——_ 48 ity in the exposed population, compared with the unexposed, has not been found on the basis of incidenceorseverity of diseases. Absorption of Radionuclides From Environment A low level of radioactive fallout contamina- - tion has persisted on Rongelap. Body burdensof the Rongelapese were evaluated by radiochemical urine analyses until 1957, when gammaspectroscopy by use of a low-level counting chamber was added to the techniques of radiochemicalanalysis. The return of the Rongelapese in 1957 to their homeisland (which after careful survey was considered safe for habitation, despite a persisting low level of radioactive contamination) was reflected in arise in their body burdens and urinary excretion of certain radionuclides. During 4 years after the original contaminating event, additional weaponstests held in the area contributed slightly to the fission products in the environment. Since their diet includes a variety of imported foods, the people may have been delayed in reaching equilibrium with the environmentalfission products. Body burdens of gamma-emitting fission products (such as 137Cs and ®Zn) were measured ina whole-body counter and checked by radiochemical analysis of urine specimens. The levels of internal contamination per unit weight appeared to be about the samefor juveniles as for adults, male and female, but wide variations in levels were found, apparently due to differences in diet, metabolism, and age. Body burdens of 9Sr were estimated from urinary excretion as determined by radiochemical analyses. Both the external dose measurements on RongelapIsland andthelevels of radioactive isotopes in the food on theisland indicated that some librium with their environment. The ®Zn level dropped to 0.071 Ci in 1959. With a larger detector and a longer counting time than previously employed, it was possible to identify and quantify 60Co for the first time in these people; the mean level of 8°Co was about 11% of the ®Zn level (7.6 nCi). A small amountof residual activity remainedafter subtraction of *#°K and the above radionuclides from the total spectrum. In 1960 the mean level of urinary excretion of 9°Sr was 7.2 pCi/liter, 14% higher than in 1959. In 1962 the mean urinary 9°Sr level was 114 pCi/g Ca,giving an estimated body burden of 12.0 nCi. Analysis of bone samples from a deceased Rongelap woman (1962) gave an estimated body burden of 11.4 nCi. These levels represent about a 6-fold increase in 80Sr over the 1958 levels. The levels of 9°Sr in 1962 and 1963 hovered around the 12.0-nCilevel in adults and about 22 nCiin children, about 5 and 10% respectively of the maximum permissible level (for members of the population at large). The bone marrow dose from natural and residual radiation sources was estimated in 1958, a peak year, to be as follows: Dose to bone marrow, mrad/yr Internal 90Sr (11 nCi bady burden) 137Cg 4 65Z.n Natural (4°K,etc.) External Residual gamma (0.03 mr/hr) Natural (cosmic, etc.} Total 11.3 120.0 44 175 250 134 384 5359 body burden was 0.67 Ci, which is slightly higher than that of a similar group obtained in 1959; it It thus appeared that by the early 1960’s the body burdensof 9°Sr and 137Cs in the Rongelap population had reached equilibrium with the environment. Little or none of the body burden in the exposed people at that time could be considered residual from the initial exposure, since little difference was noted between the body burdens of the exposed and unexposed groupsliving on RongelapIsland. Radiochemical analyses of urine collected during the 1967-1969 surveys showedlevels of 157Cs and 9Sr definitely lower than in 1965. It is spec- at the same time for comparison. It appeared at this time that the people were approaching equi- on the basis of greater consumption of imported food with less dependence on home-grownitems. increase in 137Cs, &5Zn, and 9°Sr body burdens was to be expected when the people returned there in 1957. The 137Cs body burden in 1958 was about 0.68 wCi, about 60 times as great as in 1957, and the urinary 137Cslevel rose by a factor of 140; the mean body burden for 1959 was 0.57 pCi. The mean body burden of ®Zn estimated from wholebody counting data in 1958, after the return to Rongelap,was 0.36 “Ci, 8 times as high as in 1957, and 0.44 pCi in 1959. In 1961 the mean 137Cs was 300 times that of the medical team, measured 3006305 ulated that these lowered levels may be explained