|Fig. 1: Atomic detonation in Nagasaki. (Source: Wikimedia Commons)|
The 1945 atomic bombing of Hiroshima and Nagasaki (shown in Fig. 1) left an unprecedented number of casualties. While the immediate impact was clear and quantifiable - a number of killed and wounded in the excess of 200,000 - follow-up studies of the long-term effects on the survivors are still ongoing today. In fact, not only the survivors but also their children are experiencing health issues caused by the detonations. Some of the important metrics used include cancer rates and birth defects.
The most reliable method of studying these long-term effects is cohort study, and the most notable one is known as the Life Span Study (LSS). Started by the Atomic Bomb Casualty Commission and the Radiation Effects Research Foundation, the LSS consists of 120,000 persons who were exposed at different levels, in addition to 77,000 children who are born in the immediate aftermath with at least one parent who had radiation exposure.  These studies not only describe a general distribution of health effects, but also provide an estimate for median lethal dose (LD50) from whole-body gamma radiation. With little or no medical assistance, the median lethal dose is 2.5 Gy (Gy is a gray unit for measuring irradiation resulting in the absorption of 1 joule per kilogram).  With extensive medical care, this number goes up to 5 Gy. For comparison, the yearly limit for general public is 1 mGy.
Furthermore, it is shown that the cancer rates are higher for people who were in town when compared to those who were not in town at the time. Other factors that show clear correlation with increase in cancer include proximity to the detonation site, age, and sex. Moreover, the population that was exposed to higher radiation dose of 1 Gy is shown to have 44% higher risk of cancer. 
Having many data points for convoluted problems--such as health effects of one incident over a lifespan--is essential; however, it is difficult to draw concrete conclusions for all health issues for different groups. For instance, it is quite clear that being exposed to higher doses of radiation, i.e., over 0.1 Gy, increases risk of cancer; however, such statement cannot be made for lower doses below 0.1 Gy.  Also, it is worth noting that the risk of hereditary malformations or cancer did not increase for the children. Many of the studies are still ongoing and remain to be seen. In the meantime, these studies can be used to not only understand the health effects, but also to serve as a warning and cautionary tale for workers in the nuclear industry.
© Daniel Shin. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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.
 E. B. Douple et al., "Long-term Radiation-Related Health Effects in a Unique Human Population: Lessons Learned From the Atomic Bomb Survivors of Hiroshima and Nagasaki," Disaster Med. Public Health Prep. 5 S122 (2011).
 B. R. Jordan, "The Hiroshima/Nagasaki Survivor Studies: Discrepancies Between Results and General Perception," Genetics 203, 1505 (2016).
 K. Kamiya et al., "Long-Term Effects of Radiation Exposure on Health," Lancet 386, 469 (2015).