for radium might have to be lowered by a factor of 2. Nevertheless, for purposes of calculation, the tolerance level of 0.1 microgram of radium as the maximum permissible concentration still stands as the commonly accepted value. The Radio-strontium Problem. Strontium-90 is produced in con- siderable quantity in nuclear detonations, the currently accepted value being approximately 1 gram of stron%ium-90 per KT of fission yield. Theoretical analysis and actual experimental findings discussed earlier suggest that strontium-90 is produced in a manner which makes it easily available for incorporation into the biosphere. Adding these character- istics of relatively lerge scale production and physico-chemical availability to the previously indicated properties of the physical half-life, relatively high bodily ingestion and absorption, and its bone seeking properties which give it a long biological half-life, it is evident that the importance of this element is quite well established with regard to internal hazard. For large populations the greatest hazard will be to those who have to bear the burden of isotopes for the longest period of “time, i.e., the fetus and child. In addition, the sensitivity of young, rapidly growing tissue to radiation damage is greater than that of mature, more slowly growing tissue, and this adds to the risk of the younger age groups. Strontium-90 has never been administered to human beings in dangerous amounts so far as is known at present. The work done on this problem has attempted to correlate strontium-radium effects in animals and to apply this relationship to the maximm permissible concentration of 0.1 microgram of radium in humans, thereby establishing the maximum permissible concentration for strontium-90. Brues' experi- ments with strontium-89 are the only ones applicable to the problem of establishing the maximm permissible concentration for strontium-90 in humans. On the basis of Brues' work with strontium-89 in mice, and mouse radium data, a strontium-89 to radium toxicity ratio of 1:10 on a microcurie basis was established. It has been stated that the 100