the long period required for equilibrium) would become unavailable to plants and the aVailabl soil-p tamin: to obtain actual yearly fission product production rates to refine further predictions of Surta the ra about 10 with a continued average test rate of 10 megatons of fission yield per year Seems clides absorl equilibrium levels would be only about 8 times the present values.’ Attempts arebeing made. levels under continued testing. Until then, an equilibrium build-up factor for Sr and cgit a reasonable. Table 4 shows future average maximum surface deposition levels of Sr®, ¢ gi3? and Pu’*® calculated, on the above basis, from the data in Table 2. , Table 4— AVERAGE MAXIMUM SURFACE DEPOSITION LEVELS OF Sr®’, cs'3', AND Pu? (ASSUMING A CONTINUING TEST RATE OF 10 MEGATONS OF FISSION YIELD PER YEAR) gr8? Cg it Py238 Region mec/sq mile* mc/sq mile* me/sq milet Northern USA North temperate latitudes South temperate latitudes 350 190 55 460 250 70 40 24 6 Rest of world World average : . . 35 80 50 100 3 11 * At equilibrium in about 100 years. ft In about 100 years, not at equilibrium. Others have made similar estimates of Sr®” surface deposition levels. Libby!? estimated equilibrium levels for the United States at 400 to 600 mc/sq mile. Neuman! estimated a United States deposition level of about 400, and Machta‘‘ 350 to 850 mc/sq mile. 4 INCORPORATION OF NUCLEAR DEBRIS INTO THE BIOSPHERE AND MAN Radionuclides from fallout may enter the body through inspiration of the contaminated atmosphere and by ingestion of contaminated food and water. Stewart et al.‘ estimated the mean Sr™ and Pu?”® concentrations in air at ground levelin England during 1952-1955 as 4 x 107'6 and 3 x 107!" yic/ee, respectively.* Assumingtheratio of Cs'°"/sr™ in air is the same as their ratio of total production, the mean Cs!*" concentration in air during the same period would be 5 x 107'* pc/cc. The respective occupational maximum permissible air concentration of Sr”, Pu’*®, and Cs'*? recommended by the International Commission on Radiological Protection”! are 2 x 107*°, 2 x 107", and 2 x 107" uc/cc. The esti- mated mean values are 5 to 8 orders of magnitude lower than the maximum permissible air concentrations recommended for the general population. Since the tropospheric fallout time is 20 to 30 days, the mean air concentration values during 1952-1955 probably approximate equilibrium conditions with the past 5-yearrate of biospheric contamination from stratospheric fallout.° In this case, continued weaponstests at the past rate should not increase the mean air concentrations greatly. As suggested by Stewart et al: and Bryantet al.,'! inhalation of nuclear debris is not a major factor in the potential hazards of world-wide fallout. Comparison of measured and estimated concentrations of the principal long-lived radionuclides in water with the maximum permissible concentrations recommended by the International Commission on Radiological Protection”! suggest also that ingestion of contaminated drinking water is relatively unimportant.!! Ingestion of food contaminated through soil integration and plant uptake of long-lived radionuclides seems to pose the major potential hazard. When nuclear debris is deposited on the earth’s surface and incorporated in the soil, the individual nuclides are taken into plants through the root system accordingto their individual * Their calculated value agrees reasonably well with the average measured yalue of 3 x 107" ye/ee (for the same period at Washington, D, C.) reported by Martell.”? 290 tained prope) A since conce) 4.1 § (a polica: ecolog calciu: It calciu: may be all Sr® vidual Menze soil ty: tained A Alexan from n Tr diet mi calciur States about € basis ¢ DF,) = libriun 5 per c It apply c matica depth o dilutior that it: ference (b) equilib: crimin: predict: AS i mc of If all of gives tl of avail specifi modelii concen!