(2) Based on limited sampling in the upper atmosphere, it appears possible that the order of 20 MI fission product yield may currently be in the stratosphere and settling slowly to the earth. This value is one of the major uncertainties of our knowledge in the study of world-wide fall-out. Available evidence indicates that strontium-90 is the critical radioisotope to consider in evaluating the cancer formation hazard of fall-out. From the point of view of world-wide distribution, a large degree of uncertainty exists because of the unknown amount of bomb debris remaining in the upper atmosphere and the possible rate at which this material will reach the biosphere. The phenomenon of fractionation ide effective in influencing the amount of strontium-90 in the worldwide distribution system and thus further affects the amount available in the slow fall-out. (£) The long-term strontium-90 hazard in the local fall-out area of land-surface nuclear weapons in all cases will exceed the worldwide hazard from the same detonation(s). for this local hazard. There is no yield threshold It exists in the local fall-out area of each land-surface burst nuclear weapon, regardless of yield, and affects those individuals who live off the produce of the contaminated area. Strontium-90 deposits in human bones principally by way of uptake by the cow and passage to the milk. Hazard considerations de- pend upon several variables which cannot be quantitated at present. C«) Radio-iodine has been shown to deposit in human thyroid tissue through inhalation and ingestion of fission products from distributions at locations far from the point of detonation. Due to the nature of the injury to the gland and to the relatively short half-lives of the radio-iodine series, the consequences of such depositions as far as can be determined from information to date will be secondary in importance to the radiostrontium hazard. Current evaluation of the hazard due to internal deposition of various radioisotopes involves many uncertainties. 131 At present, the