THE SHORTER-TERM BIOLOGICAL HAZARDS OF A FALLOUT FIELD Physiol., 188, 585-592, 1957. 8. Sacnmr, G. A. Mathematical analysis of the behavior of cellular systems. (Abstract) Radiation Research 5, 593, 1956. Sce also: Mathematical analysis of the behavior of celular systems. Argonne National Laboratory, Divi- sion of Biological and Medical Research Quar- terly Report, ANL 5486, pp. 47-52, 1955. 9. Konn, H. and R. Kauuman. Age, growth and the LDy of X-rays. Scienve 124, 1078, 1956. 10. Sacnen, G. A. Dependence of acute radio- sensitivity on age in the adult female mouse. Science, 126, 1039-1040, 1957. IL. Sacwgr, G. A. and D. Gras, Work in progress. 12. Koan, H. f. and R. F. Kauuman. The influence of strain on acute X-ray lethality in the mouse. I. LDw and death rate studies. Radiation Research, 5, 309-317, 1956. 13. Graun, D. Genetic variation in the response of mice to total body X-irradiation. Journ. Exptl. Zool. 185, 39-61, 1954. 14, Sacnen, G. A. 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University of Chicago Motallurgieal Laboratory RECOVERY FROM LATENT RADIATION INJURY IN RELATION TO PERMISSIBLE HUMAN EXPOSURE ' Report C3900, 1950. Also in: Biological Effects of External X and Gamma Radiation, part 2, pp. 435-463. KR. E.Zirkle, editor. Office of Technical Services, Dept. of Commerce, Washington, D. C., 1956. . Buarz, H. A. A formulation of the injury, life span, dose relations for ionizing radiations. Ea) covery of blood neutrophils in the dog after acute peripheral depletion. Amer. Jotern. 1. Application to the mouse. University of Rochester Atomic Energy Project Report UR206. 1952. . Buarr, H. A. A formulation of the injury, life span, dose relations for ionizing radiations. TI. Application to the guinea pig, rat and dog. University of Rochester Atomic Energy Project Report UR207. 1952. . Sacuzn, G. A. Remarks on the relation between single LDs and the mean survival times of decedents. Argonne National Laboratory Division of Biological and Medical Research Quarterly Report ANL 4571, pp. 106-114, 1951. . Watton, H. H. and A. M. Brues. Radio-ruthen- ium toxicity studies. Argonne National Laboratory Division of Biological and Medical Research Quarterly Report ANL 5378, pp. 162~105, 1955. . Moi, R. H. On wasted radiation and the interpretation of experiments with chronic irradiation. Journ. National Cancer Inst., 15, 907-914, we 112 1955. By WW. A. Buarr Department of Radiation Biology, University of Rochester School af Medicine and Dentistry It is well established that following whole body exposure to ionizing radiation recovery from the consequent latent injury frequently occurs nearly exponentially with a half-time in the range from 3 to more than 20 days in the species which have been studied. Tite criterion used to measure recoverydirectly is the increase in size of a second dose sufficient. to produce lethality as this dose is applied at greater intervals after a first sublethal dose. The injury so measuredis called latent because it precedes the clinical syndrome of radiation injury and is measurable at present only in terms of radiation dose. That recovery does not go to completion but leaves an irreparable residual is evidenced in either of two ways, by a permanentdecrease of the lethal dose, or, by a shortening of life-span [i]. According *to all indications recovery takes place similarly during, as well as following, exposure. For this reason it is a determining factor in how often successive doses may be given, or a protracted dose such as a fallout field maybe sustained, without exceeding a given level of injury such as that caused bya single brief dose of selected magnitude. Application of this type of calculation to human populations requires a knowledge of recovery rate in man; but this is not known and no direct nonlethal method has yet been devised to obtain it. Presumably recovery in man resembles that in someof the other species but there is no way ' Phis paperis based on work performed under contract withthe United States Atomle Energy Commission at The University of Rochester Atomic Energy Project, Rochester, New York. yet known to becomeassured on this point and there is a further complication in that measurements on animals present difficulties of interpretation which will be discussed now, Until recently it was assumed that an animal subjected to whole body irradiation would recover in all parts, except the skin, at the same rate. However, Carsten and Noonan have shown in the rat that exposure of the abdomenandlowerlevels only, leads to recovery with half-time just over 1 day[2], while exposure of the remainder of the body only, leads to recovery with half-time of 3 to 4 days [3]. Hagen and Simmions, [4] showed that the wholly exposed rat recovers with half-time about 7 days. These data suggest that recovery rate is possibly a function of volume irradiated. Data by Storer [5] in which the whole bodyof the mouse is exposed, but in which the dose is adjusted to give so-called intestinal death in about 4 days, also show a fast phase of recovery presumably associated with the abdominal region. Similar observations have been made. by others. These data, contrary to those cited above, indicate the possibility that segments of the bodyrecover, or tendto recover, at their own rates independently of whether or not other segments are irradiated. Non-homogencous recovery raises problems of measurement and interpretation which are illustrated in Figure 1. Assuming for simplicity that there are but. two segments of the animal with different recovery rates it will be seen that the recovery curve for the whole animalas defined bytest: dases will fall rapidly, initially, because of the fast segment A and 113