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HYDROFLUOROCARBON-23 36 EXPOSURE GUIDANCE LEVELS The Navy proposes to use the same exposure guidance levels for HFC-23 that were set for chlorofluorocarbons CFC-12 and CFC-114 (1-hr EEGL of 2,000 ppm, 24-hr EEGL of 1,000 ppm, and 90-day CEGL of 100 ppm), but did not provide an adequate rationale for doing this. To evaluate the validity of the proposed guidance levels, the subcommittee reviewed the available toxicity data on HFC-23 to determine what levels would be adequately protective of submariner health. A comparison of those results is presented below. Because the submariner population is all male, young, and healthier than the general population, the subcommittee did not use an uncertainty factor to account for intraspecies differences in its calculations. Submarine Exposure Guidance Levels for HFC-23 Exposure Level NRC's Calculated Levels Navy's Proposed Levels 1-hr EEGL 20,000 ppm 2,000 ppm 24-hr EEGL 5,000 ppm 1,000 ppm 90-day CEGL 500 ppm 100 ppm To evaluate the proposed 1-hr EEGL of 2,000 ppm for HFC-23, the subcommittee considered the human exposure study by Fagan et al. (1995). In that study, eight exposures of 3 min each, interspersed with 2 min of air, did not produce any effects at a concentration of 200,000 ppm, and only one of five subjects was affected at 300,000 ppm. Although the cumulative exposure in the study was 24 min, the subcommittee used the NOAEL of 200,000 ppm without time extrapolation because absorption of hydrofluorocarbons via the inhalation route is rapid, reaching maximal concentrations in the blood within 5 min of exposure and equilibrium within the next 15 min (Azar et al. 1973; Trochimowicz et al. 1974; Mullin et al. 1979). However, it was necessary to account for the uncertainty associated with the discontinuous exposure by dividing the NOAEL by an uncertainty factor of 10 to yield a value of 20,000 ppm. Because that value is 10-fold higher than the 1-hr EEGL of 2,000 ppm proposed by the Navy, the subcommittee concludes that the Navy's value is adequately protective of health. In its evaluation of the Navy's proposed 24-hr EEGL, the subcommittee used a developmental toxicity study in rats (Munley 1997). Although developmental toxicity is not necessarily the most appropriate end point for de
