ber of the public, in the sense that the estimate is averaged over the age and sex distributions of a hypothetical closed ''stationary'' population whose survival functions and cancer mortality rates are based on recent data for the United States. Specifically, the total mortality rates in this population are defined by the 1989–1991 US decennial life tables (1989–91; 1997), and cancer mortality rates are defined by US cancer mortality data for the same period (NCHS 1993a). The hypothetical population is referred to as "stationary" because the sex-specific birth rates and survival functions are assumed to be invariant over time.

A schematic of the method of computation is shown in figure 4.8. The main steps in the computation are shown in the numbered boxes in the figure and summarized below.

1. Lifetime risk per unit absorbed dose at each age

For each of 14 cancer sites in the body, radiation-risk models are used to calculate sex-specific values for the lifetime risk per unit absorbed dose for each

Figure 4.8

Schematic of method to estimate cancer mortality risk per unit concentration of Rn-222 in drinking water, derived using the methodology of Federal Guidance Report 13 (EPA 1998).

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