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Article ID:0000264912Updated on:2022年11月28日更新印刷ページ表示

The Realities of the Atomic Bombing

原爆によるきのこ雲

Monday morning on August 6, 1945, dawned bright and clear in Hiroshima. As the summer sun climbed in the sky, the temperature rose rapidly.

The air-raid warning just after midnight at 12:25 am was cleared at 2:10 am. Having finally settled back to sleep, people were abruptly awakened by another siren at 7:09. That turned out to be a US military reconnaissance plane cruising at high altitude, so the all-clear was sounded at 7:31. Sighing in relief, people returned from their air-raid shelters and other places of refuge. They were eating late breakfasts or on their way to work, beginning the day.

Suddenly, a bell in the Information Relay Room at the Hiroshima Central Broadcasting Station signaled the notice to broadcast an air-raid alarm. Furuta, the announcer, ran to the Alarm Office, grabbed the text for the announcement and rushed to the studio to push the buzzer.

“This is a report from the Chugoku Regional Military Headquarters! Three large enemy planes advancing from Saijo..." Suddenly, he heard a terrible cracking sound, felt the ferro-concrete structure tilt to one side, and was thrown through the air.

8:15 am on August 6, 1945
The first atomic bomb used against human beings was dropped on Hiroshima.

The atomic bomb exploded 43 seconds after being dropped 600 meters over the city with a blinding flash, creating a fireball that blazed like a small sun. More than 1,000,000℃ at its center, in one second the fireball reached a radius of over 200 meters, and the surface temperatures near the hypocenter rose to 3,000 to 4,000℃.

At the time of the explosion, fierce heat rays and radiation burst out in every direction, causing the air around the fireball to expand and creating a super-high-pressure blast. The complex interactions of these three factors inflicted tremendous damage upon the city.

The damage inflicted by the atomic bomb was characterized by instant and massive destruction, indiscriminate mass slaughter, and radiation. In particular, the damage caused by the radiation led to decades of human suffering.

For more information about the realities of the atomic bombing, please visit the Hiroshima Peace Memorial Museum<外部リンク> website.

Damage due to Heat Rays

At the time of the explosion, the temperature of the detonation point exceeded 1,000,000℃, and the fireball generated in the air grew to a radius of over 200 meters one second later. Beginning 0.2 seconds after detonation, heat rays emitted in all directions by the fireball exerted powerful effects at ground level for three seconds. Surface temperatures near the hypocenter reached 3,000 to 4,000℃. For reference, iron melts at 1,536℃.

People sustained severe burns from the powerful heat rays, and many died as a direct result. The burns were only on the side directly exposed to the heat rays, with some as far as 3.5 kilometers away suffering burns on exposed skin. The surface of roof tiles within 600 meters of the hypocenter melted and blistered. Many trees ignited, and within three kilometers, electric poles, trees, and lumber were charred.

Damage due to Blast

The explosion of the atomic bomb momentarily created a super-high pressure of several hundred thousand atmospheres. The surrounding air expanded enormously, generating a shockwave followed by a tremendous blast of wind.

It is estimated that the pressure of the shockwave was 11 tons per square meter even 500 meters from the hypocenter, and the blast speed reached 280 meters per second at 100 meters from the hypocenter. When the blast subsided, the air pressure in the center was extremely low, and air rushed back from surrounding areas toward the point of explosion.

Within two kilometers of the hypocenter, most wooden houses were destroyed. Although ferro-concrete structures remained standing, they were severely damaged with all windows shattered and interiors completely gutted by fire.

The blast lifted and hurled people through the air, knocking them unconscious, injuring them, trapping them under collapsed buildings, or crushing them to death. 

Damage due to Radiation

Unlike conventional bombs, the atomic bomb emitted massive amounts of radiation that inflicted grave damage to human bodies. Penetrating deep into bodies, radiation caused damage to cells, altered blood, diminished blood generation function of bone marrow, and damaged the lungs, liver, and other organs.

Radiation damage from the bomb varied considerably with distance from the hypocenter and the presence or absence of an intervening object. The initial radiation emitted within a minute of the explosion was lethal within one kilometer of the hypocenter. Most in that area died within a few days. Many who appeared uninjured developed disorders and died days or months later.

The explosion left residual radiation on the ground for a long period of time. Consequently, many who entered the city after the explosion to search for family or co-workers, as well as those who entered to participate in relief activities, developed symptoms similar to those resulting from direct exposure. Many died.

Symptoms from heat rays, blast, or radiation that appeared soon after the bombing are called acute disorders. In addition to external injuries and burns, acute disorders included vomiting and loss of appetite, diarrhea, headaches, insomnia, loss of hair, exhaustion, vomiting blood, blood in urine, blood in stools, purpura, fever, stomatitis, reduction of leukocytes and erythrocytes, and menstrual disorders.

These were not completely new disorders, but external injuries were complicated and resistance to pathogens was lowered by radiation and malnutrition. Even in the absence of external injuries, symptoms associated with low leukocytes (hair loss, bleeding, etc.) appeared, killing many.

Acute disorders faded away approximately five months later, around the end of December. At the time, it was thought that the effects of the bombing were over. However, the effects of radiation were far from finished.

Damage due to Firestorm

When the atomic bomb exploded, the intense heat rays ignited houses in the city center. In collapsed houses throughout the city, kitchen and other fires quickly spread out of control. Peaking between 10 am and 2-3 pm, but continuing all day, the city burned as if to scorch the heavens.

Within two kilometers of the hypocenter, the entire city burned to the ground. All objects were melted and fused into sheets like cooled lava. Countless people were trapped under fallen buildings and burned alive.

Damage to Buildings

The intense blast and heat rays of the bombing destroyed and burnt almost all buildings within two kilometers of the hypocenter. Even in the area beyond two kilometers of the hypocenter, wooden buildings sustained major damage, and 90 percent of the structures in the city sustained devastating damage. According to the city's general survey in 1946, 51,787 out of 76,327 buildings were completely destroyed or burnt by the bombing, and 6,180 were partially destroyed.

After the bombing, unusable buildings were removed and then buildings which remained after undergoing temporary repairs were dismantled. In the process of recovery, old buildings continued to disappear due to renovations and other reasons.

In response, citizens began to request that the atomic bombed buildings be preserved. The City Council passed a resolution that stated, "These historical assets should be passed down to future generations of Hiroshima citizens." In 1993, the City of Hiroshima established guidelines for preserving in perpetuity buildings damaged by the atomic bomb. As a result, surviving buildings and structures located within five kilometers of the hypocenter have been registered in a special ledger. Owners have been asked to help preserve the buildings, and subsidies are provided for preservation projects.

As of March 1, 2022, there are 86 such buildings (22 public, 64 private buildings) that serve as reminders of the bombing, including the Atomic Bomb Dome, located within five kilometers of the hypocenter.

Most of the 50 bridges in the city, mainly within 2.5 kilometers of the hypocenter, also sustained major damage by the bombing. Even many of those which managed to survive the fire or collapse were washed away due to Typhoon Makurazaki in September or floods that occurred in October 1946.

Afterwards, bridges that deteriorated or failed to meet safety standards continued to be replaced with new ones and currently, there are only six such bridges which now remain within five kilometers of the hypocenter.

Aftereffects

Radiation effects from the atomic bomb extended beyond the acute effects such as fever, nausea, and diarrhea, that appeared immediately after the bombing. It caused various disorders for decades and continues to threaten the health of hibakusha today.

In early 1946, burn scars on some survivors began to swell into keloids. Also, in-utero survivors had higher death rate even after their birth, and some were born with microcephaly. 

Several years after radiation exposure, the number of deaths from leukemia and cancers increased. Excess cases of leukemia began appearing two to three years after radiation exposure and peaked seven to eight years after it. In contrast, the latency period for solid cancers is relatively long; they are assumed to have increased five to ten years after the radiation exposure.

Even today, explanations of the effects of radiation over the years are inadequate. Research and survey efforts continue.

In addition to the enormous damage inflicted on persons and objects, the bombing also destroyed societal activities. The bomb's thorough destructive power assailed families, relatives, neighbors, and acquaintances. Every kind of institution and all societal functions vanished in an instant. Unprecedented in human experience, this magnitude of destruction caused inexpressible hardships in the lives of the survivors.

The harm done to minds and bodies by the atomic bomb failed to subside normally with time, as seen particularly with the horrors of radiation. Japanese research on atomic-bomb-related disorders, which was restricted during the occupation by the Allied Forces, began making progress after Japan gained its independence in 1951. Since then, images of the many survivors who suffered horrifying aftereffects have gradually come into focus.

Number of Deaths

Various institutions have investigated the number of people who perished in the bombing, but so far, the precise number is unknown. The City of Hiroshima estimates that approximately 140,000 people had died by the end of December 1945, when the acute effects by the radiation subsided.

It is thought that approximately 350,000 people were in the city when the bomb exploded on August 6, 1945. This figure includes ordinary citizens, military personnel, and people from surrounding towns and villages who commuted to work or were mobilized for building demolition work. Those who were involved in the tragedy were not only Japanese, but also people with various nationalities, such as Japanese-Americans who were born in the US, German priests, exchange students from countries in Southeast Asia, people from Korea and Taiwan, which were then Japanese colonies, and mainland China, as well as US prisoners of war.

Approximately half of those who were within 1.2 kilometers from the hypocenter died within the day; 80 to 100 percent of those who were closer to the hypocenter are thought to have died. Even among those who were able to escape an instant death or death within the day, the closer they were exposed to the bombing, and the more serious damages they suffered, the higher mortality rate they had.

Inquires about this page

Peace Promotion Division, Citizens Affairs Bureau
Tel: 082-242-7831/Fax:082-242-7452
E-mail: peace@city.hiroshima.lg.jp


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