45 clinically observed effects of known amountsof in- ternally-deposited radium. These effects do not appear until a period of years (10 to 15) has elapsed. Thus, althoughit is possible to estimate the potenual hazardin termsof the concentration of internal emitters, clinical observations made within a few years after contamination yield no data on the degree of damage that mayultimately be produced. In evaluating the long-term effects produced by an acute internal exposure and exposureto residual contamination, Sr°° is clearly the critical element. Particular effort was therefore made to determine its levels in the urine of the Marshallese, and thus to estimate the body burdens. Of the gamma-emitting fission products, Cs"*” is of the greatest interest, even thoughit is of minor significance as an internalradiation hazard. Like Sr*°, Cs'37 has a gaseous precursor with a half-life sufficiently long to avoid early condensation in the fireball. Cs’*’ thus follows Sr? into the strato- sphere. Since the fission yields and the half-lives of the two radioelements are nearly equal, they are present in the fallout in like quantities. While they have different ecological cycles because of their different chemical properties, Cs'*’ nevertheless provides a useful tracer for studying the move- ment of Sr®° through the biosphere, since its gamma-emitting properties make it readily detectable. The body burden of fission products can be determined in three ways. The method of choice is the direct im vive measurement by whole-body spectrometry. The limitations of this method are that few whole-body counters are in existence, thev require enormously bulky shielding and thus are not easily transported to various sites, and their absolute calibration is difficult. Further, this method is restricted to analysis of gamma-emitting isotopes, since, to date, a whole-body beta counter has not been developed. A second methodfor calculating body burden, particularly for counting beta emitters such as Sr?”, is the estimation of the internal deposition from data obtained by radiochemicalanalysis of the ur >. Fin..uy, it 1s possible to make a completely in- direct estimate of the human body burden of _ radioisotopes by what may be called the environmental approach. In this method, the estimate of the body burden is based on the concentrations of the fission products present in the environment, chiefly the soil and the important components of the diet. In order to make this estimate, data must be obtained on the transfer of the fission products between successive elements of the ecological chain leading from soil to bone. For example, al- though Sr and Ca are chemically similar and thus appear together in the various componentsof the ecological chain, Ga is taken up preferentially by plants and animals so thatit is necessary to determine the discrimination factor for each step. When these factors are known, it is possible to estimate the concentration of a radionuclide in man from its concentration in any step of the ecological chain. All three of these approaches to the estimation There is also some interest in the neutron-induced radioelement Zn*, even thoughit, too, does of the body burdens in the Marshallese people will be considered in this report. The interest centers chiefly around the fact thatit EXPERIMENTAL PROCEDURES not appearin levels hazardous to humanbeings. is definitely transmitted through marine life, and thus provides a clear example of the transmission of a radioelement throughthe food chain to man. Other fission products and neutron-induced activities (Fe®*99, Co%?-35-6°, Mn3* Cel#s-Prit, Zr°°-Nb**, and Ru'**-Rh’°*) also appear in small amounts in the soil and the food chain, and thus may appear ultimately in man, but the levels are Whole-Body Counting The gamma-ray activity from the internallydeposited fission products and the neutron-induced activities in 227 of the Marshallese people were measured with a whole-body gamma scintillation spectrometer. The technique of in vivo gamma-ray measurement of human beings was Ww. The potential radiation effects that maybe produced byspecific quantities of internally-deposited radioisotopes can be onlv roughlypredicted from counting techniques madeit feasible. “ou ESTIMATION OF THE INTERNAL RADIATION HAZARD elements in the Marshallese until whole-body = « so low thatlittle effort was made to quantily these Ny Te been reflected in amiacrease in body burdens of some radionuclides..As will be shown, the increases, though manyfold, have remained far below maximum permissible levels.