1965; Batemann and Snead, 1969). Current research with rabbits exposed to single doses of various heavy charged particles or photons should provide additional information for risk estimation (Lett et al., 1980). FERTILITY Female. Sensitivity to radiation-induced sterility varies with age (NAS- BEIR, 1980); women under 40 years require larger doses to induce menopause than women over 40 years. Doses below 100 rad are likely to have no effect on fertility or may produce transient sterility for a few months. Doses on the order of 170 rad can result in temporary sterility for from one to several years. However, a small percentage of women may be permanently sterilized by doses as low as 125 rad. Doses of 200 to 650 rad are required to sterilize five percent of women, for more than five years. Doses of 625 to 2000 rad or more are required to sterilize 50 percent of women (Lushbaugh and Casarett, 1976). Protraction or fractionation of dose reduces the injury to the ovary (UNSCEAR, 1962). Fractionated doses greater than 2000 rad does not always produce sterility (Baker, 1971). Male. The seminiferous epithelium is among the most radiosensitive tissues in the adult. An acute dose of 15 rad will cause a significant decrease of the sperm count in about 40 percent of normal men within approximately two months (Langham, 1967; Paulson, 1973). A dose of 30 to 50 rad results in aspermia and temporary sterility or infertility from a low sperm count. At this dose level, all sperm counts will return to normal after 9 to 19 months (Paulson, 1973). Doses up to 400 rad cause temporary sterility and/or infertility lasting up to 30 months (Paulson, 1973; Rowley et al., 1974). With testis exposures up to 600 rad, sterility followed by infertility may last for a period of five years or more (Rowley et al., 1974); but recovery may occur without serious physiological alterations. Human data suggest that long periods of exposure to low dose rates can cause infertility. Men receiving radiotherapy for Hodgkin's disease and unavoidably receiving a daily dose of 10 to 15 rad to the testes (total of 140 to 300 rad) became sterile with no evidence of recovery for up to 40 months (Speiser et al., 1973). In occupationally exposed Roumanian workers, there was about 35 to 75 percent incidence of infertility (Popescu and Lancranjan, 1975) for periods of occupational exposure ranging from 2 to 22 years at 0.5 to 9.3 rad per year (0.01 to 0.2 rad per week). Thus, infertility could result with 90-day mission dose equivalents of 40 rem repeated over several years duration.
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