76 EFFECTS OF IONIZING RADIATION monuclear explosion. This internal contamination resulted from both inhalation andingestion of fallout material. Highlevels of activity were found in water and on the external surfaces of plants. The contamination of the internal portions of fruits and vegetables was small. Of the individual radionuclides, Sr®, because of its high solubility and relatively long radioactive half-life was probably the isotope of greatest potential hazard in the environment. Fewof the fission products present in the environment were readily absorbed from the lungs and the G. I. tract. Radiochemical analysis of the urine samples from the Rongelap people indicates that Sr. Ba and the rare earth group together constituted 75 percent of the total beta activity of the urine at 45 days post detonation. Sr® was the predominant radionuclide at this time, contributing 42 percent of the total beta activity. Assays forfissile material in the pooled urine samples were negative. The humanbody burdenof individual radionuclides was estimated from radiochemical analysis of the humanurine andof the tissues and urine of animals from Rongelap. The mean body burdens of the radionuclides in the Ailinginae group were approximately one-half those of Rongelap, and the mean body burdens of the Americans about one-fourth of the Rongelap group. While the activity excreted per unit volumeof urine was the samefor adults and children from Rongelap, the total activity excreted in the urine in 24 hours by children under 15 years of age was significantly lower than that excreted by the adults. The total amount of radioactive material in the G. I. tract at one day post detonation was estimated to be 3 mc in people from Rongelap. This activity was contributed chiefly by isotopes of short radiological and biological half-life and limited solubility, and 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 urme of most of the exposed individuals, The estimated dose to the thyroid from I and other short-lived iodine isotopes was 100 S007 bo to 150 rep for Rongelap. Iodine is probably the most hazardous internal radioemitter at early times after exposure. The dose to the thyroid, although greater than tolerance, was low compared to the partially or totally ablating doses of Iused in the treatment of hyperthyroidism or carcinoma. At one day post detonation, the concentration of Sr® was calculated to be near the maximum permissible level for this nuclide. At later times following exposure, this longer-lived fission product presents the greatest potential internal hazard. Oral administration of calcium EDTA beginning 7 weeks postdetonation to a representative group of individuals from Rongelap increased the rate of excretion of activity 2.5 times. However, the decrease of the body burden was slight, as the excretion rate was very lowat this time. Analysis of the internal contamination indicates that the dose to the tissue of the body was near, but, with exception of the dose to the thyroid, did not exceed the maximumpermis- sible dose levels, Theactivity fixed in the body decreased rapidly as a function of time. The contribution of the effects of internal contamination to the total radiation response observed appears to be small on the basis of the estimated body burden of the radioelements. In viewof the short half-life of the most abundantfission products in the situation, the possibility that chronic irradiation effects will occur is small. 5.4 Internal Contamination of Animals Tue INTERNAL Contamination of a numberof animals collected on Rongelap was studied. The activity in their urine was studied, and radiochemical analyses were made of varioustissues. These data provided the basis for estimating the body burden of the radioisotopes in human beings. In addition, hematological and pathological studies were made, and autoradiographs of selected tissues were prepared. A number of the animals are also being studied for the