GENETICALLY RELATED ILL-HEALTH At low-level radiation exposure the effects of radiation include genetic disorders which depend on changes in individual genes (gene mutations) and in chromosomes (either in total number or in gene arrangement, or chromosomal aberrations). Low-level ionizing radiation causes genetic disorders (genetic mutations) in laboratory animals, and it is assumed that the same applies in man. These effects may be manifested in man as genetically related ill-health (e.g., mental retardation in descendants of irradiated parents). Gene mutations are expected to have greater health consequences than changes in chromosomes. The genetic effects arising from either are proportional to dose. In estimating genetic disorders caused by gene mutations, the BEIR III Report (NAS-BEIR, 1980) considered two exposure situations: continuous exposure over the lifetime of an individual and single exposure of the parents only. It is estimated that since genetic effects produced are proportional to radiation dose, only about one percent to six percent of all spontaneous mutations in humans are due to natural background radiation. The incidence of human genetic disorders in the absence of any additional radiation exposure is about 107,000 cases per million (about 10.7 percent) liveborn offspring (NAS-BEIR, 1980). A small increase in radiation exposure above background will lead only to a correspondingly small relative increase in the mutation rate. The doubling dose equivalent, i.e., the radiation dose equivalent required to produce as many more mutations as are already occurring spontaneously, is estimated to be 50 to 250 rem. The lower the doubling dose equivalent, the greater is the genetic risk from a given exposure. It is estimated that at equilibrium the increase in human genetic disorders in the population could be about 60 to 1,100 per million liveborn offspring per rem of parental exposure received each generation before conception. An average parental exposure of one rem before conception is expected to produce 5 to 65 additional genetic disorders per million live-born offspring in the first generation. These estimates are taken from the BEIR III Report (NAS-BEIR, 1980). Since the risk is conservatively taken to be linear at the dose levels of concern, the risk can be scaled readily for any dose equivalent. DEVELOPMENTAL ABNORMALITIES IN THE NEWBORN Among the somatic effects of low-level ionizing radiation other than cancer-induction, developmental abnormalities in the newborn are of greatest concern (NAS-BEIR, 1980). Exposure to high doses (hundreds of rads) of the embryo or fetus during gestation causes death, developmental abnormalities, retardation of growth, and functional impairment. There appears to be a threshold level for the induction of gross malformations in the newborn depending on the developmental stage
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