4. Lifetime cancer risk for chronic intakes

For purposes of computing a risk coefficient, it is assumed that the radon concentration in the drinking water remains constant and that all persons in the population are exposed to that concentration throughout their lifetimes. It is assumed that the lifetime average drinking water consumption rate is 0.6 L d^-1.

For each cancer site and each sex, the lifetime cancer risk for chronic intakes is obtained by integration over age x of the product of the lifetime cancer risk per unit intake at age x and the expected drinking water consumption at age x.

Except for the calculations of the time-dependent organ activities and absorbed-dose rates, each of the steps described above is performed separately for each sex and each cancer site. A total-risk coefficient is derived by first adding the risk estimates for the different cancer sites in each sex and then calculating a weighted mean of the coefficients for males and females. The weighted mean of coefficients for males and females involves the presumed sex ratio at birth, the sex-specific risk per unit intake at each age, and the sex-specific survival function at each age.

The cancer mortality risks associated with lifetime ingestion of ²²²Rn dissolved in drinking water at a concentration of 1 Bq m-3 are given in table 4.7; the total average over both sexes is 1.9 × 10^-9. The uncertainty in this estimate is associated largely with the estimated dose to the stomach and with the epidemiologic data used to estimate the risk. Cancer of the stomach is a major late effect in