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SUMMARY 7 by 1/4 (to account for exposure for 6 hr per day) and by 5/7 (to account for exposure five times per week), resulting in a 90-day CEGL of 900 ppm. In 1996, the COT reviewed the available toxicity data on HFC-134a and proposed a 1-hr EEGL of 4,000 ppm, a 24-hr EEGL of 1,000 ppm, and a 90-day CEGL of 900 ppm. Since that review, additional data on HFC-134 have become available. One of the new studies was an ascending-concentration safety study in humans, in which subjects were exposed to HFC-134a at concentrations up to 8,000 ppm for 1 hr with no adverse effects. The subcommittee believes that, on the basis of that study, a 1-hr EEGL of 8,000 ppm for HFC-134a is justified. For the 24-hr EEGL, the subcommittee used a 13-week toxicity study in rats, in which the highest concentration tested of 50,000 ppm was the NOAEL. Dividing the NOAEL by an uncertainty factor of 10 to account for interspecies variability yielded a 24-hr EEGL of 5,000 ppm. That exposure level is higher than the EEGL of 1,000 ppm recommended by the NRC in 1996. The reason for the difference is that in 1996 the NRC was determining exposure levels for use aboard Navy ships with female crew members and, therefore, based the NOAEL of 10,000 ppm on a developmental study in which fetal toxicity was observed. However, fetal toxicity is not as a relevant an end point for setting an exposure level for use on submarines, which have no female crew members. For the 90-day toxicity study, the subcommittee agreed with the NRC's earlier proposal of 900 ppm. That exposure level was based on a 2-year toxicity study, in which the NOAEL was 50,000 ppm. That value was divided by an uncertainty factor of 10 and then adjusted for the discontinuous exposure regimen used in the study by multiplying it by 1/4 (to account for exposure 6 hr per day) and by 5/7 (to account for exposure five times per week). Exposure Guidance Levels for HFC-404a Using the equation presented earlier and the exposure levels calculated above for the individual components of HFC-404a, the 1-hr EEGL, 24-hr EEGL, and 90-day CEGL for HFC-404a were calculated to be 12,900 ppm, 4,300 ppm, and 800 ppm, respectively.
SUMMARY 8
