The effect of prenatal radiation exposure on the developing human brain.

Recently reported dose responses in prenatally exposed Japanese bomb survivors for severe mental retardation (SMR), reduced intelligence, and reduced levels of school performance, are compared. The characteristics of, and differences between, severe and mild mental retardation in man are critically important for such comparisons. The meaning of linearity of dose response is not identical for these different forms of damage. When findings on tissue changes in the brain and in functional tests of irradiated experimental animals are taken into account, the dose response for SMR would be expected to have a threshold as is found using DS86 dosimetry. The dose responses for IQ and for school performance seem doubtfully valid: their underlying assumptions need re-examination.