Many people believe Hiroshima, known for being the target of the first atomic bomb used in warfare, remains dangerously radioactive today. This is a widespread misconception. Present-day Hiroshima is a thriving, safe city with radiation levels comparable to any other urban area globally. This article explains the scientific reasons for this, detailing the nature of the radiation released, the factors that prevented long-term contamination, and the distinct health impacts experienced by survivors versus the environmental safety today.
The Nature of Radiation from an Atomic Bomb
The atomic bomb detonated over Hiroshima on August 6, 1945, “Little Boy,” was a gun-type fission weapon fueled by uranium-235. Its explosion released energy as blast, heat, and two types of radiation: initial and residual. Initial radiation, comprising prompt gamma rays and neutrons, was emitted at detonation and for about one minute afterward. This intense burst caused severe damage and fatalities to those within its lethal radius. The explosion also created radioactive fission products, the atomic fragments left after uranium nuclei split. These products, along with unfissioned uranium, constituted the bomb’s residual radiation. Residual radiation is typically associated with radioactive fallout, which refers to particles lifted into the atmosphere that then settle back to Earth. However, the “Little Boy” bomb’s characteristics and deployment influenced how this residual radiation behaved.
Factors Minimizing Residual Radiation
A primary reason for the lack of lasting radiation in Hiroshima is the bomb’s airburst detonation. The “Little Boy” bomb exploded approximately 580 to 610 meters (around 1,900 feet) above the city. This airburst strategy maximized the blast effect and minimized residual radiation on the ground. Because the fireball did not touch the ground, it did not draw up significant soil and debris that would become highly radioactive through neutron activation and fall back as local fallout. In contrast, ground-level detonations cause substantial neutron activation of surrounding materials, leading to greater and more persistent local contamination.
The type of fission involved also played a role. The uranium fission in the Hiroshima bomb produced fission products, but most radioactive material dispersed high into the atmosphere. Less than one kilogram of the 64 kilograms of uranium-235 in the “Little Boy” bomb actually underwent nuclear fission. The radioactive particles produced were carried by winds into the stratosphere, dispersing globally rather than settling locally.
Furthermore, the radioactive isotopes created by the bomb had relatively short half-lives, meaning they decayed rapidly. The most intense, short-lived isotopes quickly diminished. Research indicates that approximately 80% of all residual radiation was emitted within 24 hours of the bombing. Within a week, the quantity of residual radiation at the hypocenter had decreased to about one-millionth of its initial level. This rapid decay, combined with atmospheric dispersion, ensured that environmental radioactivity quickly diminished.
Immediate and Long-Term Health Impacts on Survivors
While the environment in Hiroshima did not suffer long-term radioactive contamination, the initial burst of radiation had severe and lasting health consequences for those directly exposed. Survivors experienced acute radiation syndrome (ARS), characterized by symptoms such as nausea, vomiting, diarrhea, fatigue, and hair loss, appearing within hours or days of exposure depending on the dose. This syndrome results from damage to rapidly dividing cells, such as those in the bone marrow and gastrointestinal tract.
Beyond the immediate symptoms, atomic bomb survivors, known as ‘Hibakusha,’ faced increased risks of various cancers and other health problems over their lifetimes. Leukemia, for instance, increased about two years after the bombings, peaking around four to six years later, particularly among children exposed at a young age. Increased risks of solid cancers, such as lung, breast, and thyroid cancers, became evident approximately ten years after the bombings and have persisted. These long-term effects stem from the direct exposure of individuals to the initial radiation at the time of the bombing, causing cellular and genetic damage, rather than from lingering environmental contamination.
Present-Day Environmental Safety in Hiroshima
Today, Hiroshima is a modern, vibrant city, demonstrating remarkable recovery. Radiation levels in Hiroshima are now comparable to natural background radiation found in any other city globally. Natural background radiation originates from cosmic rays and naturally occurring radioactive materials in the Earth’s crust, such as radon gas. Scientific organizations, including the Radiation Effects Research Foundation (RERF), a cooperative Japan-U.S. organization, have continuously monitored radiation levels for decades. These comprehensive studies confirm that residual radiation from the 1945 bombing does not pose a health risk to current residents or visitors. The city’s environment is considered safe; living or visiting Hiroshima today does not involve exposure to harmful levels of residual radiation.