The atomic bombing of Hiroshima on August 6, 1945, released immense energy, creating a complex pattern of radiation exposure that unfolded over distinct phases. The radiation hazard lasted from less than a minute to several weeks, followed by a rapid return to near-normal conditions. The high air burst detonation, using a uranium-235 core, played a significant role in limiting the city’s long-term contamination.
The Immediate Burst (Initial Radiation)
The first and most intense phase of radiation exposure occurred instantaneously, lasting less than one minute after the detonation of the “Little Boy” bomb. This immediate energy release, known as initial or prompt radiation, accounted for about 5% of the total energy yield. It consisted primarily of high-energy gamma rays and neutrons released directly from the nuclear fission reaction. The exposure was extremely brief, but its intensity caused immediate Acute Radiation Syndrome (ARS) fatalities.
The neutrons were able to penetrate deeply into tissues and materials. The immediate dose of gamma rays and neutrons was so high near the hypocenter that it determined the fate of many people within a one-kilometer radius. Although the exposure ended in mere seconds, the resulting biological damage continued to cause deaths in the weeks following the attack, peaking about three to four weeks later.
Short-Term Contamination (Residual Radiation)
Following the initial burst, residual radiation lingered, which is the period most people associate with contamination. This phase includes two main components: induced radioactivity and local fallout. Roughly 80% of the total residual radiation was emitted within the first 24 hours, leading to a rapid decline in the hazard.
Induced radioactivity occurred when the intense flood of neutrons struck common elements, such as aluminum and sodium, in materials near the hypocenter. These elements absorbed the neutrons, becoming short-lived radioactive isotopes. Nearly all induced radioactivity decayed quickly, largely disappearing within a few days to a few weeks of the explosion.
Local fallout consisted of radioactive fission products that settled back down to the ground. Because the bomb was detonated high in the air (approximately 580 meters above the city), most radioactive debris dispersed into the upper atmosphere. Some fission products did descend locally, notably carried down by the “black rain” that fell shortly after the explosion.
Contamination levels dropped dramatically; research indicates that the quantity of residual radiation received at the hypocenter 24 hours later was only 1/1,000th of the quantity received immediately after the blast. Within one week, this level had declined further, decreasing to about 1/1,000,000th of the initial burst, allowing recovery efforts to begin soon after the event.
The Current Environmental Status
The public perception that Hiroshima remains dangerously radioactive is a misconception not supported by scientific data. The city is environmentally safe today, and the radiation hazard from the 1945 event ended decades ago. Current measurements show that radiation levels in Hiroshima are virtually indistinguishable from the natural background radiation found globally.
This rapid return to normal levels resulted from the specific conditions of the bombing. The high air burst prevented the fireball from touching the ground, meaning large amounts of soil and debris were not vaporized and made intensely radioactive. Furthermore, the uranium-235 bomb was relatively inefficient, dispersing a significant portion of the fuel as non-fissioned material with low inherent radiation risk.
The short half-lives of the most dangerous fission products ensured that high-level contamination decayed quickly within the first few weeks. Today, the city is monitored, and radiation levels are comparable to the average of about 2.4 millisieverts of natural background radiation exposure per year.