periods of nuclear testing. Van Middlesworth®’ reported the analysis of 175 human and 1044 cattle thyroids collected from the Memphis, Tenn., area during November 1954 to March 1956, and Comar et al.*! reported analysis of 1165 human and 853 cattle thyroids collected from several countries during the period from January 1955 to December 1956. These data show that the concentration of I'*! in cattle thyroids is about 18 to 200 times that of man. The average concentration in cattle thyroids during the period from November 1954 to December 1956 appears to be about 0.5 (reference 54) and the average peak level in man about 0.005 muc/g.*! The principal mode of entry of I'*! into domestic animals seems to be through ingestion of direct fallout on forage. Grazing animals show a much higher thyroid uptake than do lotfed. The modeof entry of I'*! into man is believed to be via direct inhalation with ingestion of contaminated milk as a secondary route. Following oral feeding to cattle, about 6 per cent of the ingested I'*! appears in the first week’s milk production.» The average milk concentration during the 1955 period of high level fallout was estimated at about 0.2 muc/liter, which is a factor of 500 below the value chosen as unsafe for public consumption during the recent United Kingdom Windscale reactor accident.*® Since I'*! does not accumulate in the biosphere, the above values may be considered crude average maximum equilibrium levels with the present rate of testing. Although large local fluctuations may be expected from time to time as a result of tropospheric meteorological variations and proximity to test sites, the average I'*! content of the thyroids of man and livestock should not increase materially with continued testing at the past 5-year rate. One muc of I'*! per g of thyroid delivers a radiation dose of about 10 mrad/day.*? The average I'*! concentrations during the 1955 peak period of fallout delivered about 35 and 0.3 mrad/week to livestock and man, respectively. The integrated dose received during 1955 was actually much lower.*! If the 1955 peak levels are maintained in people and livestock, the yearly integrated dose to the thyroid will be about 15 and 1500 mrad/year, respectively. For man, this is about one per cent of the recommended maximum permissible level for continuous exposure of large segments of the population. The external radiation dose to the neck area in infants and children that possibly has caused later thyroid malignancy is estimated at 200 to 750 r (reference 57), and about 900 rad to the thyroids of sheep chronically fed I'*! over a 6-year period failed to produce any observable damage.*® In the event of nuclear war, it is conceivable that pu could constitute a significant acute danger in localized areas. However, there seems to be very little probability that 15! levels introduced into the biosphere by continuation of weapons tests at the past rate will pose any general hazard to man and domestic animals. 5 5.1 SIGNIFICANCE OF Sr® AND Cs!3? LEVELS IN THE POPULATION Strontium-90 The potential significance of present and predicted Sr® levels in bone can be evaluated only in relation to human experience, which is indeed inadequate. Bone sarcoma has resulted from a fixed skeletal burden of 3.6 ic of pure Ra226 and nondeleterious bone changes have been observed in persons having only 0.4 yc for a period of 25 years.”? Necrosis and tumors of the bone have occurred also several years after large doses of X rays,” and consideration of human experience with leukemogenic effects of X and gamma radiation®!-® suggests that about 80 rads may double the incidence of leukemia. The only other human experience with which present and predicted levels of sr® may be compared is that arising from natural background radiation. Natural background dose to the bone (during a 70-year lifetime) may vary from about 8 to 38 rem.“ The major contribution to background variation is differences in the radium levels of soils and minerals. The average natural skeletal radiation dose rate was carefully evaluated by Dudley and Evans® and their data are shown in Table 8. Figure 7 shows a general summary of estimated skeletal radiation doses from accepted maximum permissible levels and from present and predicted Sr® burdens in relation to human experience. The maximum permissible level of Sr(100 puc per gram of Ca) is estimated to 300