Immediate Effects of Radiation

(I) Sickness and death due to initial radiation

The 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 die who have received a whole body dose of 400 rad, which is known as semi-lethal dose, and that those exposed to whole body radiation of, 700 rad or more can hardly escape deiith. For the purpose of radiological 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 hypocenter; and to the semi-lethal dose of 400 rad, approximately I ,025 meters. Anyone exposed without shield-ing within this distance is said to have been seriously affected by the initial radiation, and approximately 20% of deaths are said to be attributed 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 hypocenter.

(2) Sickness and death due to residual radiation

Residual 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 hypocenter within 100 hours of the explosion was considerably affected by external exposure to gamma rays of induced radiation; therefore, those who were not exposed to initial radiation but entered the city to aid victims or search for relatives and those who lived in the range of radioactive 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 hypocenter, strong residual radioactivity was detected and considerable damage was sustained.