Atomic bombings of Hiroshima and Nagasaki
Investigations into brain development

Despite the small sample size of 1,600 to 1,800 persons who came forth as prenatally exposed at the time of the bombings, that were both within a close proximity to the two hypocenters, to survive the in utero absorption of a substantial dose of radiation and then the malnourished post-attack environment, data from this cohort do support the increased risk of severe mental retardation (SMR), that was observed in some 30 individuals, with SMR being a common outcome of the aforementioned microencephaly. While a lack of statistical data, with just 30 individuals out of 1,800, prevents a definitive determination of a threshold point, the data collected suggests a threshold intrauterine or fetal dose for SMR, at the most radiosensitive period of cognitive development, when there is the largest number of undifferentiated neural cells (8 to 15 weeks post-conception) to begin at a threshold dose of approximately "0.09" to "0.15" Gy, with the risk then linearly increasing to a 43-percent rate of SMR when exposed to a fetal dose of 1 Gy at any point during these weeks of rapid neurogenesis.

However either side of this radiosensitive age, none of the prenatally exposed to the bombings at an age less than 8 weeks, that is prior to synaptogenesis or at a gestational age more than 26 weeks "were observed to be mentally retarded", with the condition therefore being isolated to those solely of 8–26 weeks of age and who absorbed more than approximately "0.09" to "0.15" Gy of prompt radiation energy.

Examination of the prenatally exposed in terms of IQ performance and school records, determined the beginning of a statistically significant reduction in both, when exposed to greater than 0.1 to 0.5 gray, during the same gestational period of 8–25 weeks. However outside this period, at less than 8 weeks and greater than 26 after conception, "there is no evidence of a radiation-related effect on scholastic performance."

The reporting of doses in terms of absorbed energy in units of grays and rads – rather than the biologically significant, biologically weighted sievert in both the SMR and cognitive performance data – is typical. The reported threshold dose variance between the two cities is suggested to be a manifestation of the difference between X-ray and neutron absorption, with Little Boy emitting substantially more neutron flux, whereas the Baratol that surrounded the core of Fat Man filtered or shifted the absorbed neutron-radiation profile, so that the dose of radiation energy received in Nagasaki was mostly that from exposure to X-rays/gamma rays. Contrast this to the environment within 1500 meters of the hypocenter at Hiroshima, where the in-utero dose depended more on the absorption of neutrons which have a higher biological effect per unit of energy absorbed. From the radiation dose reconstruction work, the estimated dosimetry at Hiroshima still has the largest uncertainty as the Little Boy bomb design was never tested before deployment or afterward; as such, the estimated radiation profile absorbed by individuals at Hiroshima had required greater reliance on calculations than the Japanese soil, concrete and roof-tile measurements which began to reach accurate levels and thereby inform researchers, in the 1990s.

Many other investigations into cognitive outcomes, such as schizophrenia as a result of prenatal exposure, have been conducted with "no statistically significant linear relationship seen". There is a suggestion that in the most extremely exposed, those who survived within a kilometer or so of the hypocenters, a trend emerges akin to that seen in SMR, though the sample size is too small to determine with any significance.
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