express risk information and its related uncertainties to broad, nontechnical audiences who may have little contact time with the subject. For the various reasons discussed, communication about the uncertainty of the risks calculated here will remain as one of the most challenging of endeavors.

Discussion And Recommendations

The committee identified the issues of uncertainty and variability as likely to have important scientific and policy implications for the health effects attributable to radon in drinking water. One overarching issue is how uncertainty and variability can affect the reliability of estimated health effects of a given standard and the health benefits of alternative standards and control strategies.

The approach used in the EPA uncertainty analysis, which is summarized in Appendix F, was fully consistent with emerging EPA guidelines and protocols for uncertainty analysis. Moreover, the EPA document, which transmitted these results, has defined the state of the art for uncertainty analysis within EPA. The explicit separation of uncertainty and variability and the resulting two-dimensional Monte Carlo analysis express uncertainty and variability separately on the same graph. Those methods are innovative and useful for understanding the distribution of risk among populations and the impact of various mitigation strategies.

In reviewing the EPA effort, 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. The uncertainty range used by EPA reflects parameter uncertainty associated with the EPA risk model. However, this model is constrained by the assumption that radon is instantaneously and uniformly distributed in the stomach after ingestion. The uncertainty range in the EPA results does not appear to have been set up to reflect the bias and uncertainty associated with this assumption. Nevertheless, the 95% confidence interval of the uncertainty range developed by the committee for this report is essentially contained within the 99% confidence interval suggested by the EPA results. That suggests that the EPA did not underestimate its confidence interval. However, the committee's uncertainty range is at the lower bound of the EPA range, and this suggests the likely upward bias of the EPA risk estimate.

Because current risk models must rely on epidemiologic relationships, it is difficult to accurately represent individual and subpopulation variations in susceptibility. However, the study of molecular and cellular mechanisms of radiation-induced cancer brings to the risk-assessment process important insight about the nature and magnitude of the uncertainties associated with the dose-response models discussed in this report. In particular, the introduction of biophysical cellular models to the risk-assessment process reveals both the limited reliability and the potential bias of the existing models.

One critical issue in defining the potential risk associated with waterborne



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