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SUMMARY 6 known, the exposure guidance levels (EGLs) can be determined using the following equation: where f is the fraction of each particular component. The component's corresponding EGL is expressed in units of milligrams per cubic meter (mg/ m3). To use this equation, it was necessary to calculate the exposure guidance levels for the individual components of HFC-404a. Exposure Guidance Levels for Components of HFC-404a The three halocarbons that comprise HFC-404a are HFC-143a, HFC-125, and HFC-134a. For component HFC-143a, the subcommittee estimated a 1-hr EEGL of 25,000 ppm on the basis of a cardiac sensitization study in dogs. The NOAEL for the study was 250,000 ppm, and an uncertainty factor of 10 was applied to account for interspecies variability. For the 24-hr EEGL, a 4-week toxicity study in rats was used; the highest tested concentration of 40,000 ppm was the NOAEL. To extrapolate from animals to humans, the NOAEL was divided by an uncertainty factor of 10, yielding a 24-hr EEGL of 4,000 ppm. A 90-day toxicity study in rats was used to calculate the 90-day CEGL for HFC-143a. The NOAEL for the study was 40,000 ppm, which was divided by 10 to account for interspecies variability; the resulting value of 4,000 ppm was adjusted to account for the study's discontinuous exposure regimen by multiplying it by 1/4 (to account for exposure for 6 hr per day) and by 5/7 (to account for exposure five times per week), which yielded a 90-day CEGL of 700 ppm. For component HFC-125, a cardiac sensitization study in dogs was used to derive a 1-hr EEGL. The NOAEL for the study was 75,000 ppm, and an uncertainty factor of 10 was applied to account for interspecies variability, yielding a 1-hr EEGL of 7,500 ppm. The subcommittee calculated a 24-hr EEGL of 5,000 ppm on the basis of a 4-week toxicity study in rats. The highest tested concentration of 50,000 ppm was the NOAEL, and that value was divided by an uncertainty factor of 10 to account for interspecies differences. For the 90-day CEGL, a 90-day toxicity study in rats was used. The NOAEL for that study was 50,000 ppm, and an uncertainty factor of 10 was applied for to account for interspecies differences. To account for the discontinuous exposure regimen used in the study, 5,000 ppm was multiplied
