23 Pdlonium-210 (Natural ) _ Polonium-210 is a daughter of radon-2z2 and occurs naturally in the air. “ Human exposure, however, occurs largely through the food chain rather than by inhalation. The data are too scattered to present a tabulation of ody burdens but UNSCEAR has assumed burdens of 200 pei in soft tissue plus 200 pCi in the skeleton. A dose estimate is given in the answer to Question 3. Other Nuclides (Man-made) A few additional nuclides have been studied sufficiently so that their contribution to radiation exposure can be evaluated. " tron-55, krypton-85 and strontium-89. . contribution. These are plutonium, None of these have made a significant a We do not consider that our understanding of low-dose radiation effects is hampered by an insufficiency of historical data on exposures of either individuals or population groups to man-made radioactive nuclides. Wedo not believe that in the foreseeable future epidemiological techniques would ' be capable of providing information on the effects of exposures of the ° general public to radiation doses within the range of "permissible doses." . Even with experimental animals, which afford a much more feasible basis for relating effects of radiation, the numbers of animals required to establish signifigant differences between irradiated and unirradiated populations make the studies prohibitively expensive long before we get down to the range of "permissible dose". The "permissible dose” is derived by extrapolation from doses where statistically significant effects can be detected. The assumption has to be made that nothing unusual happens at the very low dose. The data