the survivors of the atomic bombings in Japan, where the high susceptibility appears to be related to the high baseline rate. The basic data used in the risk calculations for the present report were derived from the atomic-bomb survivors and applied to the US population with a relative-risk model in conjunction with the US stomach-cancer experience. It is noted that the high baseline stomach-cancer incidence among the Japanese and the declining incidence in the US contribute to the uncertainty in the risk estimate. An estimate of the uncertainty in the risk was derived on the basis of judgment that the absorbed dose to the stomach is probably not greater than three times the base case of table 4.4 (divide by 20 to obtain absorbed dose) and probably greater that one-fiftieth (2%) the base case. The asymmetric bounds reflect the judgment that the base case estimate is taken to be conservative; however, at this time sufficient information is not available to further refine the model and its parameter values. Similarly, it was judged that the stomach cancer mortality coefficients are probably not greater than three times the values of table 4.6 while they probably are greater than one-tenth the tabulated values. On the basis of those judgments, it is concluded that the risk posed by ingestion of water containing 222Rn at 1 Bq m-3 probably lies between 3.8 × 10-10 and 4.4 × 10-9, with 1.9 × 10-9 as the central value.

Special Populations At Risk

No information is available to identify the characteristics of individuals or to suggest that any population group that might be at increased carcinogenic risk because of the presence of radon in drinking water. A number of environmental factors have been associated with stomach cancer, although the incidence of gastric cancer has been declining during the last 50 y. Stomach cancer is essentially a disease of the poor, not only in developing countries, but also in the West, where there is an inverse correlation between stomach-cancer risk and socioeconomic status. A strong link appears to exist between the ubiquitous bacterium Helicobacter pylori and stomach cancer, but there is no known interaction between H. pylori and radiation or radon (McFarlane and Munro 1997).

Sikov and others (1992) investigated the developmental toxicology of radon exposures in the rat. They did not find any teratogenic or reproductive effects in pregnant rats exposed to airborne radon at high concentrations. Radon in the maternal blood would flow to the placenta and, depending on the relative solubilities of radon in maternal and fetal blood, could be absorbed by the fetus. However, the regional blood flow to the uteroplacental unit during the period when most teratogenic effects are possible (1st trimester) is very small (Thaler and others 1990). The same exchange is possible for either inhaled or ingested radon. Thus, it appears unlikely that radon in drinking water would have substantial teratologic or reproductive effects.

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