B= rate of intake of '%7Cs (picocuries per day) a«= iological mean time (days), Ar=absorstion factor, assumed to be 1.0. With biological mean time expressed as a function of age, equation (8) becomes: | B=0.018 R (xi+e-%) (9) where: x=age (vears) Equation (9) may be used, for example, to estimate the average body burdens of cesium437 in the U.S. population from milk consumption. The US. Public Health Service pasteurazed milk retwork results for 1963 and 1964 averaged 114 and 109 pCi/liter, respectively (20, 21). These data indicate that the average level of cesiuin-137 in U.S. milk supplies has decreased slowly over the past 2 years. Considering the smal! change in these levels, it can be assumed ° that the various age groups in the population <r in equilibrium with the cesium-137levels in milk they consume for the portion of their body burden due to milk intake, and their average body burdens for 1964 may be determined from an average value of daily cesium-137 intake. If it is assumed that milk ingestion for males less than 1 year of age resulted in an average intak. of 55 pCi/day during 1964, and using the s,ormalization factors in table 2 to estimate milk consumption by other age groups in the population, estimated body burdens may be determined from equation (9). The resulting values are shown in figure 7. r 4 Beteten The dos: to males less than 1 year of age from at-home milk consumption, using equation (5), is about 0.002 rad or 2.0 millirads in 1964. The poini should be stressed that these estimates apply only to that portion of the cesium-137 body bura-na of the U.S. population due to milk consumpt.on at home. The contribution to body burden and whole body dose from milk con- sumption outside the home and from other items of the diet arc additive. December i965 oo WeMALE et 1. Cy A % —t. 2 Wi a _t »- = B= body sarden (nanocuries) s- where: CU (8) AVERADE CESAIMH1TT BODY BURDEN INU. $, POPULATION (nime--ntes! B=RT, Adit! Figure 7. Estimated cesium-137 body burdens from milk consumption for U.S. population in 1964 For cesium-137 in U.S. milk supplies, estimates of cesium-137 body burden in figure % indicate that males 15 to 20 years of age had the highest Jevels; those less than 1 year of age, male and female, had the lowest levels; and females of all ages had a lower average level than males of the same age. Sunmary The observations of biological half-time for cesium-137 in man indicate that half-time is a function of age. The average biological halftimes for cesium-187 from birth through adult life vary by a factor of about 10. A simple mathematical expression has been presented which relates half-time of retention of cesium— 137 and age. Though the model]is crude, it has utility in predictions of doses and body burdens from cesium-137 for various ages within an exposed population. Additional measurements of cesium-137 retention are needed, particularly for younger age groups, to further refine the model and to improve prediction capability. More data are also needed for the elderly. The combination of changes in half-time and body weight results in a dose variation, for a given cesium-137 intake, from birth through adult life, of a factor of about two. The addi- tion of assumptions regarding patterns of milk intake as a function of age and resultant cesium-137intake leads to a different distribution ' of dose with age than is foundfora fixed intake