74 EFFECTS OF IONIZING RADIATION Onthe basis of an assumeduptake of 20 percent per 24 hours, the integrated dose to the thyroid from ['*t and othershorter-lived iodine isotopes was calculated by the USNRDILto be about 100 rep. The LASL has estimated that this dose was about 150 rep for Rongelap group and 50 rep for the Americans, The differing approaches used by the USNRDLand the LASL for estimating the body burden gave results which, except for Ba™, are veryclose. The mean body burdens of the individual nuclides presented in Table 5.6 were calculated for the Rongelap group. Values for the Ailinginae group were approximately half those of the Rongelap group, and values for Americans, about one-fourth those of the Rongelap group. The total amount of radioactive material present in the G. I. tract at one day post detonation in the members of Group I was estimated as approximately 3 me. This activity was contributed chiefly by isotopes of short radiological and biological half-life and limited solubility. Thus the levels of activity in the tissues of the body were relatively low. The concentration of radioisotopes at 6 months post detonation was barely detectable in the urine of most exposed individuals. Iodine, which is quite soluble, is probably the most hazardous internal radioemitterin the early period following exposure (10). The dose to the thyroid was appreciable, but low compared to the partially or totally ablating doses of I'*' used in therapy of hyperthyroidism or carcinoma, .At one day post detonation Sr* was calculated to be near the maximum permissible level (5) for this nuclide. At later times following exposure, this longer-livedfission product presents the greatest potential internal hazard. The present study confirms the observation made in animal experiments that most of the radioactive elements formed in fission as well as the fissile material itself, are not readily absorbed from the lungs and the G. I. tract. Only I, Sr, Ba and a fewof the rare earth elements co c wi were absorbed to anysignificant degree. An attempt to measure bone-fixed radioactive emitters by meansof sensitive film badges taped belowthe knee, over the epiphysis of the tibia on a number of persons, yielded no positive results. No correlation could be obtained between the degree of internal contamination andthe clinical and hematological findings. Inview of the short half-life of the most abundant fission products deposited internally in this situation, the possibility that chronic irradiation effects will occur is quite small. Thus, an evaluation of the data on the internal contamination, including that of Sr®, leads to the conclusion that the internal hazardto the contaminated inhabitants of the Marshall Islands is minimal both from the acute and the long range point of view. 5.33 Source of Internal Contamination The fallout material consisted largely of calcium oxide and calcium carbonate. The fission products were adsorbed mainly on fairly large particles. The material was 10 percent soluble in water, and completely soluble in acid. Internal deposition of fission products resulted from inhalation and ingestionof the fallout material. Ingestion appears to be the more important of the two routes of entry into the body. The activity in the airsettles out fairly rapidly, but contaminated food, water and utensils retain their activity for long periods of time. The amountoffission products reaching the bloodstream through the respiratory tract is a function of particle size and solubility of the airborne contaminants. The particles with which the activity was associated were considerably larger than the optimum size for deposition in the alveolar tissue of the lung. Thus, the probability of the retention of inhaled airborne contamination was not appreciable during the exposure period. The hypothesis that ingestion was the chief source of internal contamination is supported by the finding that the gastro-intestinal tract, its contents, and the liver of autopsied chickens and pigs sacrificed at early intervals following