5 nearly one 1/10000 the minimum amount that is known to have produced injury to human beings. Natural radium and strontium-90 in fallout have differences in properties which may influence the ease with which are sufficiently sensitive so that it is feasible to detect this isotope at concentrations comparable with that of radium and other naturally occurring isotopes. Measurements of this type have been undertaken and have succeeded in demonstrating the absorption of strontium-90 in foods. The concentration of this iso- tion to the very much larger concentra- A more direct method of evaluating the significance of the strontium-90 fall- out is to measure the presence of this iso- . calcium content of the food, and forthis concentration would thus be 1000 micromicrocuries per gram of calcium, or tion of radium that could besafely tol- I. reason the results can be expressed best as strontium-90 activity per gram of cal3 micromicrocuries of strontium-90 per gram of calcium. One microcurie ofstrontium-90 (7) is the commonly accepted permissible content for the adult skele- erated. References and Notes tope, as expected, is dependent on the they pass from soils into biological systems; therefore, one cannot concludethat, for any given soil content of strontium-90, the equilibrium human burden would be the same as the equilibrium body burden of radium at the same soil level. However, it is worth noting the minuteness of the present strontium-90 values in relation to the amount of radium present in all soils and, more particularly, in rela- thousandth of what is considered a permissible dose.” we radium content of the upper 1 foot of sou. This amount (10microcuries) is 1 1000 the maximum permissible radium burden of 0.1 microcurie and more nately modern radiochemical techniques of radioactive strontium in their bodies. The amount, however, is quite small—a cium. Onthis basis, milk in the United States during early 1956 contained about ton (8). The skeleton contains about 1000 grams calcium, and the permissible about 350 times the presently observed concentration in milk. According to the National Academyof Sciences (9), “Already some children have accumulated a measurable amount = relation to the general environmental tope, not only in soils, but in plants, animals, and human foodstuffs. Fortu- Suan 10-4 microcuries of radium, which may be taken as the biological equilibrium of the radium burden of human beings in 9. M. Eisenbud and J, H. Harley, Science 117, 141 (1953). , bid. 121, 677 (1955). We express our appreciation to our colleagues who participated in this program of fallout collection. In particular, A. E. Brandt is responsible for the IBM reporting of computations as well as the statistical analyses. Edward P. Hardy, Jr., and Robert S. Morse performed the soil analyses, and Naomi A. Hailden assisted in developing the procedure for computation of the gamma dose. C. L. Dunham made a number of helpful suggestions in the preparation of the manuscript. The continued cooperation of Lester Machta and his staff at the U.S. Weather Bureau has been invaluable. H. F, Hunter and N. E. Ballou, Nucleonics 9, No. 11, C-2 (1951). G. M. Dunning, Sci. Monthly 81, 265 (1955). W. F. Libby, Science 122, 57 (1955), Natl. Bur. Standards U.S. Handbook 52 (1953). The permissible concentration of strontium-90 is probably lower by a factor of 10 than the concentration that would produce injury. On the other hand, the 1-microcurie level was established for occupational exposure, and the National Committee for Radiation Protection recommends that such levels be reduced to 10 percent for public exposure. Biological Effects of Atomic Radiation. Report to the public (Natl. Acad. Sci.—Natl. Research Council, Washington, D.C., 1956); Science 124, 60 (1956).