instantaneously and uniformly distributed in the stomach after ingestion. The uncertainty range in the EPA results does not appear 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 (in chapter 4) is essentially contained within the 99% confidence interval suggested by the EPA results. This reveals that the EPA did not underestimate their confidence interval. However, this committee's uncertainty range is at the lower bound of the EPA range, suggesting the likely upward bias of the EPA risk estimate.

The EPA risk assessment made no effort to assess the contribution of soil relative to that of water to indoor radon levels. Therefore, the study affords little input to the analysis of how any standard or policies can affect the risk associated with all radon exposures. Uncertainty of the ingestion: inhalation risk ratio is an important factor that was not addressed in the EPA analysis.

Issues In Uncertainty Analysis For Radon

For an agent like radon, which is ubiquitous, total exposure might reflect concurrent contacts with multiple media instead of continuous or multiple contacts with a single medium. Multimedia pollutants give rise to the need to address many types of ''multiples'' in the quantification or measurement of exposure and dose, such as the multiple media themselves (air, water, soil): multiple exposure pathways (or scenarios), multiple routes (inhalation, ingestion, and dermal), and multiple exposure target tissues for dose and effect.

There are many sources of uncertainty and variability in the process of exposure and human-health assessment. The variability and many of the uncertainties cannot be reduced. One common approach to addressing uncertainty in exposure and risk assessments is contrary to the accepted principles of decision-making in the presence of uncertainty. This is the practice of compounding upper-bound estimates as a means of basing decisions on a highly conservative estimate of exposure. Such compounding of upper-bound estimates leaves a decision-maker with no flexibility to address margins of error, to consider reducible versus irreducible uncertainty, to separate individual variability from true scientific uncertainty or to consider benefits, costs, and comparable risks in the decision-making process. Because the compounding of conservative estimates does not serve the exposure-assessment process well, there is a growing effort to include uncertainty analyses in the risk assessment process. EPA has taken the latter uncertainty-analysis approach in its risk assessment for radon, and the committee believes that it is important to continue this precedent.

For human populations, total-exposure assessments that include time and activity patterns and microenvironmental data reveal that an exposure assessment is most valuable when it provides a comprehensive view of exposure pathways and identifies major sources of uncertainty. In any issue involving uncertainty, it is important to consider a variety of plausible hypotheses about the world, to

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