Present and future levels of Cs!" in the population of various regions, estimated from ecological considerations, are shown in Table 7. Measurements of Cs!37/K*" gammaratios of the United States population during 1956 averaged about 0.5 (reference 7), which corresponds to 41 pyc Cs'*’ per gram of body potas- sium or about 0.0055 yc of Cs!*" in the total body assuming 133 g of potassium in a 70-kg man. Measurements during 1957 (reference 47) gave average Cs!’ concentrations of 45 and 50 uuc/g of body potassium for the general United States population and the population of the northern states, respectively. Levels in the United States population might be expected to show little variation because of the equalizing effect of general food distribution systems. The value of 62 yuc per gram of K for the population of the northern United States, esti- mated from ecological considerations, agrees very well with the average value of 50 pc per gram of K derived from measured Cs!37/K* ratios. The agreement may be purely coincidental and could result from direct fallout on vegetation, fortuitously making up for non-equilibrium of Cs'3? with exchangeable soil potassium. The estimates of future levels given in Table 7 are predicated on the assumption that Cs}57 is entering the biosphere largely through the soil and that the contribution of direct fallout on vegetation is negligible. In this case, Cs'3" levels in people might be expected to rise somewhat in accordance with the estimated values. If present levels represent a quasi- equilibrium with direct fallout, population levels (with cessation of testing) might be expected to start dropping immediately with a half-time comparable to the half-time of stratospheric fallout. In this case, continued testing at the past 5-year rate will produce little or no increase in the average Cs'3? of the population. Present levels in the biosphere actually may be a result of significant contribution from both direct fallout and ecological integration, in which case the truth will be somewhere in between. It should be possible to decide among these alterna- tives within the next few years. 4.3 Plutonium-239 ESR. Ecological Incorporation and Discrimination. Although the presence of naturally occurring Pu"? in pitchblende concentrate has been reported,*® its existence in the biosphere can be at- tributed entirely to the detonation of nuclear weapons. Unlike Sr” and Cs!*", it is chemically unrelated to any essential constituent of plants or animals. When plutonium is deposited in soil it is extremely tightly bound, and the establishmentof uniform distribution to the depth of the plant feeding zone may require years. A plutonium deposition level of 1 mc/sq mile would be equivalent to about 5 x 107° je per gram of soil when uniformly mixed to a depth of 2.5 in. Absorption of plutonium by barley from a sandy soil was studied by Rediske,‘* who found that the ratio of plutonium concentration in dry plant material to the concentration in the soil was 9 x 1074. When ingested by man and domestic animals, absorption of plutonium is only about 0.01 per cent. Once it is absorbed, about 85 per cent is fixed in the skeleton and largely retained throughout the life-time of the animal. The apparent half-time of plutonium elimination by man is about 200 years, which meansit is essentially completely cumulative on absorption. Its high fixation in the skeleton of domestic animals, however, provides an additional discrimination factor of about 107 in meat and dairy products. The over-all discrimination ratio in going through the ecological cycle from soils to man is at most 5 x 1078. The estimated present average maximum plutonium deposition level for the north temperate population belt would lead to a plutonium uptake of the order of 107? of the recommended maximum permissible level from the consumption of a 3000-calorie diet for 70 years. With such a large discrimination, it is quite unlikely that incorporation of plutonium fallout into man via the ecological chain can be of any consequence. Incorporation via inhalation and direct fallout on vegetation, although insignificant also, probably would be much greater than incorporation via ecological transport. 4.4 lodine-131 Radioactive iodine from weapons tests has been reported in human thyroids’’*! and in the thyroids of domestic animals.°°-* Because of its 8-day half life, I'*' cannot integrate in the biosphere and its concentration in thyroids fluctuates in relation to tropospheric fallout during 299