since 1952 from testing of weapons in the megaton class. United States Pacific tests have been held under conditions that maximized local fallout (where Sr® is of no concern because of the vast calcium reservoir of the ocean) and minimized world-wide contamination. Soil data suggest that about 1.6 megacuries of Sr® have been distributed as long-rangefallout. The present stratospheric reservoir is estimated at about 2.4 megacuries by Libby,’ and at 1.1 + 0.93 in this report. Present integrated surface deposition levels are such that the rate of sr® decay on the ground is almost equal to the rate of stratospheric fallout. If weapons tests were to stop, integrated surface deposition levels in the north temperate latitudes would probably increase by no more than 10 per cent, reaching a maximum in about 1963-1970. Unfortunately, because of the locations of the United States and USSRtest sites and tropospheric and stratospheric meteorological phenomena, long-range fallout is maximized in the north temperate latitudes where over 80 per cent of the world’s population lives. The present average soil level in the northern United States is about 35 mc/sq mile, and the average level elsewherein the same general latitudes may be about 20 mc/sq mile. Deposition levels elsewhere in the world are not potentially important with regard to general world health because of population distribution. Estimates of average maximum Sr® equilibrium bone levels for the northern United States and the north temperate population belt (from weaponstests to date) vary from about 1 to 4 uc per gram of Ca. Controversy over the issue of stopping or continuing bomb tests has resulted in greater apparent public confusion over the potential hazard of world-wide Sr™ fallout than seems justified by the factor of 4 differences in estimates of average maximum equi- librium bone levels. This confusion has resulted largely from differences in choice of reference as to average maximum permissible Sr™ levels applicable to the general population and differences in opinion as to an appropriate factor of allowance for nonhomogeneity of fallout and bone uptake. Libby*” and Kulp,” before any authoritative statements regarding a Sr* MPL for the general population had been issued, used the occupational MPL (1000 puc per gram of Ca) as a reference. Later the National and International Commissions for Radiological Protection recommended that the MPL for large segments of the general population should be one-tenth (100 puc per gram of Ca) that for occupational exposure. The U. S. National Academy of Sciences-National Research Council report® inferred that 50 pyc per gram of Ca might be considered as a safe level for the general population. The British Medical Research Council report, 5 while acknowledging that the maximum allowable concentration of Sr® in the bones of the general population should not be greater than 100 puc per gram of Ca, stated that immediate consideration should be required if the concentration in human bones showed signs of rising greatly beyond one-hundredth (10 puc per gram of Ca) of that corresponding to the maximum permissible occupational level. Lapp” has stated also that the MPL for the general population perhaps should be one-hundredth of the occupational value. All of these numbers have been broughtto public attention during the controversy over continued weapons tests.! Confusion has been increased also by the use of various safety factors for nonhomogeneity of fallout and bone uptake. Articles have appeared in which no factor was used,*?® 2 and others have appeared in which factors of 5'° and 10">"5 were recommended. The effect of choice of values for the acceptable general population MPL and the choice of safety factors for nonhomogeneity of distribution and uptake are shown by the date in Table 10. These data were derived by simple proportionality (Maximum Bone Level from Present Tests: 50 megatons :: Acceptable MPL: X) and show the megatons of fission energy release (over a short period) required to bring the average maximum equilibrium bone levels of the population to the various permissible values that have been called to public attention. The table also indicates the effect of various nonhomogeneity factors on the world average population level. These data show a variation of a factor of about 1000 in the megaton equivalents of fission that could be detonated, depending on whether one wishes to be ultraconservative and use the highest safety factor for nonuniformity and the lowest value recommendedfor the general population, or be the opposite and use the occupational MPL andno safety factor for nonuniformity. The most important point to the data is that they explain the principal reasons for public confusion and show that the major areas of uncertainty are: (1) the maximum permissible level for sr® as applied to the general population; and (2) the deviation of equilibrium bone values from the mean. 304