population, the committee needed to make a set of assumptions on the following key issues.

Lung Dosimetry of Radon Progeny

Physical and biologic differences between the circumstances of exposures of male miners working underground and of men, women, and children in their homes could lead to differing doses at the same exposures. The committee estimated the value of a dimensionless parameter, termed the "K factor" in prior reports, that characterizes the comparative doses to lung cells in homes and mines for the same exposure. Using a model to estimate the dose to the cells in the lung, and incorporating new information on the input parameters of the model, the committee found that the doses per unit exposure in mines and homes were essentially the same. Thus, K is calculated to be about 1 for men, women and children (age 10 years), and slightly above K = 1 for infants (age 1). Consequently, a value of 1 was used in making the risk projections.

Extrapolation of Risks at Higher Exposures to Lower Exposures

Average exposures received by the miners in the epidemiologic studies are about one order of magnitude higher than average indoor exposures, although the lowest exposures of some miners overlap with some of the highest indoor exposures. To estimate risks of indoor radon exposures, it is thus necessary to make an assumption about the shape of the exposure-risk relationship across the lower range of the distribution of radon exposures.

The committee selected a linear-nonthreshold relationship relating exposure to risk for the relatively low exposures at issue for indoor radon. This assumption has significant implications for risk projections. Support for this assumption came primarily from the committee's review of the mechanistic information on alpha-particle-induced carcinogenesis. Corroborating information included evidence for linearity in the miner studies at the lower range of exposures, and the linearity and magnitude of risk observed in the meta-analysis of the case-control studies, which was fully consistent with extrapolation of the miner data. Although a linear-nonthreshold model was selected, the committee recognized that a threshold—that is, a level of exposure with no added risk—could exist and not be identifiable from the available epidemiologic data.

Exposure Rate

At higher exposures, the committee found evidence in the miner data of an inverse exposure-rate effect. Theoretical considerations suggested that the inverse exposure-rate effect found in the miner data should not modify risks for typical indoor exposures. Consequently, the exposure-rate effect in the lowest

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