|Fig. 1: The city of Hiroshima, devastated following the explosion of the first atomic bomb. (Source: Wikimedia Commons)|
The atomic bombings of the Japanese cities of Hiroshima and Nagasaki had long-reaching effects, not just in their major role in ending the Second World War, but also in the arena of research into the world of nuclear weapons. Between the two bombings, it is estimated that 210,000 people were killed within the months immediately following the bombings (This number is variable based on the source. It is unknown exactly how many people perished in the bombings). 
Additionally, numerous other Japanese citizens were exposed to radiation from the bombs but did not die from the acute effects of the exposure. Many of these people went on to have children afterwards. In some cases, the mothers were already pregnant and both they and the fetuses were exposed to the radiation in-utero. In other cases, however, the parents were exposed and years later had children. It is an area of ongoing research to determine what the possible effects were on the children that experiences either of these types of exposure. The bulk of the research concerning these children was conducted in the 40 years immediately after the bombing events (1945 onwards). The main risks that were evaluated in these children were (1) cancer risk, (2) risk of mental retardation, and (3) risk of congenital birth defects. Although the actual radiation dosages received by victims at both Hiroshima and Nagasaki is unknown, the radiation dosage received was estimated using methods known as DS86 or T65. 
It is commonly believed that exposure to ionizing radiation will increase the cancer risk for both the person exposed as well as for any children of the person. Yoshimoto found that from those people that were exposed in-utero (they were fetuses when their mothers were exposed to atomic radiation), in the period of 1950 to 1980, 18 cases of cancer occurred in the sample size of 1630 people.  Unfortunately, this information is not particularly illuminating given that this only follows the subjects exposed in-utero until about age 35, so it cannot provide a good lifetime risk of cancer (making comparison against a non-exposed population impossible). Yoshimoto concluded that mothers who had received greater than 0.30 Gy had children with an increased risk of cancer. In a separate clinical surveillance conducted from 1947 to 1954, it was found that there was not a significant increase in leukemia with parental exposure to radiation in Hiroshima and Nagasaki.  However, for those children whose parents were less than 20 years old when experiencing the radiation exposure (i.e. the children were born years after the incident and any exposure is solely passed on from their parents), there appeared to exist no increase in the risk of cancer.
Two separate clinical surveillances found that children of parents exposed to an estimated 0.01 Gy or more had increased risk of birth defects. In a large sample size of 70,073 pregnancies available, it was determined the increase in the risk of birth defects was 25 per 1 million pregnancies. In a more reliable, smaller sample (55,503 pregnancies), the increase in the risk of birth defects was 42 cases per 1 million pregnancies (this is the increase over the occurrence of birth defects in a population not exposed to atomic radiation) . Although the actual radiation dosages received by victims at both Hiroshima and Nagasaki is unknown, the radiation dosage received was estimated using a method known as DS86.  Ultimately however, a conclusive statement cannot be made regarding the relationship between birth defects and radiative exposure to the mother because the increase in the risk computed is so small as to be not a significant concern. 
The incidence of mental retardation with in-utero exposure to atomic radiation was found to be quite significant.  Miller found that for in-utero exposure where the mothers had been located 1200 m or less from the hypocenter, all of the children that were born and able to be observed had a head circumference far below average (indicating less brain development) and were born with mental retardation.  There was no such trend for children born to mothers who were farther than 1200 m during the bombing. However, Schull found from clinical surveillances that there is no radiation-developed retardation, or that children born to parents that had been exposed to the ionizing radiation (but were not themselves exposed) were not more likely to be born mentally retarded. 
The many studies that were conducted after the nuclear bombings of Hiroshima and Nagasaki determined that some of the risks that the public perceives to be associated with radiation exposure are non-existent, while others are in fact matters of concern. The cancer risk for children who were in-utero when their mothers were exposed is higher than it would be for others without this exposure, but children who were conceived after their parents were exposed to radiation had no increased cancer risk. Similarly for both birth defects and mental retardation, it was found that in-utero exposure possibly increased the rate of incidence of these conditions, but results from all studies are inconclusive, except for the case of pregnant mothers within 1200m of the hypocenter. All of the in-utero children born to these mothers experienced mental retardation. All three of these cases found that there is no increased risk of these conditions if the parents of the child were exposed to the ionizing radiation of the atomic bomb (child was not conceived at the time of the exposure).
© Sayuri Yapa. 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.
 J. McCurry, "Hiroshima and Nagasaki Survivor Dies Aged 93," The Guardian, 6 Jan 10.
 Y. Yoshimoto et al., "Risk of Cancer Among Children Exposed in Utero to A-Bomb Radiations, 1950-1984," Lancet 2, 665 (1988).
 Y. Yoshimoto, "Cancer Risk Among Children of Atomic Bomb Survivors," J. Am. Med. Assoc. 264, 1812 (1990).
 W. J. Schull, "The Children of Atomic Bomb Survivors: A Synopsis," J. Radiol. Prot. 23, 369 (2003).
 R. W. Miller, "Effects of Ionizing Radiation From the Atomic Bomb on Japanese Children," Pediatrics 41, 257 (1968).