systematic treatment of the relative internal hazards of the elements found in radioactive fall-out material. Consideration of the problem has led to the conclusion that the bone seeking elements are the ones most likely to be the major cause of hazard for long-term internal effects. The comparison of the various elements produced included the following parameters in calculating relative effect. 1. Fission yield 2. Natural decay constant (A, in Table 7) 3. Biological elimination constant 0, » in Table 7) 4. Oral absorption, % 5. Deposition fraction in bone 6. Average energy of emitted particles and relative biological effectiveness. For comparative purposes, Table 7 has been compiled on the basis of metabolic data taken primarily from the work of J. G. Hamilton2l/ to show the approximate relative internal hazards posed by various radioisotopes found in bomb debris. This table indicates that within a short time after detonation strontium-89, strontium-90, and barium-140 are relatively comparable in their effects. As time passes, however, strontium-90 becomes increas- ingly important and by the end of the first year it is the most important element to consider in hazard evaluations. Maximum Permissible Concentrations of Radioisotopes. Radio- active isotopes are damaging agents to humans, apparently without threshold effect, when one considers genetic damage and possibly without threshold for carcinogenesis except for the fact that low doses may have a latent period greater than the human life span. In a true biological sense, therefore, limits of acceptable damage must be set and the amounts of material which will not exceed these limits determined. 37/ Hamilton, J.G., The Metabolic Properties of the Fission Products and the Actinide Elements, Rev. Modern Physics 20:718, 1948. 97