Review of Thirty Years Study of Hiroshima and Nagasaki Atomic Bomb Survivors II. BIOLOGICAL EFFECTS A. Acute Effects OHKITA, T.* Department of Hematology, Research Institute for Nuclear Medicine and Biology, Hiroshima University, Hiroshima, 734, Japan (Received July 28, 1975) An outline of the acute effects of the Hiroshima and Nagasaki atomic bombs are summarized, based on documentary records. Acute injuries caused by the atomic bombs have been classified as thermal, mechanical, and radiation injuries. Combinations of these were most common. Many died from the immediate effects of blast and burns, but individuals often succumbed to trauma or burns before the radiation syndrome developed. Many more would have died from irradiation, had they been saved from the effects of trauma or burns. Nearly all who died within 10 weeks had signs suggestive of radiation injuries. Remarkable variation in sensitivity of body tissues to ionizing radi ation was apparent. Radiation-induced bone marrow depletion was the most critical damage leading to death. In these instances, leukopenia and thrombocytopenia, and subsequent infections and hemor rhagic tendencies were the main causes of death. The clinical symptoms and signs of radiation in juries; the RBE of the atomic bomb neutrons for acute effects ; and the effects of irradiation on spermatogenesis are also discussed. I . INTRODUCTION Radiation injury in man was recognized as early as 1900, soon after the discovery of ionizing radiation. However, human radiobiology as it is known today began only after the Hiroshima and Nagasaki atomic bombs of 1945, when people became aware of how little we knew about radiation hazards in man. The acute effects of the atomic bombs on the people of Hiroshima and Nagasaki are summarized here, based on documentary records. Some of the biological and hema tological background for radiation injuries are also included. Since many reports of the effects of the atomic bombs are available, readers who wish further details should refer to them'-"). II. INJURIOUS FACTORS AND TYPES OF CASUALTIES Although acute injuries caused by the atomic bombs are complicated by many factors, they have been classified mainly as thermal, mechanical and radiation injuries. A. Thermal Injuries The intensity of the heat generated by the nuclear explosions in Japan is estimated to have been 3,000-4,000°C at ground level near the hypocenters16>. Its duration was exceedingly short approximately 0.5 to 1 second"). The heat was markedly dissipated with increasing distances from the hypocenters, but there was evidence that it was more than 573°C, at distances of 1,000 to 1,100 and 1,600 meters from the hypocenters in Hiro *‘å–kˆÐ:•L“‡‘åŠwŒ´”š•úŽË”\ˆãŠwŒ¤‹†•Š,•L“‡Žs‰à1’š–Ú2-3•@•§734 shima and Nagasaki, respectively 18). Thermal radiation induced burns directly or in directly (i. e., from fires started by the flash). Direct burns are often called "flash burns ", since they were produced by the flash of thermal radiation from the fireball") . Everyone exposed unshielded within 4 km of the hypocenter probably received burns of some degree. Those beneath the burst were burned to death20). In addition, persons in buildings close to the hypocenter might have been burned by hot gases and dust enter ing the structures, even though they were adequately shielded from direct thermal radiation"). Severe third-degree burns with charring and necrosis were commonly observed among people who were in the open within 1 km of the hypocenter22). "Flash burns " have also been termed " profile burns " since the lesions occurred on the un shielded parts of the body exposed in a direct line with the origin of the thermal radi ation. They were usually restricted to one side of the body"), and were sharply outlined. At the times of the bombings, hot weather prevailed 24). Most people wore short-sleeved shirts without coats25). The effects of radiant heat were enhanced on the bare skin26) since clothing was protective to a variable degree, depending on its quality and color and the intensity of the heat27>. Indirect burns, referred to as "flame burns", are identi cal with skin burns that are caused by fire. They may involve any or all parts of the body and tend to penetrate much deeper than do " flash burns ". There were no essential differences in the healing processes2S> of these two types of burns . The frequency of burn injuries was exceptionally high. Burns seem to have been the majDr cause of death on the days of the bombings29), but their relative proportion among all deaths is unknown. Many who were injured by the blasts were unable to escape and died in the fires. "Flash and flame burns" were often combined, i, e., some people were burned when their clothes were ignited by the flash of heat30>. Burns occurred under clothing at least as far as 2.5 km from the hypocenters27) . The burns of those who survived, however, were largely of the "flash" type . The incidence of "flame burns" appears to have been very small , constituting no more than 5% of the total burns"). Those in the open air without appreciable protection received severe burns en bloc within 1.5 km of the hypocenter ; moderate but fatal burns, within 2.5 km; and mild burns at distances of 3 to 4 km from the hypocenters32). As shown in Table 1, the incidence of burns in Hiroshima 31) was nearly 100% among unshielded survivors at distances up to 2.5 km, beyond which it fell rapidly. Burns were most frequent in per sons outdoors and unshielded, considerably less frequent in those outdoors and shielded, and least among those who were indoors. There was little difference in the in cidence of burns among those in concrete buildings versus those in Japanese style houses34>. The thermal energy was estimated to have been somewhat higher in Nagasa ki, but the prevalence of burns was much lower"). The overall incidence of Nagasaki "flash burns " by distance from the hypocenter was similar to that of Hi roshima (Fig. 1)36). A few second degree burns (erythema and blistering) were recorded at distances from the hypocenters of 3.3 km in Hiroshima and 3.1 km in Nagasaki. No " flash burns " were reported beyond 4 km in Hiroshima; whereas, in Nagasaki, about 3% of the per sons exposed at 4 to 5 km were reported to have received first degree "flash burns "37). The influence of shielding on the incidence of burns is clearly demonstrated from the data in Table 1. People inside buildings were burned only when the rays could reach them through doors and windows. The incidence of burns was low up to 1.5 km from Table 1. Burns by distance and shielding Cases with burns/Number of investigated (%) Hiroshima individuals who were living sixty days after the bombing An individual with 1st, 2nd and 3rd degree burns is entered under 3rd degree burns. Kajitani and Hatano (1953; Ref. 8) and Joint Commission Report (1951; Ref. 3) the hypocenters due to the high mortality in this group . Survivors within this distance were probably partially shielded against radiant heat. Such a selective factor could account for the lower incidence of burns among survivors in an area where they otherwise could be expected to have been fatal. After the healing of severe burns , overgrowth of scar tissue and keloids* was frequently observed , especially among sur vivors who were burned within 2.5 km from the hypocenter38-47>. It was suggested at one time that this might have been due to nuclear radiation, but such a view is no longer accepted. It is noteworthy that appreciable regression of protruding scars and keloids had occurred in most cases by 194741.42 and 195248). It is well known that long-term small-dose irradiation produces cancer of the skin in humans49-52). The incidence of squamous cell carcinoma of the skin has been reported somewhat higher among mice exposed to nuclear tests, especially those exposed to neutrons"). Several cases of scar cancer which seemed to be related to A-bomb lac erations and burns were reported, but no evidence was obtained to suggest that the prevalence of cancer of the skin and A-bomb exposure were definitely correlated 54. 55) The term keloid is used here in the generally accepted clinical sense and is defined as a movable , overhanging profile, inflamed or acrocyanotic surface, of rubbery consistency, and characterized symptomatically by a prickly, burning itch. Hypertrophic scar is defined as a pink , white, or telangiectatic scar which is hard, taut, unyielding, and more or less fixed to deep tissues . Diffi culties in clinical interpretation can arise when either of these approaches the other in appearance or when the two coexist in the same patient. (Ref. 47) B. Mechanical (Blast) Injuries The blast pressures generated by the Hiroshima and Nagasaki A-bombs at ground zero are estimated to have been 4.5-6.7 and 6-8 tons per m2, respectively56>. The blasts consisted of two phases : compression and suction. The duration of the compression phase is estimated to have been approximately one-half to one second"). Mechanical injuries resulting from the blasts were direct and indirect, mostly the latter, and were chiefly caused by collapsing buildings and/or flying debris. Indirectly, the blasts caused many instantaneous deaths. So far, there have been no reliably established deaths attributable to direct blast effects"). The prevalence of indirect mechanical injuries among survivors is shown in Fig. 1. For proximally exposed, it was low because of the high mortality from all causes and, consequently, small numbers of survivors were available for survey. Blast injuries occurred mostly among people in concrete buildings; somewhat less among those in Japanese style houses ; less outdoors with shielding; and least frequently among those outdoors without shielding in exactly the reverse order from that of burns.