The committee's models of lung-cancer risk were based on analyses of data from epidemiologic studies of miners. There are undoubtedly errors in the estimates of exposures to radon progeny for the miners, and information was limited on other key exposures including cigarette smoking and arsenic. The committee could not identify any overall systematic bias in the exposure estimates for radon progeny, but random errors might have led to an underestimation of the slope of the exposure-risk relationship. Although 6 of 11 study cohorts had some smoking information, sparse information on smoking limited the committee's characterization of the combined effects of smoking and radon-progeny exposure and precluded precise estimation of the risk of radon-progeny exposure in never-smokers.

The committee's models may not correctly specify the true relationship between radon exposure and lung-cancer risk. The models assume a linear-multiplicative relationship without threshold between radon exposure and risk. While the miner data provide evidence of linearity across the range of exposures received in the mines, the assumption of linearity down to the lowest exposures was based on mechanistic considerations that could not be validated against observational data. Alternative exposure-risk relations, including relations with a threshold, may be operative at the lowest exposures. However, the committee's analysis showed that a ssumption of a threshold up to exposures at 148 Bqm-3 (4 pCiL-1) had little impact on the numbers of lung-cancer deaths theoretically preventable by mitigation of exposures above that level.

Additional sources of uncertainty in the risk projections reflect the approach used to evaluate possibly differing lung dosimetry for miners and for the general population, the limited information on cigarette-smoking, and the lack of data on risks of exposures of children and women.

The committee applied new quantitative methods for uncertainty analysis to evaluate the impact of variability and uncertainty in the model parameters on the attributable risk. Since not all sources of uncertainty could be characterized, this analysis was intended to be illustrative and not to replace the committee's more comprehensive qualitative analysis.

The quantitative analysis conducted by the committee provided limits within which the AR was considered to lie with 95% certainty. For the exposure-age-concentration model, the uncertainty interval around the central estimate of AR (14%) for the entire population ranged from about 9 to 25%. This range reflects a substantial degree of uncertainty in the AR estimate, although the shape of the uncertainty distributions indicated that values near the central estimates were much more likely than values near the upper and lower limits. For the exposure-age-duration model, the uncertainty interval ranged from 7 to 17% and was centered at about 10%. The committee's preferred uncertainty limits were obtained using a simple constant-relative-risk model fitted to the miner data below 0.175 Jhm-3 (50 WLM), which is based on observations at exposures closest to residential exposure levels. The latter analysis, which minimizes the degree of



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