Genetic Effects of the Atomic Bombs in Hiroshima and Nagasaki

Emily Koufakis
February 20, 2017

Submitted as coursework for PH241, Stanford University, Winter 2017


Fig. 1: Effects of the atomic bomb in Hiroshima, viewed from the top of the Red Cross Hospital. (Source: Wikimedia Commons

Besides killing around 200,000 civilians and military personnel, the atomic bombings dropped on the cities of Hiroshima and Nagasaki in Japan unleashed an invisible plague. Taking less than one millionth of a second for the bombs to drop with hyper-energetic radiation, such horrific conditions would have profound health effects for decades to come. [1] Fig. 1 shows the aftermath of the attack in Hiroshima shortly after the bomb dropped.

Today, most of the generation that was alive during the bombings has passed away. However, immense attention has turned to the children born to the survivors of the attacks. Regarding individuals who had been exposed to radiation in-utero, studies have proven that such exposure led to mental retardation, impairment in physical growth and even an increase in cancer risk. [2]

Mental Retardation

Radiation doses between 50 and 100 rad following the bombings showed that children exposed in-utero had a significant increased risk for small brain size and mental retardation. [3] Moreover, such risk escalated for women who were further along in their pregnancy - eight to fifteen weeks - at the time of exposure in addition to them being 1200 m or less from the hypocenter during the attack. This is due in part to neuronal proliferation and cell migration to be the most active during this period of pregnancy. [3] However, Miller found that between zero and seven weeks post conception in addition to mothers being farther than 1200 m from the hypocenter, there appeared to be no impact on mental development. [3] In general, sensitivity for mental retardation was found to depend on the dose of radiation and the distance from the hypocenter at the time of the attacks. Contrary to Miller's findings, Schull discovered that radiation did not have any general effect on the development of mental retardation based on clinical surveillances, in addition to children born to parents that had been exposed to radiation during the attacks. [1]

Cancer Risk

Studies have proven that both survivors of the attacks and their children, who had been exposed to high radiation, are at risk of developing cancer. [4] A mutagen like radiation has the ability to increase the likelihood of a mutation-taking place in the body, which leads to cancer. [2] Among the long-term effects suffered by children, the most deadly was found to be leukemia. Following two years after the bombings, an increase in leukemia appeared and progressed even more around four to six years later. Attributable risk, which is defined by the percent difference in incidence rate between an exposed population and a comparable unexposed one, shows how severe the impact of radiation has on leukemia incidence. [2] Yoshimoto found that 18 cases of cancer occurred in the sample size of 1630 people, over the course of 30 years (1950 to 1980). [4] However, such finding proves to be insufficient due to the fact that the subjects who were exposed in-utero were followed until age 35. This highlights the possibility of subjects older than age 35 not developing cancer. Contrary to Yoshimoto's findings, Schull conducted a separate study - 1947 to 1954 - and failed to find leukemia presence in the subjects whose parents were exposed to radiation at the time of the attacks. [1] Moreover, Schull concluded that there appeared to be no apparent increase in the risk of cancer at all when examining children whose parents were 20 years or younger at the time of radiation exposure. [4]


Various studies that were conducted following the bombings of Hiroshima and Nagasaki showed no statistically significant increase in cancer risk or other pregnancy outcomes among children of survivors. However, evidence shown by later studies proves that there were alarming genetic and health effects on both the survivors and their children. Above all, the survivors have become one of the longest studied groups in health research. The atomic bombs' immediate effects destroyed both Hiroshima and Nagasaki, killing between hundreds of thousands of innocent people. But the long-term health effects on those who survived and their children may be one of the most enduring parts of the attacks' legacy.

© Emily Koufakis. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] W. J. Schull, "The Children of Atomic Bomb Survivors: A Synopsis," J. Radiol. Prot. 23, 369 (2003).

[2] Y. Yoshimoto, "Cancer Risk Among Children of Atomic Bomb Survivors," J. Am. Med. Assoc. 264, 1812 (1990).

[3] R. W. Miller, "Effects of Ionizing Radiation From the Atomic Bomb on Japanese Children," Pediatrics 41, 257 (1968).

[4] S. Yapa, "Effects on Children Exposed to Atomic Bomb Radiation Through Their Parents," Physics 241, Stanford University, Winter 2013.