Cohn, Robertson, Conard has been made in the past to detect them or to trace their movement. Thus onl the above three fission products of greatest interest will be considered here. The body burden of fission products can be determined in three ways. The method of choice 1s the direct in vivo measurement using whole-body spectrometry. The mutations of this method are that few whole-body counters are in existence, and their absolute calibration 1s quite difficult. More importantly, this method is, of course. restricted to analysis of gamma-cemitting isotopes, since, to date, a whole-body beta counter has not been developed. A second method for calculating body burden, particularly for counting beta cinitters such as Sr#*, is the estimation of the internal deposition from data obtained by radiochenncal analysis of the urine. Finally, it is possible to make a completely indirect estimate of the human body burden of radioisotopes by what may be called the environmental ap- proach. In this method, the estimate of the body burden is based on the con- centrations of the fission products present in the environment, chiefly the soil and the important components of the dict. lu order to make this estimate. data must bo obtuncd on the transter of the fesien products between successive ciaments of the cevlogmeal chain leading from soil to bone. For example, al- though strontium and caletum are chenncally similar and thus appear together in the vanous components af the ecological chain. calcium is taken up prefetcutis ds plunt) and amin ds. se that if as necessur to deternune the diseriny Waition factor Tor cach step MN ‘hen these factors are known, it is possible to estimate the dose ta the ° critical organi mm man trom the concentration of the notope in am step of the chain. Environmental Estimate of Body Burden The environmental cstimate of internally deposited Sr** can be made in the following way. A number of rats were collected on Rongclap at 2 vcars follow- ing the 1954 accident. Thev had subsisted on a dict consisting primarily of plants. These animals were subjected to careful radiochemical analysis, as it was thought that they might serve as indicators of the internal radiation hazard that would result to human|pcings if they inhabited the same arca during this time. and subsisted on essentially the sumedict. The Si/Ca ratios for vations tissucs of these rats were measured directly and these valucs were compared tu the Sr""Ca ratios of the food and soil on Rongclap collected at the same time. Thus a comparison could be made between the environmental approach and the direct measurement. Extiapolation of the environmental data will give the equilibrium valuc. whereas the direct measurcment give the value at the time af measurement. In this manner, the per cent of cquilibrium of Sr internally deposited in the rat can be determined. It can be seen from the data that the Sr/Ca ratios for different foods on Rongclap varied greath. Further, the diet of the rats on Rongclap was too uncertain to assume an “average” dict, so that it was necessary for this estimate to use the Sr™/Ca values of the soil itself. 320