range causes severe bone marrow depletion leading to symptoms related primarily to a decrease of circulating neutrophils and platelets in the blood. This results in the bone-marrow syndrome. The signs and symptoms are infection in a variety of body tissues due to neutropenia, severe bleeding in tissues and organs due to thrombocytopenia, and possibly death within 20 to 40 days after exposure. Effective treatment consists of neutrophil transfusions, protection of the individual against infection, and fresh platelet transfusions to control bleeding. Replacement of hematopoeitic stem cells by bone marrow transfusions from compatible donors may also provide effective therapy after exposure to doses where bone-marrow damage is a lethal threat, (Bond et. al., 1965; Mathe and Schwarzenberg, 1979). LATE EFFECTS Consideration here is given primarily to those delayed or late health effects in human beings following exposure to low-LET radiation, x-rays, and gamma rays from radioactive sources since these are the ionizing radiations most often encountered on Earth (e.g., in the nuclear industry and in medicine) and about which we have sufficient human data. Little consideration is given to high—LET neutrons and alpha particles. Briefly, low-level radiation can affect the cells and tissues of the body in three important ways. First, if the lesion occurs in one or a few cells, such as those of the hematopoietic tissues, the irradiated cell can occasionally transform into a cancer cell, and after a period of time, there is an increased risk of cancer developing in the exposed individual (carcinogenesis). Second, if the embryo or fetus is exposed during gestation, injury can occur to the proliferating and differentiating cells and tissues, leading to abnormal growth and development (teratogenesis). Third, if the lesion occurs in the reproductive cells of the testis or the ovary, the genome of the germ cell can be altered, and the injury can be expressed in the descendants of the exposed individual as genetically-related ill health. There are a number of other biological effects of ionizing radiation, such as cataracts of the lens of the eye, or impairment of fertility, but the three important late effects—carcinogenesis, mutagenesis and teratogenesis—stand out as those of greatest concern. A considerable amount of information is available from epidemiological studies of exposed human populations and from laboratory animal experiments. Furthermore, we believe that exposure to ionizing radiations, even at very low levels of dose, carries some risk of such deleterious effects. As the dose of radiation increases above very low levels, the risk of these deleterious health effects increases in exposed human populations. Because some risk is present at any level of exposure, the exposure must be kept to a minimum commensurate with accomplishing the mission.
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