5,760 years}.5% 744 Of particular pediatric medical significance are the hazards of internally deposited radionuclides from tropospheric and stratospheric fallout rather than those from local or near-in fallout. Strontium-90. The principal source of strontium-90 is dietary, through ingestion of directly contaminated vegetation and through incorporation of strontium-90 into the food chain, both plant and animal.*" 1°* 19 Tn children, milk is considered an impor- tant source of strontium-90, although contaminated potable water may contribute additional amounts of the radionuclide.*? The distribution of the radionuclide in the body follows the metabolic paths of calcium.» 12? The high turnover rate of minerals in the bones of infants and children under 2 years of age is presumed to lead to rapid equili- bration of the bone minerals and consequently to uniform deposition of strontium90 throughout the skeleton.*?)13 A yearly replacement of 30 to 70 per cent or more of bone mineral has been estimated during the first and second years of life.*? 1*° In the adult, 2.0 to 3.5 per cent of bone calcium is replaced annually.** 144 The bonesofstillborn fetuses and of newborn infants have a lower strontium/calcium ratio attributed to discrimination by the placenta which favors by a factor of 2 the passage of calcium over strontium from mother to fetus.*” 1% Metabolic discrimination between strontium and calcium does not become established until the second year of life,1!*?1* the “observed ratios” of strontium and calcium in bone to those in the diet being about 0.8 at 2 to 3 months of age, less than 0.4 at 9 to 10 months, and 0.25 (adult ratio) after 2 years.°* In 1960, the mean concentration of strontium-90 (as pCi per Gm. calcium) in bones of subjects of different ages in New York City were: 6.81 in infants, 9.84 at 1 year of age, 5.03 at 2 years, 3.41 at 3 years, et C24 = "er cae! [ on ee oFG 2.41 in children from 5 to 19 years, and 1.55 in adults over 19 years of age.°?8 The higher concentration in the bones of children is related not only to higher milk consump- biological half-life of cesium in children has been reported in two studies to be about 44 and 38 days and about 100 days in adults.5* 121: The more rapid turnover partially accounts for the ratio of cesium-137/ potassium being lower in children than in adults. Observed levels of cesium-137 in man were two to four times higher in late 1962 than the average levels in 1961.57 The absorption of this chemical by plant roots is poor, so that the entry of the nuclide intc the food chain largely depends on ingestion by animals of food contaminated by direct deposition. Milk and meat are the greatest contributors to human dietary intake.*’ Large age variations in body burdensof this nuclide have not been noted, particularly when related to lean body mass or to potassium.**%122, 128, 124, 125 Alone with the relatively short-lived zirconium-95, niobium-95, and Ba-La’® (half-life,13 days), cestum-137, which gives off gamma rays, contributes an estimated 90 per cent of the whole body and genetic dose from fallout.’ Yodine-131. Iodine-131, with a half-life 7 to 8 days, is a special hazard only during the first few weeks following environmental con- gh EE tion but also to greater deposition of Sr°° in their growing bones. Being a beta emitter, the isotope does not contribute significantly to the genetic dose; the effect is localized anatomically where it is responsible for an estimated 50 per cent of the marrow dose and 80 per cent of the bone dose.The potential danger from strontium-90 is assumed to be the development of osteogenic sarcoma and possibly leukemia. Although the behavior of strontium90 has been examined on ecological and radiophysics bases,°” 1447277428 the magnitude of the biological risk remains to be determined™*; a rather high threshold dose of well above 20 rads and probably nearer 1,000 rads has been suggested for bone sarcomagenesis from radium deposition.** 77° Cesium-137. Cesium-137 is not fixed in the body andits distribution is considered to be uniform. The metabolic behavior of cesium137 resembles that of potassium.It is rapidly absorbed from the intestinal tract and is distributed uniformly in the soft tissues. The Ma are strontium-90 (half-life, 28 years), cesium-137 (half-life, 30 years), iodine-131 (half-life, 8 days), and carbon-14 (half-life, 665 a ae ager EE: Effects of tontzing radiation seme, ie en's 3 - Volume 67 Number 4+