Health impacts of large releases of radionuclides. Biological effects of prenatal irradiation.

After large releases of radionuclides, exposure of the embryo or fetus can take place by external irradiation or uptake of radionuclides. The embryo and fetus are radiosensitive throughout prenatal development. The quality and extent of radiation effects depend on the developmental stage. During the preimplantation period (one to 10 days postconception, p.c.), a radiation exposure of at least 0.2 Gy can cause the death of the embryo. Malformations are only observed in rare cases when genetic predispositions exist. Macroscopic, anatomical malformations are induced only after irradiation during the major organogenesis (two to eight weeks p.c.). A radiation dose of about 0.2 Gy is a doubling dose for the malformation risk, as extrapolated from experiments with rodents. The human embryo may be more radioresistant. During early fetogenesis (8-15 weeks p.c.) a high radiosensitivity exists for the development of the brain. Radiation doses of 1.0 Gy cause severe mental retardation in about 40% of the exposed fetuses. It must be taken into account that a radiation exposure during the fetal period can also induce cancer. It is generally assumed that the risk exists at about the same level as for children.