mended maximum level. If testing is continued at the present rate for 30 years, the average level of Sr®° in the population of the northern United States may be about 10 to 15 per centof natural background. This may result in a few people approaching body burdens about 50 to 75 per cent of the recommended maximum. Strontium-90 burdens in the weighted world average population will be essentially the same. The average Cs!*" levels presently in the population of the United States is about 45 pyc per gram of K. This amount of Cs'*" is delivering a radiation dose of about 1 mr/year, or about 1 per cent of the natural background dose. The present population weighted world average may be about 32 yuc/g. Continued testing at the past 5-year rate until equilibrium may result in an average world population Cs'*’ radiation dose of about 7 per cent of background, depending on whether Cs'*" is entering the biosphere largely via ecological transmission from the soil or by direct fallout on vegetation. In either case, C38? appears to be relatively less important than Sr® as a potential internal hazard from world-wide fallout. Other long-lived radionuclides, including Pu®*?, appear to be orders of magnitude less significant than Srand Cs**'" These considerations suggest that the past rate of weaponstesting, if continued for several years, will not produce internal radiation levels that will exceed the general population maxi- mum permissible levels recommended by the National”and International’! Commissions on Radiological Protection. Although this leads to the conclusion that the present rate of biospheric contamination poses no serious potential somatic hazard to world health, the great uncertainties involved make it imperative that the problem be kept under constant scrutiny if weapons tests are to continue. Fortunately, present levels are not critical and the slow rate of biospheric build-up affords time for continued intensive and extensive study. REFERENCES 1. Hearings before the Special Subcommittee on Radiation of the Joint Committee on Atomic Energy, Congress of the United States, Eighty-Fifth Congress, First Session on The Na- ture of Radioactive Fallout and Its Effects on Man, Part 1, May 27-29 and June 3, 1957, and Part 2, June 4-7, 1957. 2. R. E. Lapp, Hearings before the Special Subcommittee on Radiation of the Joint Committee on Atomic Energy, Congress of the United States, Eighty-Fifth Congress, First Session on The Nature of Radioactive Fallout and Its Effects on Man, June 4—7, 1957. Part 2, pp. 1261-1262; 1277-1286. 3. Worldwide Effects of Atomic Weapons, Project Sunshine, Rand Corporation Report AECU3488, August 6, 1953. 4. N. G. Stewart, R. N. Crooks, and E. M. R. Fisher, The Radiological Dose to Personsin the U. K. Due to Debris from Nuclear Test Explosions Prior to January 1956, British Atomic Energy Establishment (Harwell), AERE/HP/R 2017 (1956). 5. W. F. Libby, Radioactive Strontium Fallout, Proc. Nat. Acad. Sci. 42, No. 6, 365-390 (June 1956). 6. J. L. Kulp, W. R. Eckelmann, and A. R. Schulert, Strontium-90 in Man. L, Science 125, No. 3241, 219-225 (February 8, 1957). 7. E. C. Anderson, R. L. Schuch, W. R. Fisher, and W. H. Langham, Radioactivity of People and Foods, Science 125, No. 3261, 1273-1278 (June 28, 1957). 8. C. L. Comar, B. F. Trum, U. S. G. Kuhn IN, R. H. Wasserman, M. M. Nold, and J. C. Schooley, Thyroid Radioactivity after Nuclear Weapons Tests, Science 126, No. 3262, 16-18 (July 5, 1957). 9. J. B. Hartgering, Hearings before the Special Subcommittee on Radiation of the Joint Committee on Atomic Energy, Congress of the United States, Eighty-Fifth Congress, First Session on The Nature of Radioactive Fallout and Its Effects on Man, May 27-29, and June 3, 1957. Part 1, pp. 725-741. 10. E. C. Anderson, R. E. Schuch, W. R. Fisher, and M. A. Van Dilla, Barium-140 Radio- activity in Foods, Science 127, No. 3293, 282-284 (February 7, 1958). 11. F. J. Bryant, A. C. Chamberlain, A. Morgan, and G. S. Spicer, Radiostrontium in Soil, Grass, Milk, and Bone in the United Kingdom, 1956 Results, British Atomic Energy Re- search Establishment (Harwell), AERE/HP/R 2353 (1957). 306