best central estimate of the transfer coefficient that can now be obtained. Further, because of the uncertainty in the value of the ventilation rate and its distributional characteristics, the committee recommends that the transfer coefficient be assumed to be in the range 0.9-1.2 × 10-4. The committee is not assigning a specific uncertainty to the central estimate, but rather suggesting that it has the highest likelihood of lying within this range.

Those are not particularly large changes, suggesting that because the parameters have remained stable even as the amount of data relating to the transfer factor has increased, the uncertainty might now be lower than that suggested by the EPA analysis (EPA 1995). Further studies on transfer factors will not reduce the uncertainty substantially. The committee did not find a compelling need to go to a three-compartment model; it is not particularly more effective in characterizing either the uncertainty or the variability of the transfer-factor calculation.

An important issue is the integration of the dosimetry model with the model of radon-progeny buildup in the bathroom and in the rest of house volume after water uses. More studies on the buildup and distribution of radon progeny will have much more importance with regard to the overall uncertainty in the link between the concentration of radon in water and inhalation dose. These issues have been discussed in more detail in chapter 5.

One important issue regarding the transfer factor is the question of whether there is a correlation of the distribution of variability and uncertainty in the transfer factor with the distribution of ambient radon levels. For example, there is a need to consider further whether there is a joint occurrence of high radon-in-water levels with geographical regions with high temperature so that both increased tapwater intake and higher radon-in-water concentrations might correspond. Similarly, there is the question of whether high radon-in-water levels occur in regions with low annual temperatures and more tightly sealed homes so that the high radon levels in water would yield to the higher water-to-indoor-air transfer factors.

Inhalation Risk per Unit of Radon-in-Water Concentration for Inhalation

The committee did not conduct its own detailed uncertainty analysis for the risk model used for radon inhalation. Instead it reviewed the uncertainty analyses that have been carried out previously by EPA (1995) and by the BEIR VI committee (National Research Council 1999) to estimate the uncertainties associated with inhalation exposures.

As has been noted by the BEIR VI committee, it is not feasible to conduct a complete quantitative analysis of all potential sources of uncertainty and variability in the estimate of the lung-cancer risk associated with the inhalation of radon and its progeny. A key limitation of such an analysis is the difficulty in enumerating all factors that could influence the lung-cancer risk associated with indoor exposures to radon. An additional limitation is that existing information does not

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