The probability of dying directly from radiation exposure depends on the dose received. A commonly used index is the dose at which 50% of a population dies (LD₅₀ = 50% lethal dose). Acute death is defined as death within about two months of exposure. At the LD₅₀ level, bleeding and infection due to immunodeficiency resulting from bone marrow depletion are the main causes of death. Recovery from such depletion sufficient to prevent death usually occurs within two months.

Early estimates from survivor interviews measured the LD₅₀ in terms of the distance from the hypocenter at which 50% of people survived: 1,000 to 1,200 meters in Hiroshima and 1,000 to 1,300 meters in Nagasaki. Dose estimation was not possible at that time because of insufficient shielding information. Later analyses of extensive records at RERF were able to make estimates of shielding and to calculate that a bone marrow dose of 2.7 to 3.1 Gy caused 50% mortality within 60 days (with the new DS02 dosimetry system, the corresponding doses would be 2.9 to 3.3 Gy). The data came from about 7,600 survivors in 2,500 households exposed inside Japanese houses located within 1,600 meters of the hypocenter in Hiroshima. Survivors inside Japanese houses received special scrutiny because the homogeneity of such housing structures allowed better estimation of individual radiation doses. The closer one was to the hypocenter, however, the higher the radiation dose received and the more severe the effects of blast and heat in terms of destruction of houses and subsequent fires. It was thus impossible to classify deaths that occurred within a few weeks after the bombings as due to radiation, injuries, or burns. To avoid deaths from injuries and burns, the above RERF analyses therefore focused mainly on delayed deaths; such deaths peaked at about one month after exposure.

Based on this information from A-bomb survivors, together with other information from cases involving exposure to accidental radiation or radiation therapy, the United Nations’ Scientific Committee on the Effects of Atomic Radiation has estimated the bone marrow LD_50/60 at around 2.5 Gy when little or no medical assistance is available and at 5 Gy or more with extensive medical care.

For number of deaths caused by the bomb, see Frequently Asked Questions, Number 1.