i. approached the lethal range. In the dog and mouse, approximately 50 to 100 rare reqiéired to lower the neutrophile count by 1000 cells min® it the high sublethal dose range. TP these data can be applied to man, an additional 50 to 100 r would have placed the dose in the lethal range. On the other tand, however, it is clear from the present data that neutrophile counts between L000 and 2000 in human beings are well talerated. Human beings with these levels of neutrophiles show ao elinieal evidence of illness, are physically active, and do not need prophylactic antibiotic therapy. The Rongelap people are estimated to have received 175 ras calculated from dose rate readings measured in air in the plane fission product field. From the preceding paragraph it Tee : is seen that an additional 50 to 100 r of laboratory radiation or an average of 75 r, probably would have resulted in somefatality. Correcting this average value geometry,* it follows that the minimal lethal dose (MLD) for man exposed in a fission product field is approximately 225 r measured in air, It is possible also to estimate the added increment of dose that would have resulted in some mortality among the Rongelap people from consideration of the minimumplatelet counts observed, the platelet levels in dogs exposed in the high sublethal range 55 and the estimated rate of decrease of platelet level with increasing dose in this dosage range. Such an analysis leads to the same conclusions as those derived from neutrophile data, 4.13.7 Peripheral Counts as an IndexofSeverityof Exposure The relative value of the several hematological determinations in estimating the degreeof exposure, as well as the approximate dose ranges over which maximumsensitivity for each determination exists, can be cstimated by comparing the degree of hematological change among the several exposure groups. The relative degree of change in neutrophiles, lymphocyte. and he platelets can be seen in Tables 4.2 to 4.5 and Figs. 4.9 to 4.11. Lymphocyte counts were de-_ pressed appreciably even in the low-exposure Utirik group. In the higher dose groups, however, with widely different physical estimates of exposure the lymphocyte counts showed essentially identical degrees of depression. The lymphocyte counts of-the Rongelap and Ail‘nginae roups were constantly depressed at a level of approximately 2000 cells. Thus while sensitive at very low doses, this endpoint may be a poor index of the degree of exposure at higher doses. The total neutrophile count of the Rongelap group was consistently more depressed than was that of the Ailinginae group and the difference was of the order of 500 to 1000 cells. However, day-to-day wide fluctuations in the neutrophile counts occurred. Accordingly this endpoint appeared to be of limited usefulness as an index of relative exposure severity except when counts on groups to be compared are performed at the same time. The platelet count showed a more systematic trend than did the neutrophile count. Differences between the low-dose Ulirik group and controls at the time of maximumdepressionfor all groups could be detected, and appreciable differences existed between the means for the Marshallese higher exposure groups. Platelet counting is as easily carried out and more re- producible than leukocyte counts,"5:5 Thus the platelet count may prove to be a useful index of degree of exposure throughout the sublethal range. The above considerations are in accord with previous findings on human beings and ani mals, *From geometric and depth dose considerations set forth in chapter 1, 1 r measured in air in a fission product field is equivalent in its effect on man to approximately 1.5 r of penetrating x- or gamma radiation under geometric conditions usually used in the laboratory. Thus the eataee . ese we e . e eoave minimal lethal dose for man exposed to penetrating radiation under the usual laboratory conditions would be approximately 335 r. - weasre We we ee J Wop neulrophile counts below 2000 at some time during the observatian period, and bO per cent had counts below 1000, By this criterion, then, the effective dose received by the Ronrelap people a r