bomb. The report of the Committee on the Biological Effects of Atomic Radiation, of the Na- tional Academy of Sciences (hereafter called the NAS report) states that “U. S. residents have, on the average, been receiving from fallout over the past five years a dose which, if weapons testing were continued at the same rate, is estimated to produce total 30-year dose of about one tenth of a roentgen: and since the accuracy involved is probably not better than a factor of five, one could better say that the 30-year dose from weaponstesting, if maintained at the past level, would probably be larger than 0.02 r and smaller than 0.5 r.” The average world-wide 30-year dose is estimated to be considerably lower. 3 ACCUMULATION OFSr”? Strontium-90 fallout has been determined in a great variety of samples collected from different parts of the world. The item of chief interest in the present discussion is the concentration in human bones, especially those of children. However, the more extensive data on soil and milk show the trends with respect to time and are useful in the estimation of future concentrations in human bones. 3.1 Soil The Sr*° content of soil has been increasing considerably since 1954. Neglecting minor fluctuations, largely attributable to the periodicity of weapons tests, the increase in this period has been roughly proportional to time. At the end of 1956 the average surface concentration in the United States amounted to approximately 25 mc of Sr®’ per square mile. Merril Eisenbud estimates that the average for the North Temperate Zone is 9.4 mc/sq miand that for the South Temperate Zone 2.6 mc/sq mile. 3.2 Milk The concentration of Sr®® in milk has increased steadily with time and reached 5.6 pyc per gram of Ca in New York State milk in November 1956. It dropped to 3 ppc per gram Ca in April 1957, but the decrease may be due in part to seasonal variations (e.g., indoor feeding of cows during the winter). 3.3. Human Bones The Sr*° concentration in human bones is higher in children than in adults, as would be expected. Until whole skeletons, or sufficient representative samples thereof have been analyzed, quantitative figures must be regarded as rough approximations. In the meantime it seems reasonable to assume that, at least in the United States, the steady state concentration in bone per gram of calcium will be about one half of that in milk. (The values for milk are quite reliable because large samples can be used in making the measurements). Accord- ingly, in very young children it should be approximately one half of that in milk averaged over their lifetime (including fetal life). The concentration in milk ranged from 1-2 pye of Sr®? per gram of Ca. It seems unrealistic to assumethat tests of weapons of the present type will continue for generations. For the present purpose we shall use estimates of the predicted concentration of Sr®® in the average human skeleton in the United States in equilibrium with fallout, under the following conditions: (1) if tests were stopped now, (2) if tests were to continue for 30 years at an annual rate equal to the average of the past 5 years, (3) if tests were to continue at this rate for many generations. If weapons tests by all nations were stopped now, the concentration of Sr®’ in the skeletons of children in equilibrium with the concentration in milk, would gradually rise to an average value of 4 puc per gram of Ca in the 1970’s and would decrease slowly thereafter. This is based largely on estimates made by Merril Eisenbud with Wright Langham. If weapons tests of ali nations were to continue for 30 years at an annual rate equal to the average of the past 5 years, (about 10 megaton equivalent of fission yield per year) the equilibrium concentration of Sr*° in bone would reach an average value of about 15 ue per gram of Ca in 100 years (as estimated by Langham) and would remain substantially at this level so long as testing continued at the same rate 276