cree 2). The use of the lower soil factor derived by equation (2) receives some independent support from the fact that a value of 0-14 has been derived from the results of tracer experiments? in the United Kingdom in which strontium-89 was distributed in the soil in a manner which is broadly comparable with that observed in surveys of fall-out in permanent pasture in the recent past?*?°, The adoption of equation (2) causes lower levels of strontium-90 in milk to be predicted when the cumulative deposit is an important source of contamination, the reverse being true when the contribution of the recent deposit, especially that in the second half of the previous year, is large. To illustrate the extent of the difference between predictions based on the two equations, calculations have been made for two hypothetical models of ange future fall-out, namely: Model 4, the rate of fall-out will decrease by a factor of 2 annually. Model B, the rat« of fall-out will remain constant. Im both models it has been assumed that the annual deposit in the first year is 45 per cent of the cumulative deposit measured at mid-year (that was, the approximate situation in 1963, the year in which the highest levels of fall-out were observed) and that the rate of fall-out was similar in the secondhalf of the previous year. The decrease in the rate of fall-out in . 3> mal OY 3 | Boao aa 3 = Pa q Model A is greater than that anticipated in the absence of ee further weapon trials}* while the amount of fall-out assumed in Model B would over a few years much exceed that caused by all weapon trials hitherto. Future situations resulting from weapons testing may thus be expected to lie between the two models. Predictions made by the two equations are compared in Table 3. In considering the situation over long periods it is necessary to take account of loss of strontium-90 from the soil which arises mainly from leaching or from ecemper 5 1 8 9 4 4 6 3 wv us 6 2 3 0 7 3 5 6 : absorption by plants; following the United Nations Scientific Committee on the Effects of Atomic Radiation’ cee ee an average loss of 2 per cent per annum has been assumed. For the first year the level of contamination in milk predicted by equation (1) is about 20 per cent lower. Table 3, COMPARISON OF PREDICTED LEVELS OF CONTAMINATION OF MILK, BASED ON DIFFERENT Equations, FOR TWO HYPOTHETICAL MODELS OF ALL-OUT First year Years 1-10 Years 1-50 Tntegrated value Integrated value Model Both models A B A B Ratio of strontium-90 to calctum in milk (relative to year 1 calculated by equation 2) Based on equation 1 46 12 10 110 ee ae mn ep nueveae > te 4-1 12 7 90 Caestum-137 per Ll. of milk (relative to year 1 calculated by equation 4) Based on equation 3 8 19 40 4 L3 2. 8 10 3-0 54 Model A, fall-out rate halved each year. Model 2B, fall-out rate remains constant. The rate and cumulative total of fall-out in year 1 are taken to be the same in both models, the annual deposit being 45 per cent of the cumulative deposit at mid-year (that is, the relationship in 1963); it is further assumed that the tate was similar in the previous year. 5