Table 6— POSTULATED AVERAGE MAXIMUM EQUILIBRIUM Sr? BONE LEVELS IN THE WORLD POPULATION (upe/g Bone Ca) Mid~-19$57 United States North temperate latitude South temperate latitude Rest of world World averaget About 1963* About 20507 Ecol. Bone Ecol. Bone Ecol, Bone data data data data data data 3.1 3.2 0.6 0.8 (2.8) 1.7 1,7 0.5 0.3-—0.5 (1.5) 3.5 3.6 0.7 0.9 (3.1) 1.9 1.9 0.6 0.5-—0.8 (1.7) 31 32 6 8 (28) 17 17 5 3-5 (15) * Assuming no more Weapons tests. + At equilibrium with a continued test rate of 10 MT equivalents of fission per year. ¢ Population weighted average. An alternative method of estimating average maximum equilibrium bone levels involves the use of current Sr® bone analyses, by adjusting the data for the pronounced variation in Sr®/Ca ratio in bone as a function of skeletal age. Langham and Anderson™estimated the . fraction of Sr*’/Ca skeletal equilibrium from the rate of skeletal accretion® and the rate of increase in integrated fallout shown in Fig. 4. It was assumed that each yearly increment of Skeletal growth contains Sr® at a concentration corresponding to the sr™ build-up in the biosphere for that year. For a first approximation, the skeleton was regarded as a unit and the Sr” burden averaged over the entire skeleton. Calculated values for the apparent fraction of equilibrium sr”/ Ca ratio as a function of age, based on skeletal growth rate alone anda yearly doubling time of the Sr® level are shown by the solid curve of Fig. 5. The points represent Kulp’s 1955-1956 data® normalized to the 0- to 4-year age group as representing 59 per cent of equilibrium Sr® concentration. At age 24 (4 years beyond the age at which skeletal growth stops) these data show that 7 to 10 per cent of the skeletal calcium was involved in bone remodeling plus exchange during the period of environmental contamination. If an equivalent fraction of the skeletal calcium of growing subjects is involved in exchange plus remodeling, then the Sr®*® levels in children would be proportionally higher (dashed line, Fig. 5) than the curve based on skeletal calcium accretion alone. This indeed appears to be the case and indicates that the major factors have been considered in constructing the model. The upper curve in Fig. 5 permits the use of adequate bone data from any age group to predict the average maximum equilibrium Sr® bone level and indicates a value of 0.9 pyc per gram of Ca by the end of 1955. Strontium-90 content of skeletons of stillborns” during 1955 averaged about 0.5 pyc per gram of Ca, which gives an average maximum equilibrium level of 1.0 when the placental discrimination factor of 0.5 is considered. Bryant et al.** in England reported analyses of 28 bone samples from subjects of all ages collected about January of 1956. Eight samples from persons ranging from 3 months to 3%, years old (average 1'4 years) averaged 0.9 ype of Sr® per gram of Ca, and 11 subjects ranging from 20 to 65 years of age (average 36 years) averaged 0.07 yuc of Sr™ per gram of Ca (after dividing all rib results by 2).° The predicted average maximum sr” equilibrium levels about January 1956, based on these age groups, are 1.0 and 0.9 ywuc per gram of Ca, respectively. On the assumption that surface deposition levels had a doubling time of one year, an aver- age maximum bone equilibrium level of 1.8 puc per gram of Ca was predicted for the north temperate latitudes for the fall of 1956.** Data on Sr™ fallout from pot collection samples in New York and Pittsburgh® show, however, that fallout did not double but increased by only about 50 per cent. On this basis, the predicted level for the north temperate population belt in the fall of 1956 would be 1.4 pyc per gram of Ca. Kulp’® applied the same age weighting method to 1956-1957 milk data (assuming a bone-to-diet discrimination ratio of 0.25) and estimated average maximum equilibrium bone levels (by the end of 1956) of 1.1, 0.9, 1.1, and 0.5 for North America, Europe, Asia, and the rest of the world, respectively. A crude estimate of present and future average maximum equilibrium bone levels can be made from the 1956 data 294