247 T. M. BEASLEY, E. E, HELD and R. M. CONARD approximately 1/100th of the maximum permissible body burden which has been established for non-occupationally exposed individuals considering the total body as thecritical organ.4%) Previous measurements of ®5Fe body burdens during a period of increasing **Fe fallout generally showed that 5°Fe body burdens of females were higher than those of males.@:5) Presumably this is due to higher turnover rates of iron in females than in males, with the result that females are more nearlyat equilibrium with their environment. As environmental levels of 55Fe decrease, females should, on the average, reflect this change by exhibiting lower Fe body burdens than those of males. Figure | shows that more female body burdens tended toward values <0.4 wCi, while male body burdens were more normally distributed, about a mean of 0.43 uCi. Regression analysis of age on body burdens showed a significant correlation (P < 0.001); older individuals had higher *5Fe body burdens. Table 2 shows a tabulation of the average ®°Fe body of males and females by age groups. The number of samples per age group is admittedly small yet the general increase of **Fe body burden with age appears qualitatively consistent with earlier data obtained by analyzing blood from Seattle, Washington males in 1966. Comparison of the 55Fe body burdens of peoples of different countries) requires knowl- edge of the turnoverrates of 9°Fe in the environ- ment and in humans. Jennincs’?) has shown that the 55Fe specific activities of salmon taken Table 2. Average body burden of *°Fe in Rongelapese residents of different ages Males Females Number of Body burden Age samples (ui) 16-20 8 0.31 21-31 32-42 43-53 54-64 > 64 16-20 21-31 32-4? 43-53 54-64 > 64 4 5 2 6 3 6 12 5 7 2 2 0.33 0.52 0.58 0.53 0.48 0.23 0.34 0.33 0.66 0.57 0.66 from the northeast Pacific Ocean decreased eightfold between 1S64-1£67. Assuming that a first order reaction governed the removal of *5Fe from the mixed layer of the ocean (upper 100 m) he calculated the effective half-life for 55e loss as 11 months. Measurements in cattle and rain waters show decreases, but at lesser rates. Tron-55 body burdens of adult males in Richland, Washington, decreased approximately 4 fold between 1067 and 1970, corresponding to an effective **Fe half-life of 1.5 yr. If the ®*Fe turnover rates of Richland, Washington, residents are similar to those of insular populations, we conclude that people from maritime cultures would exhibit similar and perhaps faster turnover rates of Fe because of the short “‘ecological half-life’? of this radionuclide in the marine environment. The highest **Fe body burdens previously measured were in female natives at Bethel, Alaska, during 1966.) The average body burden of 18 females was 1.1 wCi. During the same year, the average 5°Fe body burden of females and males at Tokai-mura, Japan was 0.92 wCi and 0.63 wCi respectively.’ We determined the ®°*Fe body burdens of 32 females and 37 males from Tokai-mura from blood collected in October 1970; the average values were 0.12 and 0.17 respectively. Thus, not only do the Rongelapese havesignificantly higher 55Fe body burdens than those of the Tokai-mura residents, but the decrease in the 55Fe body burdens of this latter group from 1966 to 1970 appears comparable to that for Richland, Washington, males. As previously stated, all of the donors of the Rongelap study were subjected to external radiation during the accidental contamination of Rongelap Atoll in 1954. Because of the high levels of radioactivity at the Atoll, the Rongelap natives were moved to Majuro Atoll where they resided for 3.5 yr. Following exposure in 1954, whole body counting and urinalysis disclosed measurable quantities of internally deposited fallout radionuclides. By 1957, however, the only radionuclides present in the Rongelapese in significantly measurable quantities were °°Zn, 137Cs and %Sr.49) No 5°Fe analyses were per- formed at that time so body burdens of this radionuclide are not known. However, based