radon is the rate of diffusion of radon through the stomach wall toward the stem cells surrounding it. This rate is critical in defining the relative importance of the risks associated with waterborne radon compared with the risks associated with indoor airborne radon.

There remains insufficient information to quantify interindividual variability in the cancer-risk models that are available. As a result, the cancer-risk models for inhalation described in this report are characterized only in terms of uncertainty, not variability. In contrast, radon exposure data—including concentrations in water and in indoor air, transfer factors, and equilibrium factors—have been collected with sufficient resolution to explicitly represent population variability within the United States. However, uncertainties in the parameters—that is, distributional moments—describing this variability are not yet known with precision. The uncertainty in the parameters describing exposure variability can be measured with methods used by EPA (1995) and Rai and Krewski (1998), which use combined uncertainty and variability analysis to characterize the relative importance of the two sources of variance in risk estimate.