Science Update: Hiroshima and Nagasaki

  Last updated March 6, 2017 at 3:17 pm


Science Update is a monthly written series from Dr Paul Willis tackling some of the controversial topics in the public and aims to provide the current research behind these subjects. 

There is great confusion surrounding the effects of radiation on the populations of the two attacks using atomic weapons. It is a popular myth that many, if not most, of the deaths from the bombing of Hiroshima and Nagasaki were caused by exposure to radiation, and that the long-term health effects of exposure include a staggering increase in deaths from cancer and an increase in birth defects in later generations. Time for a Science Update on the public health effects from radiation generated by the Hiroshima and Nagasaki bombings.

Nuclear issues

Earlier this year The Royal Institution of Australia, in collaboration with the Australasian Radiation Protection Society, hosted a discussion around many of the issues surrounding the disposal of nuclear waste. During this discussion it became apparent that there are many myths and misunderstandings about the effects of radiation.

This article is the first of a series of Science Updates on nuclear issues with articles to follow on the nuclear incidents at Chernobyl and Fukushima, which are fundamentally different from what happened at Hiroshima and Nagasaki.


Professor Geraldine Thomas from Imperial College London argues that the atomic attacks on Hiroshima and Nagasaki have created a dark legacy over modern discussions around nuclear issues.

It is important that we understand exactly what happened at Hiroshima and Nagasaki if we are to proceed with rational discussions around current nuclear issues.

Hiroshima and Nagasaki: what happened?

On the 6th August 1945 the US Airforce dropped a 16 kiloton atomic bomb on the city of Hiroshima in Japan during World War II. Three days later, a second atomic bomb with a yield of 21 kilotons was dropped on the city of Nagasaki.

Estimates of the death tolls for both attacks range from between 90,000–146,000 people killed in Hiroshima and 39,000–80,000 killed in Nagasaki. For the purpose of this article, we will use a median total score of 150,000-200,000 deaths for both attacks combined.

Contrary to popular belief, the majority of deaths from the atomic bombs in Hiroshima and Nagasaki were caused by flash burns or other injuries. Half the people killed in both events died on the first day of the attacks. These deaths were mostly caused by the blast effects of the bombs or incineration from the extreme heat generated by the detonations.

Most of the other half of the deaths occurred within months of the blasts and were caused by effects of burns and blast injuries, illness and malnutrition caused by the breakdown of the provision of services in the devastated cities and the effects of radiation sickness. Of this half, 21,000 – 40,000 can be attributed to the effects of exposure to radiation.

As horrendous as these figures are, they are comparable to bombings of other cities in WW2 using conventional weapons such as Tokyo (up to 125,000 deaths in one raid), Hamburg (42,600 deaths) and Dresden (up to 25,000 deaths).

There are some differences in these comparisons:

  • the difference in bombing effort (Hiroshima and Nagasaki were single plane raids dropping a single bomb whereas Tokyo, Hamburg and Dresden were raids involving many planes and bombs)

  • the most significant difference in the death tolls are the proportion of deaths ascribed to radiation exposure in Hiroshima and Nagasaki.

Death by Radiation

There are two groups of deaths arising from radiation exposure associated with the two blasts in Japan – radiation-induced deaths in the period immediately after the detonations and longer-term deaths from increased risks of diseases induced by exposure to the radiation.

Deaths from Acute Radiation Syndrome

Around 15-20% of the Hiroshima and Nagasaki population died as a result of acute radiation sickness (ARS) within the two-month period following the blasts.

Victims of ARS would present with symptoms such as loss of hair, bleeding into the skin and other haemorrhaging, lesions in the mouth and throat, vomiting, diarrhoea, and fever. More severe cases would die within 10 days of exposure whereas around half of the more moderate cases proved fatal after a month. Less severe cases were likely to make a full recovery. Deaths attributed to exposure to radiation began about a week after the attacks and reached a peak in 3 to 4 weeks and they practically ceased to occur after 7 to 8 weeks.

Radiation dosage

To understand the effects of radiation from the blasts, we first need to understand the dose of radiation that survivors experienced. The threshold dose for ARS is around 1Gy (Gy is Gray, an SI Unit that measures absorbed doses of radiation. In other words, 1Gy of radiation is the minimum dose of ionising radiation that will produce a detectable degree of any given effect.

Location of the survivors on the day of the Hiroshima blast showing levels of radiation experienced. Red >1Gy; Orange .5-1Gy; Yellow .2-.5Gy; Green .1-.2Gy; Brown .05-.1Gy; Pink <.05Gy. Rings represent 2 and 3 km from epicenter. From Douple et al

Clearly the closer to the epicentre of the blast, the greater the dose of radiation received. Almost all of the radiation received by the survivors was gamma radiation.

Longer-term deaths from radiation exposure

In 1950 the Life-Span Study was set up following the long-term health of the survivors of the two atomic detonations over Japan and has continued monitoring these populations ever since. It is now the most comprehensive database on the long-term effects of exposure to radiation currently available. See here, here, here and here.

Of the 120,000 original subjects in this study, 54,000 were within 2.5 km of the epicentre of the detonations and 45,000 were located further away. 40% of these people are still alive. These people are compared to a control population of 26,000 individuals who were not present at the detonations but lived in Hiroshima or Nagasaki between 1951 and 1953.

Health effects: adults

In the years after the attacks a spike in extra cases of leukaemia was identified. The risk of getting this disease was related to how close the patient was to the centre of the blast. These leukaemias were first reported about 3 years after the bombings and reports peaked 6-8 years after the incident.

In cases of solid cancers, the data shows a linear relationship between the dose of exposure to radiation and the likelihood of developing a cancer. In other words, the closer they were to the blast, the higher the dose of radiation they received and the more solid cancers were reported over their lifetimes. But this pattern is difficult to see in people exposed to lower doses of radiation.

While these are relative risks surrounding solid cancers, the overall increase in solid cancers due to radiation exposure is rather small. From a total of 17,448 solid cancer cases reported in the study from 1958 to 1998, 853 (around 11%) could be attributable to radiation exposure from the detonations.

At higher doses there was an increased risk of cardiovascular diseases and some other non-cancer diseases. Survivors who were exposed to high doses of radiation may show a deterioration of the immune system and may have minor inflammatory reactions.

Health effects: children and infants

Survivors who were children at the time of the blasts have a higher risk of developing cancer than those who were older at the time. Infants who were in the womb at the time of the blasts showed some problems with the development of the central nervous system and growth as well as a higher risk of cancer related to the size of the radiation dose the mother received. There has been no detectable increase risk of hereditary malformations, cancer or other diseases in children of parents who were exposed to the blasts.

Life span effects

An analysis of the data shows average life expectancy decreases with increasing doses of radiation at a rate of about 1.3 years/Gy. The same study estimated that, at 1 Gy, the proportion of total life lost was roughly 60% from solid cancer, 30% from diseases other than cancer, and 10% from leukaemia. This equates to an overall effect in terms of decreased life expectancy of 2.6 years for those who received the highest doses and 21 days for those who received the lowest doses.

Present day

There is no residual radiation from the nuclear detonations in either city today and both have been rebuilt as functional modern cities.


Attempting to reconstruct how many people have died as a result of radiation from the two atomic attacks on Japan at the end of WW2 has proved difficult.

Of the 150,000 – 200,000 people killed by both events, the best estimates are that 21,000 – 40,000 (between 15-20% of the total death toll) can be attributed to exposure to radiation. Most of these deaths occurred within two months of the detonations and a small number (around 1,000) died from cancer or other diseases induced by exposure to radiation. These deaths occurred over the decades after the events. There are no records of birth defects in subsequent generations that can be attributed to exposure to radiation from the two attacks.

Caution must be exercised when reviewing these figures and comparing them to health effects from radiation caused by the nuclear incidents at Chernobyl and Fukushima. These are fundamentally different events and the profile of health effects from radiation exposure is also completely different from that of the atomic attacks on Hiroshima and Nagasaki. That will be the subject of a future Science Update.

Australia’s Science Channel wishes to acknowledge the assistance of Professor Geraldine Thomas from Imperial College London in reviewing early drafts of this blog and we would like to thank her for her help.

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About the Author

Paul Willis
Paul is a respected leader in the science community with an impressive career in science. He has a background in vertebrate palaeontology, studying the fossils of crocodiles and other reptiles. He also has a long history as a science communicator, with a career spanning as Director of The Royal Institution of Australia, presenter and host for Australia’s Science Channel, working for the ABC on TV programs such as Catalyst and Quantum as well as radio and online. He’s written books and articles on dinosaurs, fossils and rocks and is finding new ways to engage the people of Australia with the science that underpins their world. Follow him on Twitter @fossilcrox.

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The Royal Institution of Australia is an independent charity, and the sister organisation of the prestigious Royal Institution of Great Britain, tasked with promoting public awareness and understanding of science.

The Royal Institution of Australia is passionate about building and connecting communities engaged with science, and as such works closely with scientific organisations, institutions, universities from Australia, and leaders to inspire the next generation of innovators and to create a lasting legacy for Australia.

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