bqm-3 (4 pCiL-1) (USEPA 1992c).

The BEIR VI committee thus faced the task of characterizing the risks associated with indoor radon across the full range of exposures and providing an indication of the uncertainty to be attached to risk estimates across that range. At the higher end of the range, exposures of the general population overlap those received by miners, and the extensive findings on risks in miners provide a reasonably accurate picture of the risks likely to be sustained by the population. At the lower end of the range, risks are estimated by extrapolating from the miner data and are consistent with the results of a meta-analysis of domestic-exposure studies (Lubin and Boice 1997). The extrapolation requires assumptions about the relationship between exposure to radon and lung-cancer risk and a careful exploration of the comparative doses from alpha particles delivered to the lung in the mining and indoor environments. The extrapolation also requires assumptions on the potential modifying effects of cigarette-smoking, age at exposure, and sex. These assumptions are a source of uncertainty in estimates of the risk of indoor radon, but choices can be supported by epidemiologic and experimental data. In preparing this report, the BEIR VI committee considered the entire body of evidence on radon and lung-cancer, integrating findings from epidemiologic studies with evidence from animal experiments and other lines of laboratory investigation.

Radon, of course, is only one of the causes of lung-cancer (Table 1-1). In fact, the epidemic of lung-cancer in the United States and many other countries largely reflects trends in cigarette-smoking, the dominant cause of lung-cancer (USDHHS 1989; Burns 1994; Mason 1994). Of the approximately 170,000 lung-cancer cases in 1996, most will be in cigarette smokers and thus avoidable in principle through smoking prevention and cessation. Synergism between radon and tobacco smoking implies heightened risks from radon in ever-smokers but cases caused by the joint effect of smoking and radon can be prevented by avoidance of smoking. Thus, risk projections of the number of lung-cancer cases attributable to radon should be interpreted with acknowledgment that most lung-cancer cases and deaths can be prevented by eliminating smoking.

PRIOR REPORTS ON THE RISK ASSOCIATED WITH RADON

There have been numerous assessments of the risks posed by radon, in both mining and indoor environments. Some have been generated by previous BEIR



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