+8 ity in the exposed population, compared with the unexposed, has not been found on thebasis of incidenceor severity of diseases. Absorption of Radionuclides From Environment A low level of radioactive fallout contamina- tion has persisted on Rongelap. Body burdens of the Rongelapese were evaluated by radiochemical urine analyses until 1957, when gammaspectros- copy by use of a low-level counting chamber was addedto the techniques of radiochemical analysis. The return of the Rongelapese in 1957 to their home island (which after careful survey was considered safe for habitation, despite a persisting low level of radioactive contamination) wasreflected in a rise in their body burdens and urinary excre- tion of certain radionuclides. During 4 years after the original contaminating event, additional weaponstests held in the area contributedslightly 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 !37Cs and ®°Zn) were measured in a librium with their environment. The ®°Zn level dropped to 0.071 pCi in 1959, With a larger detector and a longer counting time than previously emploved,it was possible to identify and quantify 60Co for the first time in these people; the mean level of 5°Co was about 11% of the ®5Zn level (7.6 nCi). A small amount of residual activity remained after subtraction of 49K and the above radionuclides from the total spectrum. In 1960 the mean level of urinary excretion of 99Sr 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 11.4nCi. These levels represent about a 6-fold increase in %Sr over the 1958 levels. The levels of 9°Sr in 1962 and 1963 hovered around the 12,.0-nCi level in adults and about 22 nCiin children, about 5 and 10% respectively of the maximum permissible level (for members of the population atlarge). The bone marrow dose from natural andresid- ° ual radiation sources was estimated in 1958, a peak year, to be as follows: Dose to bone marrow, mrad/vr whole-body counter and checked by radiochemical analysis of urine specimens. Thelevels 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 9°Sr were estimated from urinary excretion as determined by radiochemical analyses. Both the external dose measurements on RongelapIsland andthe levels of radioactive isotopes in the food on the island indicated that some increase in !37Cs, 65Zn, 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 137Cs level rose by a factor of 140; the mean body burden for 1959 was 0.57 pCi. The mean body burden of §5Zn estimated from whole- body 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 body burden was 0.67 pCi, whichis slightly higher than that of a similar group obtained in 1959; it was 300 times that of the medical team, measured at the sametime for comparison. It appeared at this time that the people were approaching equi- Internal 90Sr (11 nCi body burden) 137Cs 4+ 65Zn Natural (49K, etc.) 11.3 120.0 44 External Residual gamma (0.03 mr/hr) Natural (cosmic, etc.) Total = 175 250 134 384 559 It thus appeared thatby the early 1960’s the body burdens of 99S5r and 137Cs in the Rongelap population had reached equilibrium with the environment. Little or none of the body burdenin the exposed people at that time could be considered residual from the initial exposure, since little difference was noted between the body bur- dens of the exposed and unexposed groupsliving on Rongelap Island. Radiochemical analyses of urine collected during the 1967-1969 surveys showedlevels of 137Cs and 99Sr definitely lower than in 1965. It is speculated that these lowered levels may be explained on the basis of greater consumption of imported food with less dependence on home-grownitems.