doses below 100 rad, but can be useful for estimating cancer risk from higher doses of low-LET radiation. The recent calculations of the BEIR III Report (NAS-BEIR, 1980) provides the most current basis for estimating the carcinogenic risk from radiation in SPS workers in the space environment. This report (NAS-BEIR, 1980) chose three exposure situations for illustrative computations of lifetime cancer risk of low-dose, low-LET, whole-body radiation: 1. A single exposure of a representative (life-table) population to 10 rad; 2. A continuous, lifetime exposure of a representative (lifetable) population to 1 rad per year; 3. An exposure to 1 rad per year over several age intervals exemplifying conditions of occupational exposure. These three exposure situations reflect circumstances that might normally occur—i.e., the general and worker populations and single and continuous exposure. The selected level of chronic exposure of one rad per year, although only one-fifth the maximal permissible dose for conventional occupational exposure is nevertheless consistent with the occupational exposure experience in the nuclear industry. The United States 1969— 1971 life-table was used as the basis for the calculations (NAS-BEIR, 1980). The expression time was taken as 25 years for leukemia and the remaining years of life for other cancers. Two projection models were used, the absolute risk and the relative risk models. The absolute risk is the expression of excess cancer risk due to radiation exposure as the arithmetic difference between the risk among those in the exposed population and that obtaining in the population in the absence of radiation exposure. The relative risk is the expression of excess cancer risk due to radiation exposure as the arithmetic ratio of the risk among those in the exposed population to that obtaining in the population in the absence of radiation exposure. In the absence of any increased radiation exposure, among one million persons of life-table age and sex composition in the United States, about 164,000 persons would normally be expected to die from cancer, according to present cancer mortality rates. For a situation in which these one million persons are exposed to a single dose increment of 10 rad of low-LET radiation, the linear-quadratic dose-response model (LQ-L)* predicts increases of about 0.5 percent to 1.4 percent (about 750-2,300 cancer cases) over the normal expectation of cancer *Linear-Quadratic (LQ) for low-LET radiation, linear (L) for high-LET radiation.
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