20 naturally occurring radionuclide in the environment does not change with , time. . A smaller number of measurements of Ra-228 are made from time to time. The data are not listed here, but the estimated doses are given in the repiy to Question 3. Strontium-90 (Man-made) Strontium-90 appears to have the greatest biological Significance of the radionuclides produced in weapons tests. There have been many studies of its deposition and transfer through the food chain to man. A large number of bone samples are analyzed each year by the Atomic Energy Commission and the Bureau of Radiological Health. A summary of these measurements for 1958, 1963 and 1968 are given in the following table. - " Mean Body Burden of 5r-90 United States pci in the Body Age (years) o-4k 5 - 19 over 19 . 1958 1963 1968 260 600 ‘540 1800 420 1900 200 1300 900 Since strontium-90 essentially did not exist in 1944 it could not have . been present in the skeleton. Measurements were not made in 1951 but our knowledge of fallout and the transfer mechanisms mentioned above would indicate that the levels were below 100 pCi of strontium-90. The uptake of strontium-90 is greater in children. This is apparent in the next table, which gives the concentration of Sr-90 rather than the body burden.