Laboratory animal experiments (Lushbaugh and Casarett, 1976; Langham, 1967), and limited human data on effects of protracted or fractionated low-dose exposures on spermatogenesis, indicate that fractionation of dose at low levels may be more effective on spermatogenesis than acute exposure because of the cyclic process of spermatogenesis (Lushbaugh and Casarett, 1976; Kramer et al., 1974). OTHER HEALTH EFFECTS OF RADIATION As stated above, carcinogenesis, mutagenesis, and cataractogenesis are the health effects of principal concern. At low doses of radiation, cancer induction or promotion probably accounts for all excess mortality in experimental animals and man (NAS-BEIR, 1980). However, low doses of radiation may accelerate the appearance time of some non- lethal conditons (e.g., vascular or renal diseases) and thereby contribute to late life morbidity. The impact of such morbidity from non-neoplastic diseases is thought to be small, in comparison with neoplasia, based on life-span studies on mice (Storer et al., 1979; Sacher, 1976). Vascular disease occurs as a consequence of aging in man and experimental animals. The vasculature of the mouse heart appears quite sensitive to high-LET neutron radiation, even at low doses (20 rad), and the expression of vascular disease occurs earlier than in aged controls (Yang et al., 1978; Steamer et al., 1979). Also, the degree of vascular injury is increased, in comparison with the same single dose, by fractionation of fission neutron doses over several months. Vascular sensitivity of man to HZE particles is unknown, but neither vascular injury nor other adverse health effects are considered to be of significance for space workers. HZE Particle Radiation Effects The potential health hazard of HZE particles (stripped atomic nuclei with Z > 2) from galactic space was suggested soon after their discovery. Tobias (1952) predicted that a single HZE particle could cause light sensation in the retina and suggested that a single HZE particle might kill or modify a column of living cells in tissues. Neon and argon particles at energies of about 0.5 GeV/nucleon have been shown by several authors (Worgle, 1980; Malachowski, 1978; A. Nelson, 1980) to produce lesions in various eye structures of the rodent which appear to be track related phenomena as predicted by Tobias. Some workers have unsuccessfully tried to relate neuronal killing in the CNS to such track related events. The significance of this track and track structure related phenomenology to human hazard remains to be evaluated. It appears
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