new data included a National Research Council re-evaluation of the relative dosimetry of radon decay products in mines and homes (National Research Council 1991b). On the basis of the new data, the total (all-pathways) lifetime risk estimate for radon was changed to 7 × 10-7 per picocurie per liter. Although the total risk by pathway did not change substantially, the allocation of risk contributed by each pathway did. However, perhaps a more important component of the revised risk numbers was the inclusion of a detailed uncertainty analysis (EPA 1995). Appendix F provides the committee's summary and evaluation of the EPA uncertainty analysis. Our focus is on how the explicit analysis of uncertainty and variability can influence the process for setting standards and for setting priorities for intervention and future research.

In reviewing the EPA uncertainty and variability analysis, the committee found that the EPA approach demonstrated innovative methods and consistency with emerging policies, and provided an adequate characterization of the uncertainty in cancer risk factors. However, our analysis of and proposed revision in the risk models will result in changes in both the magnitude and the uncertainty ranges of some of the parameters in the EPA model. In particular, the magnitude of the risk associated with radon ingestion has been lowered in the committee's analysis, but the resulting uncertainty range is contained within the uncertainty range used by EPA (1995).

Although the EPA analysis was an important initial effort at uncertainty assessment, results of that analysis can be misleading. In particular, because the variability in the risk-per-dose factors cannot be specified, the variability in risk derived from this analysis includes only variability in exposure and not the actual variability in cancer risk among the population. Moreover, in reviewing the EPA models and uncertainty analysis, the committee observed that implicit in the development of this model is the assumption that the risk factor is independent of variability in the unit dose factor. That assumption requires that the radon-gas dosimetry be independent of the breathing rate—an assumption that is not consistent with the key issues of inhalation dosimetry described in chapter 5 of this report.

The committee had a particular interest in the radon-ingestion risk model because the ratio of ingestion risk to inhalation risk is an important component of the multimedia approach to radon risk management. The committee observed that the EPA risk assessment used an appropriate approach to obtain the uncertainty factor for the population cancer risk associated with ingestion of radon in water. EPA assigned a geometric standard deviation of 2.4 to the risk factor for ingestion-cancer risk. That implies that there is a 68% likelihood that the actual risk factor is within a range of roughly 2.4 times lower to 2.4 times higher than the estimated risk and a 95% likelihood that the actual risk factor is within a range of roughly 6 times lower to 6 times higher than the estimated risk factor. That uncertainty range reflects parameter uncertainty associated with the risk model used by EPA. However, the model is constrained by the assumption that radon is

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