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The Effects of the Atomic Bombs: Excerpt from United States Strategic Bombing Survey Summary Report (Pacific War), Washington, D.C., 1 July 1946

Less than a year after the Pacific war ended, the U.S. government sought to assess the effects upon Japan of the major bombing campaign mounted in the later years of the war. The survey covered both the physical impact and casualties and the effect on morale. It provided a graphic account of the explosions of nuclear devices over both Hiroshima and Nagasaki. It was too soon, however, to assess the long-term impact of the nuclear explosions upon the health of the survivors.


On 6 August and 9 August 1945, the first two atomic bombs to be used for military purposes were dropped on Hiroshima and Nagasaki respectively. One hundred thousand people were killed, 6 square miles or over 50 percent of the built-up areas of the two cities were destroyed. The first and crucial question about the atomic bomb thus was answered practically and conclusively; atomic energy had been mastered for military purposes and the overwhelming scale of its possibilities had been demonstrated. A detailed examination of the physical, economic, and morale effects of the atomic bombs occupied the attention of a major portion of the Survey's staff in Japan in order to arrive at a more precise definition of the present capabilities and limitations of this radically new weapon of destruction.

Eyewitness accounts of the explosion all describe similar pictures. The bombs exploded with a tremendous flash of blue-white light, like a giant magnesium flare. The flash was of short duration and accompanied by intense glare and heat. It was followed by a tremendous pressure wave and the rumbling sound of the explosion. This sound is not clearly recollected by those who survived near the center of the explosion, although it was clearly heard by others as much as fifteen miles away. A huge snow-white cloud shot rapidly into the sky and the scene on the ground was obscured first by a bluish haze and then by a purple-brown cloud of dust and smoke.

Such eyewitness accounts reveal the sequence of events. At the time of the explosion, energy was given off in the forms of light, heat, radiation, and pressure. The complete band of radiations, from X- and gamma-rays, through ultraviolet and light rays to the radiant heat of infra-red rays, travelled with the speed of light. The shock wave created by the enormous pressures built up almost instantaneously at the point of explosion but moved out more slowly, that is at about the speed of sound. The superheated gases constituting the original fire ball expanded outward and upward at a slower rate.

The light and radiant heat rays accompanying the flash travelled in a straight line and any opaque object, even a single leaf of a vine, shielded objects lying behind it. The duration of the flash was only a fraction of a second, but it was sufficiently intense to cause third degree burns to exposed human skin up to a distance of a mile. Clothing ignited, though it could be quickly beaten out, telephone poles charred, thatchroofed houses caught fire. Black or other dark-colored surfaces of combustible material absorbed the heat and immediately charred or burst into flames; white or light-colored surfaces reflected a substantial portion of the rays and were not consumed. Heavy black clay tiles which are an almost universal feature of the roofs of Japanese houses bubbled at distances up to a mile. Test of samples of this tile by the National Bureau of Standards in Washington indicates that temperatures in excess of 1,800° C. must have been generated in the surface of the tile to produce such an effect. The surfaces of granite blocks exposed to the flash scarred and spoiled at distances up to almost a mile. In the immediate area of ground zero (the point on the ground immediately below the explosion), the heat charred corpses beyond recognition.

Penetrating rays such as gamma-rays exposed X-ray films stored in the basement of a concrete hospital almost a mile from ground zero. Symptoms of their effect on human beings close to the center of the explosion, who survived other effects thereof, were generally delayed for two or three days. The bone marrow and as a result the process of blood formation were affected. The white corpuscle count went down and the human processes of resisting infection were destroyed. Death generally followed shortly thereafter.

The majority of radiation cases who were at greater distances did not show severe symptoms until 1 to 4 weeks after the explosion. The first symptoms were loss of appetite, lassitude and general discomfort. Within 12 to 48 hours, fever became evident in many cases, going as high as 104° to 105° F., which in fatal cases continued until death. If the fever subsided, the patient usually showed a rapid disappearance of other symptoms and soon regained his feeling of good health. Other symptoms were loss of white blood corpuscles, loss of hair, and decrease in sperm count.

Even though rays of this nature have great powers of penetration, intervening substances filter out portions of them. As the weight of the intervening material increases the percentage of the rays penetrating goes down. It appears that a few feet of concrete, or a somewhat greater thickness of earth, furnished sufficient protection to humans, even those close to ground zero, to prevent serious after effects from radiation.

The blast wave which followed the flash was of sufficient force to press in the roofs of reinforced concrete structures and to flatten completely all less sturdy structures. Due to the height of the explosion, the peak pressure of the wave at ground zero was no higher than that produced by a near miss of a high-explosive bomb, and decreased at greater distances from ground zero. Reflection and shielding by intervening hills and structures produced some unevenness in the pattern. The blast wave, however, was of far greater extent and duration than that of a high-explosive bomb and most reinforced-concrete structures suffered structural damage or collapse up to 700 feet at Hiroshima and 2,000 feet at Nagasaki. Brick buildings were flattened up to 7,300 feet at Hiroshima and 8,500 feet at Nagasaki. Typical Japanese houses of wood construction suffered total collapse up to approximately 7,300 feet at Hiroshima and 8,200 feet at Nagasaki. Beyond these distances structures received less serious damage to roofs, wall partitions, and the like. Glass windows were blown out at distances up to 5 miles. The blast wave, being of longer duration than that caused by high-explosive detonations, was accompanied by more flying debris. Window frames, doors, and partitions which would have been shaken down by a near-miss of a high-explosive bomb were hurled at high velocity through those buildings which did not collapse. Machine tools and most other production equipment in industrial plants were not directly damaged by the blast wave, but were damaged by collapsing buildings or ensuing general fires.

The above description mentions all the categories of the destructive action by the atomic-bomb explosions at Hiroshima and Nagasaki. There were no other types of action. Nothing was vaporized or disintegrated; vegetation is growing again immediately under the center of the explosions; there are no indications that radio-activity continued after the explosion to a sufficient degree to harm human beings.

Let us consider, however, the effect of these various types of destructive action on the cities of Hiroshima, and Nagasaki and their inhabitants.

Hiroshima is built on a broad river delta; it is flat and little above sea level. The total city area is 26 square miles but only 7 square miles at the center were densely built up. The principal industries, which had been greatly expanded during the war, were located on the periphery of the city. The population of the city had been reduced from approximately 340,000 to 245,000 as a result of a civilian defense evacuation program. The explosion caught the city by surprise. An alert had been sounded but in view of the small number of planes the all-clear had been given. Consequently, the population had not taken shelter. The bomb exploded a little northwest of the center of the built-up area. Everyone who was out in the open and was exposed to the initial flash suffered serious burns where not protected by clothing. Over 4 square miles in the center of the city were flattened to the ground with the exception of some 50 reinforced concrete buildings, most of which were internally gutted and many of which suffered structural damage. Most of the people in the flattened area were crushed or pinned down by the collapsing buildings or flying debris. Shortly thereafter, numerous fires started, a few from the direct heat of the flash, but most from overturned charcoal cooking stoves or other secondary causes. These fires grew in size, merging into a general conflagration fanned by a wind sucked into the center of the city by the rising heat. The civilian-defense organization was overwhelmed by the completeness of the destruction, and the spread of fire was halted more by the air rushing toward the center of the conflagration than by efforts of the fire-fighting organization.

Approximately 60,000 to 70,000 people were killed, and 50,000 were injured. Of approximately 90,000 buildings in the city, 65,000 were rendered unusable and almost all the remainder received at least light superficial damage. The underground utilities of the city were undamaged except where they crossed bridges over the rivers cutting through the city. All of the small factories in the center of the city were destroyed. However, the big plants on the periphery of the city were almost completely undamaged and 94 percent of their workers unhurt.

These factories accounted for 74 percent of the industrial production of the city. It is estimated that they could have resumed substantially normal production within 30 days of the bombing, had the war continued. The railroads running through the city were repaired for the resumption of through traffic on 8 August, 2 days after the attack.

Nagasaki was a highly congested city built around the harbor and up into the ravines and river valleys of the surrounding hills. Spurs of these hills coming down close to the head of the bay divide the city roughly into two basins. The built-up area was 3.4 square miles of which 0.6 square miles was given over to industry. The peak wartime population of 285,000 had been reduced to around 230,00 by August 1945, largely by pre-raid evacuations. Nagasaki had been attacked sporadically prior to 9 August by an aggregate of 136 planes which dropped 270 tons of high explosives and 53 tons of incendiary bombs. Some 2 percent of the residential buildings had been destroyed or badly damaged; three of the large industrial plants had received scattered damage. The city was thus comparatively intact at the time of the atomic bombing.

The alarm was improperly given and therefore few persons were in shelters. The bomb exploded over the northwest portion of the city; the intervening hills protected a major portion of the city lying in the adjoining valley. The heat radiation and blast actions of the Nagasaki bomb were more intense than those of the bomb dropped over Hiroshima. Reinforced-concrete structures were structurally damaged at greater distances; the heavy steel-frame industrial buildings of the Mitsubishi steel works and the arms plant were pushed at crazy angles away from the center of the explosion. Contrary to the situation at Hiroshima, the majority of the fires that started immediately after the explosion resulted from direct ignition by the flash.

Approximately 40,000 persons were killed or missing and a like number injured. Of the 52,000 residential buildings in Nagasaki 14,000 were totally destroyed and a further 5,400 badly damaged. Ninety-six percent of the industrial output of Nagasaki was concentrated in the large plants of the Mitsubishi Co. which completely dominated the town. The arms plant and the steel works were located within the area of primary damage. It is estimated that 58 percent of the yen value of the arms plant and 78 percent of the value of the steel works were destroyed. The main plant of the Mitsubishi electric works was on the periphery of the area of greatest destruction. Approximately 25 percent of its value was destroyed. The dockyard, the largest industrial establishment in Nagasaki and one of the three plants previously damaged by high-explosive bombs, was located down the bay from the explosion. It suffered virtually no new damage. The Mitsubishi plants were all operating, prior to the attack, at a fraction of their capacity because of a shortage of raw materials. Had the war continued, and had the raw material situation been such as to warrant their restoration, it is estimated that the dockyard could have been in a position to produce at 80 percent of its full capacity within 3 to 4 months; that the steel works would have required a year to get into substantial production; that the electric works could have resumed some production within 2 months and been back at capacity within 6 months; and that restoration of the arms plant to 60 to 70 percent of former capacity would have required 15 months.

Some 400 persons were in the tunnel shelters in Nagasaki at the time of the explosion. The shelters consisted of rough tunnels dug horizontally into the sides of hills with crude, earth-filled blast walls protecting the entrances. The blast walls were blown in but all the occupants back from the entrances survived, even in those tunnels almost directly under the explosion. Those not in a direct line with the entrance were uninjured. The tunnels had a capacity of roughly 100,000 persons. Had the proper alarm been sounded, and these tunnel shelters been filled to capacity, the loss of life in Nagasaki would have been substantially lower.

The Survey has estimated that the damage and casualties caused at Hiroshima by the one atomic bomb dropped from a single plane would have required 220 B-29s carrying 1,200 tons of incendiary bombs, 400 tons of high-explosive bombs, and 500 tons of anti-personnel fragmentation bombs, if conventional weapons, rather than an atomic bomb, had been used. One hundred and twenty-five B-29s carrying 1,200 tons of bombs would have been required to approximate the damage and casualties at Nagasaki. This estimate pre-supposed bombing under conditions similar to those existing when the atomic bombs were dropped and bombing accuracy equal to the average attained by the Twentieth Air Force during the last 3 months of the war.

As might be expected, the primary reaction of the populace to the bomb was fear, uncontrolled terror, strengthened by the sheer horror of the destruction and suffering witnessed and experienced by the survivors. Prior to the dropping of the atomic bombs, the people of the two cities had fewer misgivings about the war than people in other cities and their morale held up after it better than might have been expected. Twenty-nine percent of the survivors interrogated indicated that after the atomic bomb was dropped they were convinced that victory for Japan was impossible. Twenty-four percent stated that because of the bomb they felt personally unable to carry on with the war. Some 40 percent testified to various degrees of defeatism. A greater number (24 percent) expressed themselves as being impressed with the power and scientific skill which underlay the discovery and production of the atomic bomb than expressed anger at its use (20 percent). In many instances, the reaction was one of resignation.

The effect of the atomic bomb on the confidence of the Japanese civilian population outside the two cities was more restricted. This was in part due to the effect of distance, lack of understanding of the nature of atomic energy, and the impact of other demoralizing experiences. . . .

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