On August 6 and August 9, 1945, the United States detonated atomic bombs over Hiroshima and Nagasaki. The two attacks killed an estimated 140,000 people in Hiroshima and 70,000 in Nagasaki by the end of 1945. Eighty years of follow-up research on the survivors has produced the most extensive dataset on real nuclear-weapon effects anywhere in the world. Almost everything we now know about acute and long-term radiation effects on humans comes from this work.
August 6, 1945: Little Boy over Hiroshima
At 8:15 a.m. on August 6, 1945, the B-29 Enola Gay dropped Little Boy — a gun-type uranium-235 bomb yielding approximately 15 kilotons — over Hiroshima, Japan. The bomb detonated at 580 m altitude, optimized for maximum blast spread.
Hiroshima at the time had a population of roughly 350,000. The detonation point was near the Aioi Bridge in the city center. Within seconds, the fireball had vaporized everyone within about 200 m of ground zero. Within minutes, the blast wave had collapsed nearly every wooden building within 2 km, and thermal radiation had set fires throughout the city. Within a few hours, those fires had merged into a city-wide firestorm.
By the end of 1945, an estimated 140,000 people had died — roughly half from the immediate blast and thermal effects, and half from injuries, burns, and acute radiation sickness in the weeks that followed.
August 9, 1945: Fat Man over Nagasaki
Three days later, the B-29 Bockscar dropped Fat Man — an implosion-type plutonium-239 bomb yielding approximately 21 kilotons — over Nagasaki. The original target had been the city of Kokura, but heavy cloud cover forced a diversion to the secondary target.
Fat Man detonated at 503 m altitude over the Urakami Valley, about 3 km from the city's industrial center. Nagasaki's hilly geography somewhat shielded parts of the city from the blast, reducing total casualties relative to the flatter Hiroshima. Even so, an estimated 70,000 people died by the end of 1945.
Both bombs were air bursts — chosen specifically to maximize blast spread at the cost of fallout. Local fallout in both cities was minimal. The "black rain" that fell on Hiroshima after the detonation was a mixture of soot, ash, and condensed moisture; it carried some radioactivity but not at lethal levels.
The Atomic Bomb Casualty Commission
In 1947, the United States Atomic Energy Commission and the National Academy of Sciences established the Atomic Bomb Casualty Commission (ABCC) to conduct long-term medical follow-up of the survivors. In 1975 the ABCC was reorganized as the Radiation Effects Research Foundation (RERF), a joint US-Japan institution that continues operations today.
The RERF Life Span Study (LSS) is the largest and longest-running epidemiological study of radiation exposure ever conducted. It tracks roughly 120,000 people — survivors and their non-exposed control population — from 1950 to the present. The study's findings underpin essentially all modern radiation-protection standards used in medicine, nuclear power, and arms control.
What the data showed
The Life Span Study established the dose-response relationship for radiation-induced cancer. Survivors who received high doses had clearly elevated rates of leukemia (peaking 5-10 years after exposure) and solid cancers (rising over decades). The data fit a roughly linear dose-response model down to the lowest measured doses, which is why modern radiation-protection regulations assume that there is no safe threshold for ionizing radiation.
But the study also overturned some pre-1945 assumptions. Earlier expectations had been that radiation effects would be far more catastrophic than they turned out to be — that survivors would suffer mass genetic abnormalities, infertility, or population-level health collapse. The data did not show those effects. Genetic damage was not detected in the children of survivors, and overall survivor mortality from causes other than cancer was not significantly elevated.
The implication is dual: nuclear weapons are even more devastating in their immediate effects than feared (the casualty toll matched or exceeded estimates), but their long-term genetic legacy is less catastrophic than feared.
The acute radiation syndrome data
The Hiroshima and Nagasaki survivor cohort also produced the foundational dataset on acute radiation syndrome (ARS). The dose-response curves for ARS — including the LD50/60 (the dose at which 50% of victims die within 60 days) of about 4 sieverts — come almost entirely from the 1945 cohort, supplemented by later accidents at Chernobyl, Tokaimura, and Goiânia.
For the simulator's casualty calculations, ARS is implicit in the mortality rates assigned to each blast zone. Inside the 20 PSI severe-blast zone, where neutron and gamma fluxes were highest, mortality is essentially 98% — a combination of blast injury, burns, and acute radiation. Outside the blast zones, ARS is rare in air-burst scenarios because the radiation flux drops below the ARS threshold by the time it reaches survivable distances.
What the data does not capture
Hiroshima and Nagasaki are the only two real-world data points we have on nuclear weapons used against real cities. Both bombs were small by modern standards (15 and 21 kt). Both were air bursts in flat or moderately hilly terrain. Both were used against early-1940s wood-and-paper urban construction.
A modern strategic warhead (100-500 kt) detonated over a modern city would produce different effects: larger blast zones, greater thermal impact, and — depending on construction type — either reduced firestorm risk (steel-and-concrete buildings) or different patterns of injury. A surface burst, which neither attack used, would also add a major fallout component.
The simulator extrapolates from the 1945 calibration data using the Glasstone-Dolan scaling laws, which themselves were validated against test-data at higher yields. The methodology page details the formulas. The bottom line is that we have good first-order knowledge of what nuclear weapons do — but we have not used a nuclear weapon against a city in 80 years, and we hope never to again.