(Reprinted from Nature, Vol. 209, No. 5028, pp. 1062-1065, March 12, 1966) PREDICTION OF FUTURE LEVELS OF LONG-LIVED FISSION PRODUCTS IN MILK By B. O, BARTLETT and Dra. R. SCOTT RUSSELL Agricultural Research Council Radioblological Laboratory, Wantage, Berkshire INCE 1958, when the United Nations Scientific Committee on the Effecte of Atomic Radiation! made its first comprehensive review of the consequences of world-wide fall-out, many attempts have been made to derive formulae for calculating the level of radioactivity in foodstuffs which is likely to be caused by different patterns of environmental contamination. Information has been obtained mainly by analysing relationships between measurementsof fall-out and of dietary contamination made in environmental surveys. Controlled experi- ments on food-chain mechanisms and information on agricultural practices are, however, important supplementary sources of guidance; besides indicating whether relationships suggested by survey results are causal or fortuitous they can reveal factors which, though unimpor- tant under conditions so far experienced, may require greater consideration in the future. Improved information can be expected as the period during which observations have been made becomes longer because both climatic factors and variations in the seasonal distribution of fall-out influence the transfer of radioactivity to diet. Conclusions previously reached in the United Kingdom?-* have, therefore, been reviewed in the light of the more extensive data which are now available. Consideration has been limited to two nuclides, strontium-90 and caesium-137, in a single foodstuff, milk. These two nuclides are mainly responsible for the internal radiation doses to which long-lived fission products give rise and it is well established that the contamination in milk has provided a good guide not only to the exposure of children but also to that of adults who consume a mixed diet of the type normal in Western countries’. Moreover, the data on the contamination of milk are considerably fuller and more reliable than those for other foodstuffs. : The annual average levels of contamination in milk throughout the United Kingdom have alone been con- sidered; predictions for smaller areas or shorter periods would be subject to much greater uncertainty. The country-wide average deposition of strontium-90 used in the present calculations has been derived, from the