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REVIEW OF ACUTE HUMAN-TOXICITY ESTIMATES FOR GF 45 INHALATION VAPOR EXPOSURE Lethal Effects (LCt50) CDEPAT's proposed LCt50 estimate for inhalation exposure to GF vapor is 35 mg-min/m3, assuming exposure durations of 2 to 10 min, moderate temperatures, and minute volumes of 15 liters. There is no existing LCt50 estimate (CDEPAT 1994). No data are available concerning the toxicity of GF in humans following inhalation exposures, and the animal data are insufficient for deriving an LCt50 estimate (CDEPAT 1994). LCt50 studies were conducted in rats and monkeys. The LCt50s for male and female Wistar rats exposed for 1 min were 181 mg-min/m3 and 110 mg-min/m3, respectively (Callaway and Blackburn 1954). In a study using 44 rhesus monkeys, the LCt50 was determined for 2-min and 10-min durations. The reported LCt50s for GF were 130 mg-min/M 3 and 75 mg-min/m3 for 2-min and 10-min exposures, respectively (Cresthull et al. 1957). These studies are considered inadequate for deriving human LCt50 estimate. The proposed estimate of 35 mg-min/m3 is based on the assumption that GF is as potent as GB or GD (CDEPAT 1994). That assumption is supported by animal studies (Cresthull et al. 1957) that show that GF, GB, and GD have equal potencies for this effect via inhalation vapor exposures (CDEPAT 1994). The subcommittee recommends that the proposed LCt 50 estimate for GF be lowered to correspond to that recommended for GB. The subcommittee also recommends that further research on GF be conducted to establish this estimate with a greater degree of confidence. ECt50 for Severe Effects CDEPAT's proposed ECt50 estimate for severe effects from inhalation exposure to GF is 25 mg-min/m3, assuming exposure durations of 2 to 10 min, moderate temperatures, and minute volumes of 15 liters. There is no existing ECt50 estimate (CDEPAT 1994). No data are available on severe effects in humans, and the studies on severe effects following inhalation exposures in animals are inadequate. In the absence of data in humans and animals, CDEPAT's proposed estimate of 25 mg-min/m3 is based on the assumption that GF is as potent