How to assess natural risks

A person in high background radiation prefectures accumulates more than 2R in 40 years than in low radiation prefectures. Excess collective exposure amounted to 3.35 million person R in 1975. Excess crude death rate of all cancers in high background radiation prefectures seemed significant indicating a rate of risk per R 50 times as high as ICRP's risk factor. Excess rate of crude cancer death disappeared when the age construction of high background radiation prefectures was adjusted. Excess loss of life expectancy due to cancer in high background radiation prefectures was negligible. Excess rate of childhood cancer death and the sum of foetal, neonatal and infant deaths was zero at the level of natural background radiation. Least dose which detect a possible excess may range from 6 to 10R. (author)