support a fully quantitative characterization of the uncertainty and variability in some of these factors.

The BEIR VI committee focused its quantitative uncertainty analysis on the population attributable risk (AR) associated with radon. Because the AR is a measure of population risk rather than of individual risk, the variability among individuals was not quantified in the BEIR VI analysis. The uncertainty analysis was applied in BEIR VI to the BEIR VI committee's two preferred models—the exposure-age-concentration model and the exposure-age-duration model. BEIR VI uncertainty factors reflecting only uncertainty in the parameters of the BEIR VI risk models provide the geometric range of uncertainty associated with the BEIR VI model. For males, the ratio of the high to low values in the 95% confidence interval of AR is 2.7 for the exposure-age-concentration model and 2.3 for the exposure-age-duration model. The ratios are similar for females. On the assumption that those uncertainty ranges can be represented by log normal distributions, the BEIR VI committee derived from these ratios a GSD of approximately 1.3 for the exposure-age-concentration model and 1.2 for the exposure-age-duration model. From those results, we select an uncertainty factor of 1.3 to be applied to the inhalation risk factor for situations when the equilibrium factor used by BEIR VI applies.

There remains inadequate information to measure and characterize inter-individual variability in the inhalation-risk models that are available for this study. As a result, the cancer-risk models for inhalation described in the BEIR VI report are characterized only in terms of uncertainty, not of variability. However, when the AR is used as a measure of population rather than individual or subpopulation risk, the inter-individual variability in cancer risk is effectively averaged out in the analysis. A problem arises when population-based risk factors are applied to small populations or individual households (such as a small number of houses with high radon). In such cases, the failure to know the appropriate risk factors for this small population—where interindividual, variability may not average out—constitutes an important uncertainty.

Ingestion Risk per Unit of Water Concentration for Ingestion

One of the important uncertainties in our analysis involved the issue of radon gas behavior in the stomach. During the information-gathering phase of our analysis, the committee heard conflicting information about the potential of inert gases, such as radon, to be transferred from the contents of the stomach through the mucus layer and to the stem cells surrounding the stomach. The extent to which radon is transferred into and through the stomach wall has a large effect on the predicted radiation dose associated with water ingestion. Previous efforts were based on assumptions that either there was no diffusion through the stomach wall or that the entire stomach wall contains radon at the same concentration as the stomach contents (see chapter 4 for more discussion). These bounding as-



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