ABSTRACT
Analysis of epidemiological data on cancer mortality among the Japanese A-bomb survivors and the Chernobyl cohort of emergency workers_exposed to different doses of ionizing radiation made it possible to suggest with assurance that there is a threshold of the radiocarcinogenic effect in the region about 200 mGy (mSv). Analysis of the excess of deaths due to solid cancer over the expected value for the dose range 5-200 mSv observed in the Japanese cohort led us to the conclusion that this excess presents a ( quasi)plateau and is presumably of a non-radiogenic nature. The epidemiological data from the Chernobyl cohort enabled the conclusion that the dose dependence of leukemia risks in a limited low level radiation (LLR) dose range cannot be used to determine real coefficients of excess absolute or relative risk when the level of adequate control is not taken into account. To corroborate the principle of single particle hit per cell nucleus as the basis for the microdosimetric definition of low radiation doses, an approach was proposed to objective separation of delimitation regions between low, intermediate, and high radiation doses on the dose scale. By this approach, the low dose upper limit of sparse ionizing radiation for a cell nucleus of 8 J..lm in diameter was evaluated at 0.65 mGy. It can be used in assessing dose rate thresholds in respect to safe environmental chronic radiation exposure for man.
