Facts about the Atomic Bomb:

  • Length: 3 metres
  • Diameter: 0.7 metres
  • Weight: 4 tons
  • Element: Uranium 235
  • Energy: Equivalent to 20 kilotons of TNT explosive power.
    (It has been estimated that the yield was equivalent to approx. 13 kilotons.)

Death Toll

About 140,000 +/- 10,000 (including 20,000 soldiers) were dead by the end of December 1945; 90% of these are thought to have been killed within 2 weeks after the bombing.

Destruction of Buildings

There were approx. 76,000 buildings in the city at the time, and 92% of these were destroyed by the blast and fire. The blast was so powerful that it did a great deal of damage to 60% of the buildings as far as 5 kilometers away from the hypocentre. It is said that only 6,180 buildings (8%) remained suitable for use in and around the city. An area of 13 square kilometers was transformed into a wide stretch of A-bomb-affected ruins.

Comparison of the Atomic Bomb between Hiroshima and Nagasaki
Height of burst point
280 +/- 20m
500 +/- 25m
Area where roof-tiles melted
(radius from hypocentre)
Area where granite stone melted
(radius from hypocentre)
Total death rate and rate on the day of exposure (Hiroshima)
Distance from
hypocentre (km)
death rate
Death rate
on day of exposure
Average 0 – 20
(surveyed by medical group of Tokyo University)


Effects of Radiation

  • Immediate Effects
    • Sickness and death due to initial radiationThe main components of initial radiation emitted in the air within 1 minute of the explosion were gamma rays and neutrons. Man is exposed to approximately 0.1 rad of natural radiation per person per year. It is said that 50% of persons who receive a whole body dose of 400 rad, which is known as a semi-lethal dose die, and that those exposed to whole body radiation of 700 rad or more have little chance of escaping death. For the purpose of radio�logical protection, the International Committee on Radiological Protection (ICRP) stipulates the permissible dose for man in general as 0.5 rad per year to the whole body, gonads, or bone marrow. The location that was exposed to the lethal dose of 700 rad was a point approximately 925 meters away from the hypocentre (in Hiroshima); and in the case of the semilethal dose of 400 rad, ap�proximately 1,025 meters. Anyone exposed without shielding within this distance is said to have been seriously affected by the initial radiation, and approximately 20% of deaths are said to be attributable to these effects. The location that was exposed to the permissible dose for human beings (0.5) rad was determined to be a point approximately 2.3 kilometers away from the hypocentre.
    • Sickness and death due to residual radiationResidual radioactivity was present on the ground for a long period of time from 1 minute after the explosion. It consisted of induced radioactivity produced secondarily as a result of the nuclear reaction of neutrons when the initial radiation collided with the soil and building materials; of radioactivity of nuclear fission products; and of radioactivity of unfissioned bomb materials.Anyone staying within 1 kilometer of the hypocentre within 100 hours of the explosion was seriously affected by external exposure to gamma rays of induced radiation; therefore, those who were not exposed to the initial radiation but entered the city to aid victims or search for relatives and those who lived in the range of radio�active contamination were affected by residual radiation.Black rain fell on the western part of the city from 20 minutes after the explosion for two hours in an oval range with a major axis of 19 kilometers and a minor axis of 11 kilometers. Due to the Black rain, even in remote areas far from the hypocentre, strong residual radioactivity was detected and considerable damage was sustained.
  • Delayed Effects of Radiation
    Delayed effects of radiation were caused by the complicated involvement of three factors; namely, burns due to thermal radiation and fire, injuries due to the blast, and radiation effects.The effect of radiation had a synergistic action with the burns due to thermal radiation and fire and the injuries due to tFf~ blast and generally aggravated the condition of the diseases of those exposed.

    Delayed effects of radiation are divided into acute sickness and long-term illness according to the time of their development

    • Acute sicknessAcute sickness is a symptom that developed in the period from the time of the explosion to the end of December the same year, and most of the people are said to have recovered from the symp�toms 5 months after the explosion. They are: digestive symptoms (nausea, vomiting, and diarrhea); nerve symptoms (headache, de�lirium and insomnia); a dynamic symptoms (epilation, weakness, and malaise); bleeding symptoms (hematemesis, bloody discharge from the bowels, hematuria, nosebleed, gingival bleeding, genital bleeding, and subcutaneous bleeding); inflammation symptoms (fever, pharyngalgia, stomatitis, and dermatitis); blood symptoms (leukocytopenia and erythrocytopenia); genital organ symptoms (aspermia and menstrual abnormalities).Of the total of 140,000 deaths in the period of acute sickness, approximately 20% were from injuries due to the blast, approxi�mately 60% from burns due to thermal rays and fire, and the re�maining 20% due to radiation disturbance.
    • Long-term illnessMost of the people who did not die of acute sickness super�ficially appeared to be in good health at the end of December of the year of the bombing. However, medical effects due to the atomic bomb did not end.Burns due to the atomic bomb, healing once, formed keloids in 1-4 months with the protuberance of their scars. Keloid inci�dence shows a peak in the period of 1946-47. Ophthalmological disorders such as traumatic cataracts, and blood dyscrasias such as leukemia appeared around 1947. Leukemia incidence, especially, shows a peak in the period of 1950-60. Incidence of malignant tumors such as thyroid cancer, breast cancer, lung cancer, and sali�vary gland tumor generally increased from around 1960 after the peak of leukemia incidence, and is still observed.Adding to these, nerve problems such as easy fatigability, dizzi�ness, and insomnia, aging and sickness due to in utero exposure have been observed. As to genetic effects of radiation, no definite evi�dence has been obtained. This is a problem that requires further study and research.