sumptions lead to disparate results regarding the estimated risk. To confront this issue of uncertainty better, the committee elected to develop a stomach model that allowed exploration of a range of diffusion conditions in the stomach and a model characterizing the behavior of radon dissolved in blood and body tissues. Once the radon has entered the blood, through either the stomach or the small intestine, it is distributed among the organs of the body according to the blood flow to the organs and the relative solubility of radon in the organs and in blood. Radon dissolved in blood that enters the lung will equilibrate with air in the gas-exchange region and is removed from the body; this model is described in detail in chapter 4 and in Appendixes A and B. The dosimetry model indicates that any radon absorbed in the stomach results in a higher risk per Bq than in the intestines. The need for the new models also arose from the lack of directly applicable experimental observations and from limitations in the extent to which one can interpret results of existing studies. Risk relevant to ingestion of radon in water depends heavily on the extent to which radon penetrates the stomach wall. With the new model, the committee was able to conduct a broader set of sensitivity and uncertainty analyses. The committee notes that limitations in the model structure with regard to the relative locations of the microvasculature structure (and its fractional capture of the diffusing radon) and stem cells are the major sources of uncertainty. The diffusion of radon within the stomach wall was modeled to determine the expected time-integrated concentration of radon at the depth of the cells of risk in the stomach wall. The committee's baseline (or median) estimate is based on a radon diffusion coefficient of 5 × 10-6 cm2/s. Using this value yielded an integrated radon concentration in the wall that is about 30% of the concentration in the contents of the stomach. Sensitivity and uncertainty analyses with this model helped the committee to bracket the range of risks that could plausibly be associated with ingestion of radon in water. The committee estimated that the diffusion coefficient in the stomach could have a plausible lower bound of 10-7 cm2/s and a plausible upper bound of 10-5 cm2/s (the diffusion coefficient of radon in water). That range of diffusion coefficients results in a median estimate of risk of 2.0 × 10-9 per becquerel per m3. However, the "no diffusion" and "saturated diffusion" limits in the calculations which were carried out were not intended as realistic limits and thus should not be interpreted as representing the range of uncertainty in the ingestion risk. This range was selected to reflect the current literature, in which some authors believe that diffusion is not a viable mechanism, others avoided the whole issue, and still others endorsed it with the intent of being conservative when setting a radiation-protection quantity. The committee's calculations in the extremes were largely for the purpose of illustrating the significance of this mechanism; that is, they were bounding calculations.

The committee has not carried out a detailed uncertainty analysis for the ingestion-risk model described in chapter 4. However, it has made some subjective judgments regarding uncertainties on the basis of what has been done and

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