that caesium-137 is transto milk about ten times as 4), and (6) that the average in their use to forecast situations over relatively short periods of years; much greater uncertainty besets many other phases of the assessment of the effects of environmental contamination. out 1-1 g/l., a soil factor for n be derived from the value by equation (2) (Table 1). ive deposit of caesium-137 expected to account for less ium-137 which has hitherto le fall-out. The analysis of s could not be expected to it the present time the soil only from laboratory experi- mld contribute about 20 per usions is article indicates that the trontium-90 in milk in the xed to an appreciable extent the last half of the previous it rate of fall-out and cumulai basis for predicting future k is therefore provided by an oxroportionality factor for this to the rate and soil factors equation (2). The same type to caesium-137 (equation 4) the contribution from the 80 smail that it can be ignored experienced hitherto. des in- milk which would be ; of fall-out which may be lecade as a result of weapon od when allowance is made for year the greatest difference is ) per cent and averaged over rontium-90 would be altered degree of agreement between iction encourages confidence 8 + of strontium-90 and caesium-137 in milk over 50 years would be expected on the new basis to be respectively 30 per cent lower and rather more than 50 per cent higher than those formerly predicted; this reflects the reduced soil factor for strontium-90 and the small soil factor now a decreasing fall-out the inteion (4) is 50 per cent higher. crease by a factor of two annually, the integrated levels - um-137 in milk which would f equations (3) and (4) are two models of fail-out which no allowance has been made n the soil by processes other ity because its movement in re much smaller than those of ver the first 10 years, equaralues 20-30 per cent higher n (3) for both models; over If, however, the integrated levels of dietary contamina- tion are considered over long periods, especially when the rate of fall-out is declining steadily, predictions on the new basis diverge substantially from those which were made formerly. Thus, if the rate of fall-out were to de- deduced for caesium-137. In consequence the ratio of the integrated level of caesium-137 in milk to that of strontium-90 over this period would be twice that previously calculated. Long-term predictions of this type are subject to particu- lar uncertainties owing to the lack of direct information on the behaviour of nuclides in soil over long periods. None the less, they deserve consideration as they form the basis for estimating the integrated radiation dose, or dose commitment, which will be received by the population; this is regarded as the best available guide to the total impact of radiation from weapon testing® 5. The revised proportionality factors suggest that an appreciably larger fraction of the tota] dose commitment is due to caesium137 and a smaller fraction to strontium-90 than has hitherto been assumed. In conclusion it is to be emphasized, first, that although the method of prediction here outlined appears to give a reasonable description of the average situation, appreciable divergences in individual years must be expected because of climatic variation, and secondly, that the proportionality factors here proposed have been derived for the United Kingdom only. Moreover, variations are to be expected between countries which contrast in climate, soil type and agricultural practices. Thus a recent analysis of relationships for caesium-137 in milk in Sweden?’ gives a rate factor more than three times that calculated for the United Kingdom and a lag-rate factor (which was related to the deposit in the entire previous year) similar to the factor which relates to the deposit during 6 months only in the United Kingdom. Djifferences, though of lesser magnitude, are apparent between proportionality factors calculated for milk in other countries®. 1 United Nations Scientific Committee on the Effects of Atomic Radiation , General Assembly, Officiat Records: Thirteenth Session, Suppl. No. 17 (4/3838) (United Nations, New York, 1958). ? Rusgell, R. S., Nature, 182, 834 (1958). * Burton, J. D., Milbourn, G. M., and Russell, R. 8., Nature, 185, 498 (1960), ‘ Mercer, E. R., Burton, J. D., and Bartlett, B. O., Nature, 198, 662 (1963). 9