The atomic bomb survivors of Hiroshima and Nagasaki are a significant population for the study of radiation induced human leukemia. Whole body irradiation is most likely the cause of the leukemia caused by the atomic bomb radiation.  Follow-up of Japanese adult survivors of Hiroshima and Nagasaki have revealed abnormally high rates of cancer, persistent chromosomal aberrations in circulating lymphocytes, and lenticular opacities.  Among Japanese children exposed to atomic bomb radiation impaired growth is apparent, but there has been no evidence of aging and life-shortening.  Finally, children of atomic bomb survivors, who received exposure in utero ionizing radiation, had diminished stature, small head size, mental retardation, and subsequent seizures.  Study of the aforementioned groups has provided essential knowledge on the health effects of atomic bomb radiation.
One type of leukemia that was found to be very prominent among Hiroshima survivors was chronic myelogenous leukemia (CML). CML is a form of leukemia characterized by an increase in unregulated of predominantly myeloid cells in the bone marrow and the accumulation of these cells in the blood.  Studies by Ichimaru et al. have revealed a peak in the incidence of CML in Hiroshima victims 6 years after the accident. The increase in CML occurred in groups of individuals exposed to below and above 1Gy of radiation. In a separate cohort of individuals, Ichimaru et al. compared individuals exposed to 0.5 Gy or more, 0.1-.49 Gy, and below 0.09 Gy.  Results demonstrated individuals exposed to 0.5 Gy and 0.1-.49 Gy had very similar rates of CML development while individuals exposed to below 0.09 Gy of radiation had a much lower rate of CML development.  Pooling results from both Hiroshima and Nagasaki, out of all types of leukemia (acute myeloid leukemia, acute lymphoblastic leukemia, myelodysplastic syndromes, and chronic myelogenous leukemia), CML was found to occur in 17.6% of individuals exposed to 0 Gy, 36.5% of individuals exposed to 0.01-0.99 Gy, and 29.8% of individuals exposed to 1Gy or more. Taken together, this data suggests that a threshold for exists for CML development somewhere between 0.5 Gy and 0.09 Gy.  Although the impact of atomic radiation on hemopoietic stem cells could be indicated by chromosome aberrations observed in colony forming cells induced from stem cells and peripheral T-cells, the reason for increased CML development in individuals of Hiroshima has not been pinned down yet.
Examining a cohort of individuals who were in utero during the atomic bombing of Hiroshima and Nagasaki reveals two correlations. First, Yamazaki et al. found as radiation dose to the in utero fetus before 18th week of gestation increased the size of the infant's head decreased. More specifically, a small head was classified as any head that was 2 or more standard deviations below the average circumference (about 54 centimeters at 18 years of age).  Examining the different cohorts studied by Miller et al. one finds that a minimum dosage of 0.10 to 0.19 Gy was required for any significant change in head size to be detected. Table 1 provides a summary of the data collected by Miller et al. 
|Table 1: Radiation exposure and small head size correlation |
In addition to small head size, the frequency of severe mental retardation among fetuses exposed to ionizing radiation in utero was examined. Otake et al. found that the earlier a fetus was exposed to radiation the more damaging the effect of the radiation was.  Fetuses aged 8-15 weeks that were exposed to 1 Gy of radiation exhibited a 50% chance of mental retardation while fetuses aged 16-25 weeks only about a 20% chance of having mental retardation.  Examining the data in Table 2 indicates a vulnerable period from 8-15 weeks for mental retardation for fetuses.  Compared to the population average of about 1 out 100 babies being classified as mentally retarded, the impact of bomb radiation is significant as discussed by Otake et al. Combining the data from Yamazaki et al and Otake et al one can see the significant health impacts on fetuses by atomic bomb radiation.
|Table 2: Fetal absorbed dose of radiation and percent chance of mental retardation correlation. |
Examining development of CML in adults and small head size and mental retardation in children reveals trends concerning atomic bomb radiation and disabilities arising from them. The longevity with which studies conducted by the researchers cited in this paper have provided convincing evidence of the negative health effect of atomic bomb radiation. The Hiroshima and Nagasaki accidents represent key events that, although very dreadful, have provided a wealth of knowledge on the health effects of acute radiation exposure.
© Jonathan Tran. 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.
 M. Ichimaru et al., "Atomic Bomb and Leukemia: A Review of Forty-Five Years of Hiroshima and Nagasaki Atomic Bomb Survivors," J. Rad. Res. Suppl. 32, 162 (1991)
 UNSCEAR 1988 Report: Sources, Effects, and Risks of Ionizing Radiation (United Nations, 1988).
 J. N. Yamazaki and W. J. Schull, "Perinatal Loss and Neurological Abnormalities Among Children of the Atomic Bomb," J. Am. Med. Assn. 264, 605 (1990).
 R. W. Miller and J. J. Mulvihill, "Small Head Size After Atomic Radiation," Teratology 14, 355 (2005).
 M. Otake, H. Hoshimaru and W. J. Schull, "Prenatal Exposure to Atomic Radiation and Brain Damage," Congital Abnormalities 29, 309 (1989).