102 EFFECTS OF IONIZING RADIATION week. This is in contrast to other mammals where deaths from the acute phase are uncommon after the 30th day. 6.53 Probability of Survival as Related to Symptoms Hence, individuals exposed in the lethal range (where some, but not all, will die in the first several weeks following exposure) can be divided according to symptoms and signs, into groups having a different prognosis. Thus they maybe divided into three groups in which survival is, respectively, improbable, possible, and probable, It will be apparent that there is no sharpline of demarcation among the groups. Group 1.—Survival improbable: If vomiting occurs promptly or within a few hours and continues and is followed in rapid succession by prostration, diarrhea, anorexia, fever, the prognosis is grave: death will almost definitely occur in 100 percent of the individuals within the 1st week. There isno known therapy for these people; accordingly, in a catastrophe, attention should be devoted principally to others for whom there is some hope. Group 2.—Survival possible: Vomiting may occur early but will be of relatively short duration followed by a period of well-being. In this period of well-being marked changes are taking place in the hemopoietic tissues. ILymphocytes are profoundly depressed within hours and remain so for months. The neutrophile ‘count is depressed to lowlevels, the degree and time of maximum depression depending upon the dose. Signs of infection may be seen when the total neutrophile count has renchedvirtually zero (7-9 days). The platelet count may reach very lowlevels after 2 weeks. External evidence of bleeding may occur within 2 or 4 weeks. This group represents the lethal dose range in the classical pharmacologic sense. In the higher exposure groups of this category the latent period lasts from 1 to 3 weeks withlittle clinical evidence of injuries other thanslight fatigue. At the termination of the latent period, the patient may develop purpura, epilation, oral and cutaneous lesions, infections of wounds or burns, diarrhea, and melena. The mortality will be significant. With therapy the survival time can be expected to be prolonged and if sufficient time is provided for bone marrow regeneration the survival rate will be incrensed. In groups 1 and 2 the blood picture is not as well documented asin gronp 3. There are good clinical reasons to believe that in the lethal range the granulocyte depressions will be marked and below 1,000 per mm* during the 2d week. Good observations in Japan (21, 22) confirm this contention. However, in the sublethal range it takes much longer for the granulocyte count and platelet count of man to rench minimal values, as compared to other mammals (see Chapter IV and reference 10). Despite the chaotic conditions that existed in Hiroshima, the data of Kikuchi and Wakisaka (22) shows that there was a more rapid and marked decrense in Groups 1 and 2 than in Group 3. Group 3.—Survival probable: This group consists of individuals who may or may not have had fleeting nausea and vomiting on the day of exposure. In this group there is no further evidence of effects of the exposure except the hematologic changes that can be detected by serial studies of the blood with particular reference to lymphocytes and platelets. The lymphocytes reach low levels eurly. within 48 hours, and may show little evidence of recovery for many months after exposure. The granulocytes may show some depression during the second and third week. However, considerable varin- tion is encountered. A late fall in the granulocytes during the 6th or 7th week may occur and should be watched for. Platelet counts reach the lowest on approximately the 20th day at the time when \\