• Assess the role of arsenic, silica, and other contaminants in mines on the consequences of exposure.

• Examine the uncertainties associated with the miner studies.

Analysis of Data from Studies of Residential Exposure

• Critically review studies and comment on their strengths and weaknesses and their current and future roles in risk assessment.

• If EPA and BRER agree that 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 biologic factors as aerosol size distribution, ultrafine fraction, equilibrium fraction, and hygroscopicity and deposition of radon daughters in the respiratory tract.

• 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.

Most of those projected deaths are attributable to the lower range of exposures, well below the exposures and exposure rates sustained by the miners, so there is substantial uncertainty as to the total burden of lung-cancer resulting from indoor radon. The agency's action guideline of 148 Bqm-3 (4 pCiL-1) is also in this range, as are the action levels set by other countries. The BEIR VI committee therefore characterized the certainty that could be attached to risk estimates made with its model, moving from a high level of confidence in estimates at exposures comparable with those received by the underground miners to a substantially lower degree of confidence in estimates related to typical indoor exposures.

CRITICAL ISSUES

The BEIR VI committee identified 6 issues deemed critical in characterizing the risks associated with indoor radon. Each deals with a point of uncertainty in formulating a risk model and estimating the risk posed to a population by indoor radon. In the review of the evidence, those critical issues served as points of synthesis for identifying the limits of current evidence for risk assessment. The issues are briefly described here.

Extrapolation from Higher to Lower Exposures

Early in its work, the committee recognized that the evidence on quantitative risks from studies of underground miners would remain the principal basis for estimating the risk associated with indoor radon. Most of the evidence from the



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