When the atomic bomb detonated over Hiroshima on August 6, 1945, and a second over Nagasaki three days later, the immediate destruction was unlike anything in the history of warfare. But what followed in the seconds, hours, days, and decades after the blasts shaped not only the fate of two cities but the world’s understanding of nuclear weapons. Here’s what happened, from the first microsecond to the present day.
The First Seconds: Heat, Pressure, and Light
The Hiroshima bomb, nicknamed “Little Boy,” released energy equivalent to roughly 15,000 tons of TNT in a fraction of a second. That energy took the form of light, heat, radiation, and pressure, all arriving nearly simultaneously. Temperatures at ground zero reached an estimated 7,000°F. Winds at the hypocenter exceeded 980 miles per hour, with air pressure spiking to about 8,600 pounds per square foot.
Within a third of a mile from ground zero, substantial concrete buildings were obliterated. A full mile out, every brick building was destroyed, where winds still reached 190 mph and pressure hit 1,180 pounds per square foot. The thermal flash burned everything within 9,500 feet, charring trees instantly and bleaching stone and concrete surfaces with ultraviolet light so intense it worked like a photo negative.
This is what created the famous “nuclear shadows” still visible on surfaces in Hiroshima. These aren’t the remains of people. They’re outlines left where a person or object shielded a section of stone from the heat flash, leaving that patch darker than the surrounding surface, which had been bleached pale by the blast. A shadow on the steps of a bank, for example, marks where someone was sitting when the bomb detonated. The granite around them changed color, but the spot beneath them did not.
The First Hours: Radiation Sickness Sets In
Survivors close to the blast who weren’t killed instantly faced a second, invisible threat: radiation. Within 30 minutes, virtually everyone exposed to high doses began vomiting. High fevers followed within the hour. Within one to three hours, severe diarrhea set in for many. By three to four hours, roughly 80% of heavily exposed survivors developed intense headaches, and some experienced reduced consciousness.
What made radiation sickness so terrifying was its deceptive timeline. After this initial wave of symptoms (called the prodromal phase), many people felt temporarily better, entering a latent period lasting up to a week. Then the critical phase hit: complete hair loss, immune system collapse, internal bleeding, and overwhelming infection. For those who received the highest doses, this phase was fatal. Those who survived it faced months of slow, uncertain recovery.
Black Rain
Starting roughly 20 to 30 minutes after the Hiroshima blast, a dark, sticky rain began falling over the city and surrounding areas. The mushroom cloud had sucked up massive quantities of debris, soot, and radioactive material, which then condensed and fell back to earth as large, oily black drops. People who were desperately thirsty drank it. It stained skin and clothing.
The rain carried radioactive fallout, including uranium particles from the unfissioned portion of the bomb’s warhead. These sub-micron particles were small enough to be inhaled or ingested, delivering internal radiation exposure to people who may not have been close enough to the blast to receive significant external doses. For decades, the full radiological significance of the black rain was underestimated in survivor studies.
The First Weeks: A City Without Medicine
Hiroshima’s medical infrastructure was essentially gone. Most of the city’s doctors and nurses were dead or injured. Hospitals were destroyed. Survivors with severe thermal burns, many of deep second or third degree, received little to no treatment. Nutrition was poor, infection rates were extraordinarily high, and healing was slow.
These conditions contributed to an unusually high rate of keloid scarring, where the body produces excessive amounts of fibrous tissue during wound healing. The raised, often disfiguring scars became one of the visible markers of atomic bomb survivorship. Studies later concluded that the keloids weren’t a unique effect of radiation itself but rather the result of deep burns healing under the worst possible conditions: no antibiotics, no proper wound care, and severely weakened patients. Similar scarring occurred in Japanese burn patients from other causes under comparable circumstances.
The First Years: Cancer and Chronic Illness
Within two to three years of the bombings, survivors began developing leukemia at elevated rates. This was the first wave of radiation-induced cancers, and it drew immediate scientific attention. By the early 1950s, the Atomic Bomb Casualty Commission (later reorganized as the Radiation Effects Research Foundation) had begun the Life Span Study, one of the longest-running epidemiological studies in history, tracking health outcomes in roughly 120,000 survivors and their descendants.
Solid cancers, including stomach, lung, liver, and breast cancers, followed on a longer timeline, with elevated rates appearing years to decades after exposure. The data showed a clear dose-response relationship: the more radiation a person had absorbed, the higher their cancer risk. Women were more vulnerable than men. For every unit of radiation dose (measured in grays), women’s risk of developing a solid cancer increased by about 64%, while men’s increased by about 20%. These findings became foundational to modern radiation safety standards worldwide.
Effects on Survivors’ Children
One of the most feared consequences of the bombings was the possibility that radiation damage would be passed down to the next generation. Survivors (known in Japanese as hibakusha) faced severe social stigma, with many struggling to find marriage partners because of widespread belief that their children would be born with defects.
Decades of research on the children of survivors, however, have found no indication of inherited genetic effects. Studies examined rates of birth defects, stillbirths, perinatal deaths, chromosome abnormalities, and protein alterations. None showed a statistically significant increase linked to parental radiation exposure. Cancer incidence studies in this second generation are still ongoing, but so far the evidence has not confirmed the feared intergenerational damage.
Japan’s Surrender and the End of the War
The bombing of Hiroshima on August 6 killed an estimated 70,000 to 80,000 people instantly, with the toll eventually reaching around 140,000 by the end of 1945 when deaths from injuries and radiation sickness were included. Nagasaki’s bomb on August 9 killed approximately 40,000 immediately, with the total climbing to around 70,000 by year’s end. On August 15, 1945, Emperor Hirohito announced Japan’s surrender in a radio broadcast, the first time most Japanese citizens had ever heard the emperor’s voice. The formal surrender was signed aboard the USS Missouri on September 2, 1945, ending World War II.
Hiroshima and Nagasaki Today
Both cities were rebuilt, and today they are thriving urban centers. A question people often ask is whether they’re still radioactive. The answer is essentially no. The bombs detonated high above the ground (about 1,900 feet over Hiroshima), which maximized blast damage but meant relatively little long-term ground contamination compared to a surface detonation or a reactor meltdown like Chernobyl. Current background radiation levels in both cities are so low that they are difficult to distinguish from the trace radioactivity present everywhere on Earth from decades of global nuclear weapons testing in the 1950s and 1960s.
Hiroshima’s Peace Memorial Park sits at the former commercial heart of the city, near the iconic Atomic Bomb Dome, one of the few structures left partially standing after the blast. The Hiroshima Peace Memorial Museum preserves artifacts including nuclear shadows, stopped clocks, and personal belongings of victims. Both cities have become global symbols of nuclear disarmament, and hibakusha have spent decades sharing their testimony to advocate against the use of nuclear weapons.