A number of observations on the late health effects of low-level radiation have been made about which there is reasonably good general agreement. These observations are based primarily on careful evaluation of epidemiological surveys of exposed human populations, on extensive research in laboratory animals, on analysis of dose-response relationships of carcinogenic, teratogenic and genetic effects, and on known mechanisms of cell and tissue injury in vivo and in vitro (NAS-BEIR, 1972, 1977, 1980; UNSCEAR, 1977; NCRP, 1980). 1. Cancer is the most important late somatic effect of low-dose ionizing radiation. Different organs and tissues appear to vary greatly in their relative susceptiblity to cancer-induction by radiation. The most frequently occurring radiation-induced cancers in man include, in decreasing order of susceptibility: the female breast; the thyroid gland; the hematopoietic tissues; the lung; certain organs of the gastrointestinal tract; and bone. There are influences, however, of sex, age at the time of irradiation, and age at the time of expression of the disease. 2. The effects of growth and development in the irradiated embryo and fetus are related to the gestational stage at which exposure occurs. It appears that a threshold level of radiation dose and dose rate may exist below which gross teratogenic effects will not be observed. However, these dose levels would vary greatly depending on the particular developmental abnormality and on the radiation types and qualities. 3. Estimations of the radiation risks of genetically-related ill- health are based essentially entirely on laboratory animal observations, primarily from laboratory mouse experiments, because of the lack of data on exposed human populations. Scientific knowledge of fundamental mechanisms of radiation injury at the genetic level is far more complete than, for example, of mechanisms of radiation carcinogenesis, thereby permitting greater assurance in extrapolating information on genetic mutagenesis from laboratory animal experiments to man. Mutagenic effects are related linearly to radiation dose even at very low levels of exposure at low dose rates. In spite of a thorough understanding of these late health effects in exposed human populations, there is still considerable uncertainty about the potential delayed health effects of low-level radiation. These uncertainties are discussed in the sections that follow.
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