Samet and Spengler 1991). The major source of the radon in a home is the soil gas that enters from beneath and around the structure (NCRP 1984). Radon-contaminated water and radium-rich building materials can also contribute radon. Under some circumstances, water can contribute a substantial amount of the radon in air, but water is a relatively minor source of population exposure to radon progeny.

Surveys of indoor radon concentrations in the United States and many other countries have shown that radon is ubiquitous indoors, typically at a concentration only one hundredth to one-tenth that found in the underground mines and shown to be associated with lung-cancer. For the survey conducted by the U.S. Environmental Protection Agency, the distribution of concentrations is roughly lognormal; the arithmetic mean concentration is about 46.25 Bqm-3 (1.25 pCiL-1) (Figure 1-4). On the basis of the survey information, mean exposure of the general population is about 0.0007 Jhm-3 (0.2 WLM) per year or about 0.049 Jhm-3 (14 WLM) in a lifetime (NCRP 1984, 1991).

THE COMMITTEE'S APPROACH

The BEIR VI committee was constituted after a BEIR VI phase 1 committee determined that sufficient evidence had become available since the 1988 publication of the BEIR IV report to justify a new study (NRC 1994). The charge extended to the BEIR VI committee was broad (Table 1-2). Its principal goals were to examine evidence of effects of low-level exposure to radon progeny and to develop a mathematical model for the lung-cancer risk associated with radon. In developing this model, the committee was to address key uncertainties, including the combined effect of smoking and radon, the effect of exposure rate, the effects of exposures to agents other than radon in the mines, and the consequences of errors in exposure estimates. The committee was also asked to review relevant evidence from radiobiologic studies, to reassess exposure-dose relations, and to evaluate the potential utility of the case-control studies of indoor radon.

The multidisciplinary BEIR VI phase 2 committee worked in topic-oriented groups that addressed exposure and dosimetry; molecular and cellular aspects of radon carcinogenesis, including the findings of in vitro approaches and animal studies; epidemiologic studies of miners; case-control studies in the general population; and risk modeling. When the committee began its work, it was recognized that the data on the 11 cohorts of miners would be essential for the development of a new risk model; the committee obtained the cooperation of the principal investigators for the individual cohort studies so that additional analyses of the data could be undertaken to develop a risk model. During the course of deliberations by the committee, it became apparent that a new meta-analysis of exposure in homes was powerful enough to yield useful risk estimates associated with domestic exposures.

The committee also recognized that its work could not be artificially sepa-



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