remains unaccounted for were to settle out slowly from the upper atmosphere over a period of many years and be subjected to this four-fold increase, then consideration must be given to this in future hazard calculations. This analysis is an oversimplification, since no account is taken of the decrease in world-wide distribution due to local fallout, the relative efficiency of the fall-out collecting system and the possible increase, with time, in the number of Sunshine Units in bone due to the strontium-90 already fallen out. The Lamont Laboratory aatat3/ suggest that the younger age groups in the more recent samples have a quantity of strontium-90 lodged in bone approximating 1.0 Sunshine Unit. body calcium comes from dairy products. In the United States 85% of If the young age group in an area of high dairy product intake is considered as the critical segment of the population for risk assessment, it would appear then that dairy products should be correlated with bone deposition. If the current data can be assumed to be a reflection of a maintained or increasing rate of uptake, then the bone of newborns should approach the strontium90/calcium ratio present in milk products. Due to the lower saturation of the mother cow, there is a lowering of the potential ratio of strontium/calcium in the milk and calf. The same mechanism applies in the transport of strontium and calcium from cow's milk through the human mother to the fetus. Thus the current strontium-90 values in bone and milk products are lower than would be expected for the current fall-out, if the mother cow and the human mother had both grow up in this same contaminated environment. Since the hazard calculation seeks to find the most sensitive population at risk we cannot accurately project current data to a situation of a long-term, maintained contaminated environment. The data can be extrapolated, to some extent, if allowance is made for the fact that eventually all parts of the chain transporting strontium-90 to 43/ Kupl, J.L., Project SUNSHINE Annual Progress Report, March, 1955. 114