see + ¢ o ws a) An initial rise in‘total white count (reflected in the neutrophile count) may occui, ‘There after the magnitude of depression of the total white and neutrophile counts, snd within tmits their duration are a function of radiation dose: A secondary or abortive rise in the total white count (reflected in the neutrophite or lymphocyte count) may oceur, followed by a second decrease, There is little species difference in the rate of depression of the total white or neutro phile count at comparable dosed: however, the rate of recovery and time for complete recovery is quite different in various species. Small animals (mouse, rat, hamster) show relatively complete recovery to control levels, even at doses in the lethal range, by the end of the (ifth week or earlier, Data on dogs are inadequate to indicate when recovery is complete; however, return to control levels at high dose levels had not occurred by the fifth week. Swine require 9 to 15 or more weeks for complete recovery. b) The response of lymphocytes is essentially identical in all animal species. Depression : can be detected within a few hours, and recovery from the minimumvalues (achieved in 36 to 48 hours) requires longer than does neutrophile recovery, Lymphocytes fall to very low levels at doses well below the lethal range, and increasing dose results in no or minimal further deerease in count. Lymphocyte depression appears to have no causal relationship with acute radiation deaths. wo, . - ¢) Platelet counts have been studied most extensively in dogs."8 As with neutrophiles, the rapidity and magnitude of depression ig a function of dose below the lethal range. Maximum depression occurs by the 9th or 10th day with doses in the high lethal range, by the 10th to 15th day al sublethal levels. Recovery begins during the third week, but is not complete by the 30th day when most studies have been terminated. Insuffictent data are available to indicate the time required for complete recovery. Considerable evidence including studies in the mouse using splenic homogenates, induced bacterial infections and spontaneous infections have indicated that critical neutrophile levels exist below which survival is correlated with the absolute neutrophile count following whole me body irradiation. From data on dogs, it appears that survival is likely unless neutrophile counts remain below 1200 cells for a period of time. Platelet data on dogs indicate that animals with external purpura have platelet counts of : 50,000 or below. - Insufficient data on large animals are not as yet available to quantify. the extent of maxi-mum depression of either the neutrophile or platelet counts as a function of dose in the sublethal range. The response of the platelet count in the present study was much less subject tofluctuation than were the neutrophile or lymphocyte counts. For the preceding reasons, systematic investigation of the platelet and leukocyte counts in large animals as a function of dose in the sublethal range are indicated. lt is not possible to say at present whether severity of exposure, or of radiation damage correlates better with absolute levels of peripheral blood count, or with degree of change from control or pre-exposure levels. Some evidence on this point can be gained by comparing the degree of depression of the neutrophile and platelet counts in the Ailinginae and Rongerik groups, both of which had essentially the same calculated exposure but for which control hematological values were considerably different (the lymphocyte count is not suitable for compartson since degree of depression was essentially the same in these groups and the higher-dose Rongelap group). At the time of peak depression for each element, both the neutrophile and platelet counts were essentially identical in terms of absolute counts, but considerably differ ent in terms of the respective control values. Thus some evidence is afforded that absolute counts, rather than counts relative to control values, may be the more reliable index of exposure in this dose range. 4.13.6 . ‘ . - ‘ Approximation of Minimal Lethal Dose for Man Someindication of severity of exposure can be gleaned from a comparison of minimum individual counts in Japanese groups exposed at Hiroshima and Nagasaki in which fatalities oc- to curred. In general, a significant number of deaths was encountered only in individuals whose . neutrophile count fell below 1000. In the Rongelap group 42 or approximately 50 per cent had +e : i wa ee ve r « , . . ese foe * . ‘ ae coe 4 «© e tS . be , ° es 2 On aoe e =e Pr 4 ee oe toe a & see oe © we eo. woe we ane ! we ee wo 8 7 8 oe ne