erals in the earth’s crust and from radium, K**, C“, and thorium deposited in the body constitute this so-called natural background. The amount of natural background radiation is such that persons living to an age of 70 years receive an average total dose of about 7 rem, while their skeletons (as a result of radium and other radioactive materials deposited in the bones) receive an average dose equivalent to about 10 to 12 rem. The natural background dose to some segments of the population may be at least three times the average becauseof variations in cosmic ray intensity and composition of the earth’s crust with geographic location. The net result of fallout is a smali increase in the radiation background to which all life is exposed. The problem of the potential hazard of world-wide fallout then becomes one of trying to ascertain the magnitude and significance of this increase in background dose with © regard to its potential risk to man’s health and well-being. Contamination from nuclear weapons testing may be divided on the basis of local and distant (world-wide) fallout. Local fallout is of primary significance in the event of war in which weapons with a high fission component may be detonated at or below the surface to maximize surface contamination. In this case, fission products of short and intermediate half-life are of major concern since local fallout occurs within a few hours after detonation. Distant (world-wide) fallout is of significance both with regard to continued weapons testing and in the event of nuclear war. Since months and even years are required for fission products to deposit over the earth’s surface, only the long-lived radionuclides are important. External exposure from environmental deposition of gamma-emitting fission products is of concern primarily because of the potential production of genetic changes. Internal exposure is of significance primarily with regard to the potential production of somatic effects in the tissues in which the various fission products deposit upon entering the body. This report is restricted mostly to the potential internal hazard of distant (world-wide) fallout, with emphasis on Sr”. Strontium-90 is believed to be the most important radionuclide because of its similarity to calcium (resulting in a high rate of uptake by plants and animals), long physical and biological half-life, and high relative fission yield. These factors lead to “eth high incorporation in the biosphere and a long residence time in bone. General contamination will result in the bones of the population eventually reaching an equilibrium state with Sr®™ in the biosphere. 2 PRODUCTION OF BIOLOGICALLY IMPORTANT RADIONUCLIDES FROM WEAPONS TESTS A crude estimate of production of biologically important radionuclides from past nuclear weapons tests would be helpful in assessing the potential hazard of present biospheric con- tamination and in extrapolating to future levels in the event of continued testing or nuclear war. Statements during the Subcommittee Hearings of the Joint Committee on Atomic Energy, Congress of the United States! assumed a constant nuclear weapons test rate of 10 megatons of fission yield per year, beginning in the spring of 1952. This leads to a total testing by all nations of about 55 megatons of fission yield by mid-1957. The total estimate may be reasonably realistic; however, the assumption of a constant test rate is highly questionable.” One megaton of fission energy release results in the production of about 100,000 curies of Sr® 3 which suggests a total Sr®° production of 5.5 megacuries from weaponstests by all na- tions to mid-1957. From the fission yield curve (thermal neutron fission of U?*) and the appropriate decay constants, it is possible to make a crude estimate of the total production of other radionuclides of potential biological importance. Table 1 shows estimates of total production (in terms of megacuries of initial activity) and other pertinent data for the more important intermediate- and long-lived components of fission debris. The values for total yield are crude approximations only because it was necessary to use the fission yield curve for thermal neutronfission of U’*, and isotopic abundance varies with the fissionable material and the neutron energy. None of the values, however, are incorrect by more than a factor of about 2. The total production of Pu239 was estimated from the report of Stewart, Crooks, and Fisher,‘ who postulated from analysis of bomb debris that one Pu 238 atom was formed per <- 283