1x 10° gm. Then to achieve a bone radio-strontium level of 1 microcurie, & continuous daily intake of 1.5 x 107 pe strontium-90 is necessary, since A, = luc x 107? gm -O7 gm =1.5x 1079 pe Since, at present, the bone strontium content (sr,) and daily strontium intake (Srp) are relatively constant, the maximm attainable radiostrontium content (A,) is determined by Ap: This means that the spe- cific activity, 1.e., the ratio of strontium-90 to stable strontium, in the diet is in the upper limit of the specific activity of bone strontim. Implicit is the corollary that for a fixed radio-strontium intake per day, an increase in stable strontium intake will result in @ lower bone strontium specific activity if there are no alterations in the metabolism of strontium with an increase in daily strontium intake. Under these conditions of increased strontium intake the bone radiostrontium level will decrease, with time, to a new level, fixed by the specific activity of the food strontium. Hazard Calculations. The evaluation of long-term hazard from radioactive fall-out has led to the identification of strontium-90 as the critical isotope on the basis of knowledge currently available. At present, Project GABRIEL and Project SUNSHINE are-the two most serious studies concerned with strontium-90 evaluation. These studies accept strontium-90 as the most likely critical parameter for long-range hazerd effects and have begun to accumulate and assess data concerning the actual strontium-90 situation as it exists in the world today. The RAND Corporation SUNSHINE Project report normalized its findings to a maximum permissible concentration (MPC) of 1.0 microcurie of strontium- 90 lodged in the bone of a standard man. This enabled the physical data relating to fall-out to be interpreted in terms of a definitive amount of strontium-90 deposited in human bones. 109 The actual signifi-