sufficient to reliably associate damage with body burden except in the crudest fashion. The inability to associate an increment of damage due to the mesothorium in the luminous dial workers indicates the degree of uncer- tainty in correlating damage found with particular body burdens of radium. It is fortunate that the data do strongly suggest e critical level of 0.5 - 1.0 microgram of radium body burden as the damage threshold. Other factors regarding the risk involved in the anticipated strontium-90 contaminated fall-out area also must be considered. The radium experience underlying the establishment of the MFC consists of adults with a high intake over a relatively short period of time. The strontium-90 problem on the other hand is concerned first with young people having a continuous long-term intake. How to evaluate the MFC background data in terms of the different situations involved is still unresolved. Effect of Higher Concentrations. The MPC concept has the relatively easy task of defining a safe level. Since the radium data. tend to show a damage level of 0.5 - 1.0 microgram of radium and no damage in the cases below 0.5 microgram, these facts were of direct value in determining the MFC. The analysis of the high levels of radium deposition do not show a proportional increase in amount of roentgenographic changes nor in their severity. Thus, current data do not permit a damage projec- tion to higher levels than the MFC. In addition besides the rather arbitrary MFC which includes a safety factor, the lowest damage level actually observed can be defined. For radium this is considered to be 0.5 - 1.0 microgram, and, hence, for strontium-90 this becomes 5.0 - 10 microcuries. - The Project GABRIEL report for July 1954 states: . "Using the radium to strontium-90 conversion factor of 10, one would estimate from these data that 1.0 microcurie strontium-90 adult body burden at 121