oe — ~~ seme ee ee ee eee - Seer wrens mee | 4° oe ee * for maximum permissible cose, as well as length of stay within the arca of contaminalion. With knowledze of the magnitude of the radiation levels present aud the rate of ' decay, (t)—'?, it is possible to plan and execute a short stay even in a highly conftamineted area. Planning for continuous occupancy recuires more extensive analysis. The following data mayaid in such evaluation. The fallout 1nap and Table II suggest the Tortie degree of radiation exposure received in continuous occupancy under normalliving conditions beginning with the time of initial fallout. For those entering the con- _taminated zone four montis after the first fallout, however, and then living there jndefinitely, the area encompassed by the 50-r effective biological isodose line will have shrunk from about 25,000 to 2,500 square mites. At such time (four months after fallout), an area of about 1,000 ‘square miles within the 50-r isodose line might have the highest residual contamination, amounting to about three times the dose rates at the periphery. The 0.3 r per week cut-of-doors isodose-rate line mivht extend to about the same position zs the bne marked 50 on the map. As one attempts to extrapolate such data te one year after fallout, the analysis becomes still niore difficult and uncertain. Yhe data suggest, however, that if return is postponed to one year aiter fallout, the 59-r effective biological isodose line will have disappeared. On the basis of these coiservative estimates, the 1,000 square miles of highest contanunation might have en out-of-doors dose rate of about + r per week after one year. Similarly, personnel might accumulate a dose of about 100 for the first year following exposure and an additional 90 r over the next three years, ‘independent of the biological recovery factor. It is to be expected that this factor would be relatively great for such long periods of time, thus reducing the effective biological dose below 50 r. The 0.3 r per week out-of-doors isodose-rate line might encompass an area somewhat larger than the line marked 400 on the map. Ae rye 593 GaxstA RADIATION ExrosureS FROM J'ALLOUT Vel. 66 (The weathering factor for the islands in the Pacific has been greater than the as- sumed value for large land masses, so that at one year the out-of-doors dose rate on these islands was less, by a factor oi alinmost 2, than would be predicted by tiie method sugested here.) The forcgoing anaivses are bascd on passive factors only, not taking into account the actions of persens themselves in reducing contamination. If, for example, a permanent retuum into an area were post- poned for one year after fallout, the radio- logical situation would probably have bcen adequately appraised, and decontatmina- tion operations initiated. Moreover, with the return of a populace into a knowncontaminated area, more than normal pre- cautions might be expected in regard to occupancy of the more protective types of buildings and reduction of time spent outcf-doors. It appears not unreasonable to assume that the theoretical out-of-doors dose rates for the areas of highest residual contamu- nation, calculated by means of the extrap- olations given above, actually mugiit he many times reduced. The data thus suggest that, with this type of detonation, continual occupaney even of the most heavily contaminated area need be prohibited for only about one year. The task of evaluating radiation exposures from fallout is fraught with uncertainties, and one instinctively shrinks from proposing criteria based on such variables and intangibies. Yet we would be doing ourselves a disservice if we did not attempt an analysis of the relevant factors and incorporate them into some conceptual scheme as indicated here. The analytical approaches, and certainly the quantitative values suggested, are not to be considered precise but are intended, rather, to give order-of-nagnitude estimates. It is believedthat they are, in general, conserva- tive, t.e., they do not underestimate the potential hazards involved. Division of Biology and Medicine U.S. Atomic Eucrgy Corumissioa Washington, D, C. roa Ss