total dose equivalent in about 1 O-percent of missions of a 90-day duration. Early health effects of radiation (those occurring within hours, days, or a few weeks of exposure) assume clinical significance with whole-body doses only in excess of about 150 rem. Such exposures are likely to be encountered rarely if at all in close-in space missions. A potential increase in the risk of cancer is the principal and most serious late effect of exposure to ionizing radiation. Radiation causes an increase in the cancer risk at doses greater than 50-100 rem. At lower doses, it is difficult or impossible to demonstrate an increased risk even in large, exposed populations. The low-LET radiation dose expected will be below the threshold for radiation- induced cataracts and the probability of any serious risk of cataracts from HZE particles seems low. Other effects considered are genetic and teratogenic effects, life-span shortening, and effects on fertility and the skin. The probability of an individual of having a radiation-induced cancer will depend on many factors, included would be the total lifetime dose-equivalent, dose rate, duration of exposure, and the age, sex, and host susceptibility. The majority of the lifetime dose-equivalent will be from the space radiation. Therefore for this consideration, the dose per mission is not as important as the total career dose. The dose per mission would be expected to be a constraint on the total number of missions allowable for a space worker. If a worker spends five years in space, protected by the shielding of the reference system, it is anticipated that the potential risk of having cancer may be increased about 12 percent. The health effects of ionizing radiations must be considered in the context of the potential health effects of other physical and chemical agents in the space environment. Such competing effects may interact to mask, enhance or diminish the induction of health effects which may occur under exposure to low-level radiation delivered at a slow rate.
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