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SUMMARY 5 relatively nontoxic in laboratory studies and because similar HFCs, such as HFC-23 and HFC-134a, have been shown to have low toxicity in humans, the subcommittee recommends that tests be conducted with humans to determine whether HFC-236fa affects performance skills, such as motor coordination and alertness. HFC-23 The subcommittee believes that data on HFC-23 support a 1-hr EEGL of 20,000 ppm, a 24-hr EEGL of 5,000 ppm, and a 90-day CEGL of 500 ppm. The basis for the 1-hr EEGL was a human exposure study in which subjects were intermittently exposed to HFC-23 (eight exposures of 3 min each with 2-min intervals of exposure to air only). The NOAEL for the study was 200,000 ppm. To account for the discontinuous exposure, the NOAEL was divided by a factor of 10, resulting in a 1-hr EEGL of 20,000 ppm. For the 24-hr EEGL, a developmental toxicity study was used. Although such a developmental study is not necessarily the most appropriate study for deriving a 24-hr exposure guidance level, when considering the all-male population aboard submarines, that study had the most relevant exposure duration (a total of 90 hr), and no maternal or fetal effects were observed at the highest dose tested of 50,000 ppm. The NOAEL was divided by a factor of 10 to account for interspecies differences, resulting in an exposure guidance level of 5,000 ppm. The basis for the 90-day CEGL of 500 ppm was a 90-day continuous exposure study in dogs, in which the NOAEL was 5,000 ppm. That value was divided by an uncertainty factor of 10 to account for interspecies variability. HFC-404A HFC-404a is a gaseous mixture of three halocarbonsâ52% HFC-143a, 44% HFC-125, and 4% HFC-134a. The subcommittee believes that the most appropriate way to calculate exposure guidance levels for HFC-404a is the method used by the American Conference of Governmental Industrial Hygienists to calculate threshold limit values for special cases when the exposure of concern is a liquid mixture and the atmospheric composition is assumed to be similar to that of the original material (i.e., on a time-weighted- average exposure basis, all the liquid mixture eventually evaporates). In this case, when the percent composition by weight of the liquid mixture is