by assuming they will be proportional to the predicted average maximum surface deposition levels given previously. These estimates are compared in Table 6 with those postulated from ecological considerations. Values postulated from ecological discrimination and from bone analyses differ by a factor of about 2. This discrepancy results mostly from the weighted influence of the Far Eastern countries, with low milk consumption and large populations, on the discrimination ratios. Although bone data are probably morereliable than ecological predictions, they may be low since they were predicted on the assumption that the analyses repre- sented the average bone levels for the various regions. Since the number of samples from the United States and Europe exceeded the number from countries with large populations and low relative milk consumption, it is unlikely that they are weighted adequately for population density and dietary habits. Another troublesome feature of such estimates is that they are average maximum equilibrium levels and make no allowance for such factors as local variations of fallout due to meteorological factors, variations in available soil calcium, dietary patterns and habits, nutritional state of segments of the population, and individual metabolic condition. Frequency distribution patterns have been reported for stable strontium, * natural radium,? 1 and Cs'3" (reference 7) in man. All these nuclides show essentially normal distributions with standard deviations of about 35 per cent. Libby’® has stated that (at steady state among people living in a given locality) only one person in about 700 will have more than twice the average sr® burden, and the chances of anyone having as much asthree times the average will be about one in 20 million. At present, the Sr*” measurements of bone samples from subjects of all ages show a much greater scatter than indicated by a standard deviation of 35 per cent. The greater scatter of the observed values is due largely to the fact that samples came from many localities and (because of the relatively short period of environmental contamination and the age dependence of Sr™ deposition) represent varying degrees of equilibrium conditions. The spread may be expected to decrease as equilibrium is approached. !3.28 Local meteorological conditions will result in increased intensity of fallout in certain localities. The worst possible situation that could come about would be for these “hot spots” to coincide with localities of low available soil calcium in which the population grew up and lived in provincial isolation. Libby! has considered this problem in view of the general averaging which occurs in food distribution systems and has postulated that a factor of 5 encompasses the total variation due to all factors. The question as to the applicability of the normal distribution curve to sr® equilibrium levels in bone has been raised.**-*® The observed distribution of stable strontium in bone*® appears to be log normal rather than normal; in fact the former is rather common for geo- chemical distribution.“ The great fundamental difference in the mechanisms of distribution of stable strontium and Sr® however, greatly weakens agruments based on the analogy. Whether the distribution of equilibrium levels in the bones of the population will be normal or log normal can probably be decided only by more extensive experimental evidence. 4.2 Cesium-137 (a) Ecological Incorporation and Discrimination. Cesium is chemically and metabolically similar to potassium, an essential body constituent. If it enters the food chain from the soil (rather than by direct fallout on plants), its uptake via the ecological cycle and incorporation into man should be in relation to the exchangeable or available soil potassium. It is reasonable, therefore, to consider incorporation of Cs'*" into the biosphere in terms of Cs'*"/K ratios. Like Sr®, Cs!3? may be incorporated through direct fallout on vegetation and through soil ac- cumulation and uptake by plants. When Cs!!’ comes in contact with soil, it is rapidly fixed. Leaching studies! show essentially all of the Cs'’ remains in the top inch of soil, even after 200 in. of simulated rainfall. The extent of fixation, as with potassium, is probably proportional to the colloidal content of the soil, being greatest in clays and clay loams andleastin light sands and sandy loams. Plants discriminate heavily against Cs’*' 137 with respect to potassium, even when the cesium is in an exchangeable form. Auerbach” reported uptake of Csi37 by corn grown in a lake bed once used for the disposal of reactor wastes. He found that the Cs'""/K ratio in the plants was about 1 per cent of the exchangeable cs84/K ratio in the soil. Menzel’? obtained a discrimina- tion factor of about 0.04 between Cs!*"/K in barley and corn and the ratio in available soil 296