a = the fractional deposition of the The model for females is the same as for males up to a K mass of K, = 43 g. The modelfor females is then represented by the following equations: ingested 137Cs in the short-term compartment, (1-a) T; = the fractional deposition in the long-term compartment, in Tz T2=-17.14+109K, = the biological half-life of 197Cs days in the short-term compartment, = the biological half-life of 137Cs in days in the long-term (Ky> 43g), Ty = 14 e001 Kp (Ky > 43 g), a = 0.89 e(-0.016 K,) (Kt > 43 g). compartment. The biological half-life of 137Cs_ for Marshallese children of various ages has been The biological half-life and fractional deposition for both compartments are determined by the total potassium, Kt, in the body. Values for the grams of potassium in the total body, taken from Leggett 1987, were used with a polynomial interpolation to generate Ky for other ages. The body weight as a function of measured by BNL (Lessard etal., 1979) and can be compared with those predicted using the model described above. The average T!/? for 137Cs for 14 children ages 5 to 10 was 43 d; the average from the model is 40 d. For the 11- to 15-y age group, the BNL wholebody counting of 9 children gave a T'/? of 70 d; the model value age, My, in kg, is taken from ICRP (1975) and represented with equations developed by Adams (1981). The body weight as a function of ageis shownin Figs. 1 and 2. The mass of most body organsis assumed to be proportional to total pody mass; the proportionality is established by the ratio of a specific organ mass to the body mass of a standard adult. Thus, the organ mass as a function of age is also based on the ICRP agedependent body weights. The biological half-lives and fractional depositions for males are then determined from the following equations (Leggett, 1986, 1987): T]=T2=09K, +265 (g<K< 15g), Ty =To =13 (15g <Ki< 20g), for this age group is 72 d. Strontium-90 Infants, Children, and Adults The cycling andretention of ingested 7°Sris much more complex than that for ingested 137Cs. The development and changing physiology of bone structure from birth to adulthood greatly affects retention and discrimination factors for 90Sr and also deposition patterns in the various compartments of developing bone. For example, the distinct cortical (compact bone) and trabecular (cancellous) bone compartments in adults are not nearly as well differentiated in infants. Turnover times of calcium and strontium due to bone modeling are much morerapid in T2 =-1.22 + 0.72K; T; =18 e(-0.016K,) (Kt > 20 g) a =0.6 (5¢<Kt<20g), a = 0.81 e(-0.014 Ky) (Ky> 20 g). 2000168 Thus, the model predictions are quite good for the Marshallese children. infants and children than adults, and the Sr/Ca discrimination factors and the gut transfer of 90Sr change markedly with age. Rather than attempt a detailed discussion of bone 14