oo. fe ¢ ” ee However, underconditions of decreasingfall-out (Model A) the relative magnitude of predictions based on the two equations changes progressively, the integrated values over 10 and 50 years being about 10 and 40 per cent higher respectively when derived by equation (1). For Model B the two bases of prediction differ to a smaller extent and the difference would bestill smaller if an increasing rate of fall-out were assumed. Caesium-137 Unlike strontium-90, caesium-137 is usually subject to a considerable degree of ‘fixation’ in the soil on accountof its entrapment in the lattice structure of clay minerals and it usually enters plant roots muchless readily. It has been suggested that current fall-out is the only significant source of caesium-137 in foodstuffs, but this was disproved in 1960 and 1961. In many countries the decreasing rate of fall-out then caused smaller reductions in the levels of ee a ete ee caesium-137 in milk than would have been expected if they were determined solely by the current fall-out'. An explanation of the situation seemed to be provided by the fact that caesium-137 can enter plants relatively freely from soils which contain large quantities of organic matter. The surface soil underlying permanent pastures usually shows this characteristic, and experimental. investigations have indicated that caesium-137 continues to enter the edible tissues of pastures relatively freely durimg the 2 years after its deposition®®. This led.to the suggestion that an equation similar to (1) might be applicable except that the soil factor related only to the deposit in the previous two years**, namely: C= prF, + pala (3) where C is the 12-month mean concentration of caesium137 in milk (me./km?/year), #, is the annual deposit of caesium-137 (mc./km*/year), and F¢ is the deposit of caesium-137 accumulated over the preceding two years (me.jkm*). The values of the proportionality factors (Table 4) were considerably higher than those for strontium-90 because caesium-137 is transferred from the diet of cattle to milk ten or more timesas readily as strontium90 (ref. 24). Levels of caesium-137 in milk between 1960 and 1963 calculated by equation (3) agreed reasonably with those which were observed (Table 5) and it was found that the body-burden of caesium-137 in man could be related to the pattern of fall-out in a similar manner!-*5. When, however, the proportionality factors were applied to fall-out data for 1964 the calculated level in milk differed appreciably from that which was observed (Table 5). As the manner in which caesium-137 is initially retained on edible herbage and removed from it by rain differs little from that of strontium-90 (ref. 25), it appeared possible that a lag-rate factor similar to that 6 ws -