- 8 - site in Nevada. These 22 mc/mi2 of Sr90 in the soil of the U.S. amount to about 0.040 MPC units in the top two inches of soil where most of the fallout is absorbed. As of the present time, considering the latest human bone and milkshed data, both domestic and foreign, together with the total fallout figures for corresponding periods, we find that the level of somewhat less than 0.001 MPC units now found in the bene s of young children is to be compared with a total Sr90 fallout in the soil of about 12 times higher concentration. Additionally, laboratory data -have shown that there is a three-fold discrimination against strontium as compared to calcium in the assimilation by plants from the soil,‘and that a further factor of about eight-fold of discrimination against strontium relative to calcium in the excretion of strogyiun in milk as compared to the cow feed exists. Barlieré it seemed reggonable to conclude that the human body burden of Sr7¥ might well be as high as 70 percent of the concentration in the top soil on which people live. The further evidence just citad seems to indicate that this figure is much too high and possibly should be reduced to about 10 percent. A strict application of the two discrimination factors described ._ would give § percent. Leaf retention of fallout which bypasses the soil causes the figure to be higher... Therefore, at the moment, we would expect that the body burden forchildrenbornnowinAmericaeventually, would amount to between0.004 MPC units, corresponding to 10 percent of the top soil concentration and possibly a figure two or threetimeshigher. deposition would be expected to conti The stratospheric t the expected rate which atthepresent is about..1.2_ma/ye,sothat some 15years fromnow, inthe early 1970's, amaximum additional totalstratosphericfallout ofabout6 mc/mis Willhaveoccurred. Inthe_meantime,thepresent22 me/mi2would pave beenreducedto 15by_radioactive deca justabout_compensating Sepositron, for thestratospheric. Thus, thé conclusion that the boay burden “in the U.S. from weapons fired to date would be about 0.004 MPC units, or possibly as high as 0.010 MPC units, seems justified. This level probably will not 2/ ' “Radioactive Strontium Fallout," W. F. Libby, Proc. Nat. Acad. Sei., 42, 365 (1956). (more) - a a a | (8