RECOVERY FROM LATENT RADIATION INJURY THE SHORTER-TERM BIOLOGICAL HAZARDS OF A FALLOUT FIELD cells. In any case, at this time, it is safe to assume only that recovery as determined experimentally by paired doses may be faster RADIATION DOSE 114 than that of the slowest recovering tissue and that the use of this recovery to predict. the levels of injuryfor prolonged or intermittent exposures muy underestimate them eonsiderably. For this reason in clioosing a tentative value for man probably it is advisable, in the absence of other information, to select a recovery half- TEST OQOSE time somewhat in excess of the longest known ~— ln. TIME a AFTER INITIAL DOSE Frover I—Following an initial whole body dose, about one-hatf LDDs in this case, recovery of the fastest segment is represented by curve A and of the slorest by curve BL Test doses adjusted to produce LDtill define a recavery curve somewhere between A and B strongly influenced hy A initially and later tending to run parallel to B. There are implications in these Statements with respect lo the summation of radiation effects produced in different segments of the body. problem haa been discussed (if). will later run parallel to, but below, B. This The effect will be to make recovery appear too fast in the carly stages and too complete in later stages. The first will indicate too short a half-time and the second will tend to obscure the residual injury. If curves A and B are simply exponential the experimental curve will not be. There is suggested, perhaps, somepeculiarity of abdominal radiation which alters its relative importance in some strains or species or under certain conditions since some observers find a fast early, presumably abdominal, component of recovery from whole bodyirradiation while others do not. Possibly recovery of segments is less independent in some specics or under in mammals, which, at present, is that of the guinea pig---some 20 days. There is another problem raised by Mole [6], who asserta that recovery rate per unit of injury is not a constant, as required by an exponential recovery hypothesis, but is a function of the level of injury and is slower with highinitial dores, a result perhaps contrary to that of Storer discussed above. Mole’s analysis of his data is not definitive butif bis conclusion should be correct much more investigation of recovery from different dose levels would be required before the results could be applied to unknown situations. The bulk of the present evidence indicates that recovery is not a function of initial dose for doses less than about one-half LD. Since this paper was presented verbally Davidson [7] has issued a report in which he shows a linear relation between whole body recovery rates in various mammals and the time course, following irradiation, of changes in white cells of the blood. Because blood data are available in man this relation permits a prediction of recovery half-time in man to be aboul.one month. Although there is no known biological basis for Davidson's correlation it may be @ sound one and also it may give a certain conditions, one of which may be dose size. In the gastro-intestional tract, for example, latent injury, as defined here, may have lead for search of similar empirical relation- normally undergoing rapid replacement. The objective for which the recovery halftime is used for human exposure calculations is that of avoiding a level of acute injury which will be dangerous or lethal. If recovery went no meaning with respect to those doses which are sufficiently great to kill cells whieh are Res- toration of normal eell division and proliferation is presumably a process quite different from those involved in recovery of persisting ships. In any case the half-recovery time of 28 days chosen by Davidson appears to be a fairly conservative choice for man, even in the lightof the problems raised above. to completion this factor alone would be wholly determining and it would be reasonable, when necessary, to permit exposures to levels as high as possible without incapacitation. Thepertial irreversibility of radiation injury precludes adoption of this simple point of view and. also raises the question whether it is more practical to adopt a total dose as a permissible level independently of the time, for at least a month or two, over whichit is austained. It is reasonable in comparison to other species and is indicated by the Rongelap incident (8) that 200 roentgens of whole body gamma radiation is sublethal for young adult man and probably for most. of the very young and for the moderately old. In the young adult man this dose is not seriously incapacitating even when received promptly. It appears worth- while then to consider the probable effects of 200 roentgens as a permissible dose in single episodes lasting for durations of minutes up to a month or more. Observations on rodents [1] indicate that life is shortened about 7 percent per LD, or about 1 percent per 100 roentgens for accumulated doses whose daily components do not exceed 120 roentgens. The effect with doses administered in less than a few hours is about 3 per~ cent per 100 roentgensin the 200 to 500 roentgen Tange and is greater with larger doses. This difference is not attributable to dose rate per se but to total dose within a given time. For example, using the same dose rate, Hurshet al (9] showed that 600 r shortened the life of the rat some 20 percent when administered in one day but gave a much smaller effect when administered in 10 daily doses of 60 r. These relationships require much additional study but in the rodents, at least, it appears safe to assume that doses less than 100 r per day give the smaller effect on life apan and that doses of 200 r per day or more give the larger effect. The only evidence that man may suffer fractional shortoning of life spen similar to 115 comparison to unexposed physicians. The av- . erage ages of death are 60.5 and 65.7 years, respectively. If these radiologists dying in the period 1930 to 1954 sustained on the average the equivalent of about 800 roentgens of whole body radiation in divided doses their loss of life span wouldbe similar to that in the rodent. Because this dose is in the range to be expected it is unlikely that man and rodent can differ by more than a small factor such as 2 or 3. Theeffect on life-span of large prompt doses in man is not known but presumably it will be greater than that of distributed doses as in the rodents. Assuming man and rodent to be alike 200 roentgens will shorten life about 2 percent when delivered at rates not exceeding about 100 r per day and shorten it as much as 6 or 7 percent when delivered promptly. Existing data indicate that the after effects of successive exposures are additive. There- fore, two exposures of 200 r widely separated would shortenlife twice as much as one. However, 400 r in a single prompt dose, if this is very near LD,» for man, would be expected to shorten life as much as 30 or 40 percent because life-shortening in rodents increases rapidly with the magnitude of the single prompt dose as the dose approaches the lethal range. CONCLUSIONS It appears that a limit of 200 roentgens for emergency exposures for any period up to 30 days will not entail acute lethality or significant incapacity. Congequent life-shortening would be as much as 6 percent, about 4 years, if man is like the rodent andif the doseis received over a short period. If the dose is less than a given amount, possibly about 100 r on any one day, life shortening will be about 2 percent. How- ever, there are no definitive data for any species on how small the daily level must he to cause the lesser effect. The suggested use of recovery with half-time that of rodents is that presented by Warren of 28 days by Davidson to determine “effective years in longevity of American radiologists in It is not clear at present, however, what effec- [10] whose data show an average loss of 5.2 dose” appears to be a conservative practice.