ABSTRACT The purpose of this report is to assess the magnitude and extent of the hazard to human life imposed by radioactive debris deposited in the human enviroment by large numbers of nuclear weapons. Known physi- cal facts, data from nuclear weapon test experience, and pertinent theo- retical considerations are utilized to evaluate the extent of the hazard. A discussion of the physical phenomena is presented to show the mechan- isms whereby radioactive particles may be formed. Consideration was given to the changes in the intensity of a fall-out field of radioactive debris ag variations are made in the proximity of the detonation to the earth's surface, the magnitude of the fission yield, the total yield of fission and fusion weapons and the meteorological conditions immediately before and shortly after an atomic detonation. The degree of local hazard involved will in every case at least equal or exceed that of the world-wide hazard when expressed as a function of unit areas. The biological significance of the effects of ionizing radiations of various levels on humans, the beneficial effects of shielding against high levels of radiation during the early decay of the radioactive fall-out field, the genetic effects, and the significance of internally deposited radioactive isotopes as carcinogens are discussed in the light of the presently existing data together with projected calculations of possible effects. For long-term carcinogenic effects, strontium-90 is considered to be the most hazardous of the radioisotopes spread both locally and world-wide. It appears likely that the number of nuclear weapon detonations required to cause a world-wide long-term strontium-90 hazard would be so large as to result in devastation of much of the habitable world area from the inmediate destructive effects of the weapons. iii