Absolute risk represents the total number of persons with a specific disease affected by radiation exposure, or the rate of that disease in a given population over a given period of time (usually designated as “person-years”). AR is often expressed as the number of affected subjects per 10⁴ person-years or 10⁴ person-year-Gy (i.e., per 10⁴ person-years per Gy). Whereas relative risk (RR) expresses degree of excess risk, or strength of causation, AR describes the numbers of people affected and hence the public health impact in a population. For instance, the RR for leukemia is the highest among various late effects of radiation (RR approximately 5-6), but the total number of radiation-caused cases of leukemia in the Life Span Study (LSS) survivors is estimated to be only about 90-100. In contrast, the RR for solid cancers is much smaller (RR approximately 1.5), yet the total number of survivors who have developed such cancers due to bomb radiation is estimated to be about 850.