Date: 6 August 1945
Location: Hiroshima, Japan
Type of event: combat use of nuclear weapon
Following prolonged strategic bombing of Japan during World War II, first combat use of a nuclear weapon was conducted by the United States. A B-29 bomber dropped a single Mk-I "Little Boy" atomic bomb over Hiroshima, estimated population 285,000. The Mk-I was a gun-type nuclear gravity bomb using highly enriched uranium. It detonated 580 meters over central Hiroshima at 8:15 AM local time with a yield estimated at 15 kilotons. The airburst height was selected to maximize the extent of prompt effects and to minimize residual radiation (fallout). Individuals at ground zero received combined gamma and neutron doses of perhaps 80,000 rad, although flash and blast would have been immediately fatal. The thermal flash produced fires which merged into a firestorm, razing much of the city. Flash, blast, and prompt radiation killed most people within 1.5 km of ground zero; immediate fatalities were generally from flash and blast injuries, with many otherwise injured and uninjured dying of fatal prompt radiation doses over the following weeks and months. Small numbers of people were injured by residual radiations (neutron-induced radioactivity and residual material from the weapon).
On the day of the bombing, an estimated 348,000 people were in Hiroshima, including 265,000 Japanese residents, 20,000 Korean residents, 12,000 conscripted Japanese workers, 3,000 conscripted Korean workers, 48,000 Japanese soldiers, and a small number of prisoners of war. Casualty figures are uncertain, despite many surveys (some figures presented here are extrapolated from partial surveys). Among civilians, possibly 44,000 to 59,000 were killed the day of the bombing, with another 17,000 missing. Subsequent deaths include about 25,000 through the end of August 1945, 9,000 in September 1945, 2,000 in October-December 1945, and 2,500 in 1946. Many of these subsequent deaths involved radiation injuries. Deaths among survivors after 1946 include greater fractions from natural causes. Less information is available regarding military fatalites, but at least 9,000 soldiers died through the end of 1946. The estimated 130,000 fatalities to 1950 include about 111,000 Japanese civilians, 12,000 Japanese soldiers, and 6,500 Koreans. After 1950, deaths attributed to radiation include about 60 leukaemia deaths, 300 other cancer deaths, and 145 non-cancer deaths. New cases of leukaemia peaked in 1951.
Those injured in the bombing numbered (through August 1946) 30,500 severely injured and 48,600 slightly injured. These figures do not include a possible 7,000 injuries among military personnel. Of 2,160 medical personnel in Hiroshima at the time of the bombing, 1,980 were killed or injured. Emergency response was supported by the arrival of about 3,270 medical personnel from surrounding areas and other parts of Japan, along with another 2,910 relief workers. An additional 40,000 to 60,000 are registered as having entered Hiroshima shortly after the bombing. Survivor registries include 2,300 individuals exposed in utero. Studies suggest that excess miscarriages and fetal deaths numbered in the dozens, and excess infant deaths (for those exposed in utero) also in the dozens. About 45 cases of microcephaly are known among those exposed in utero, including at least 15 with mental retardation.
Portions of Hiroshima with little or no damage were continuously inhabited, and the city was rebuilt. Hiroshima reattained its pre-attack population by 1954 and had a population of 1,066,000 in 1992.
Consequences: Estimated 130,000 fatalities (of which perhaps 40,000 are related to ionizing radiation injuries) and 86,000 injuries.
© 2004, 2005 by Wm. Robert Johnston.
Last modified 16 October 2005.
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