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INTRODUCTION 10 STATEMENT OF TASK The National Research Council (NRC) was asked to conduct an independent evaluation of the Navy's proposed exposure guidance levels for HFC-236fa, HFC-23, and HFC-404a. The NRC assigned this task to the Committee on Toxicology (COT), which convened the Subcommittee on Exposure Guidance Levels for Selected Hydrofluorocarbons, a multidisciplinary group of experts, to review (1) the Navy's toxicity assessments of the three HFCs, (2) other data relevant to establishing EEGLs and CEGLs for these substances, and (3) the scientific validity of the Navy's proposed EEGLs and CEGLs, based on the relevance of existing animal studies for evaluating the health risks to humans, the completeness of the data base, the target organs of toxicity, and the appropriateness of the methods used to derive the exposure guidance levels (e.g., correctly adjusting for exposure durations and the use of uncertainty factors). The subcommittee was asked to review the three HFCs only in the context of use aboard submarinesâvessels with male personnel only. The subcommittee was also asked to identify any deficiencies in the data bases on each HFC and to make recommendations for future research. Although products other than HFC-23 are formed during combustion of HFC-236fa or HFC-404a, HFC-23 was the only combustion product the subcommittee was asked to consider. APPROACH TO THE STUDY The subcommittee conducted a critical analysis of the available toxicity data on each of the HFCs and used the data to calculate possible EEGLs and CEGLs according to the guidelines outlined in the NRC's report Criteria and Methods of Preparing Emergency Exposure Guidance Level (EEGL), Short- Term Public Emergency Guidance Level (SPEGL), and Continuous Exposure Guidance Level (CEGL) Documents (NRC 1986). The subcommittee also reviewed the Navy's toxicity assessments of the HFCs and used information provided in a 1996 report of the NRC titled Toxicity of Alternatives to Chlorofluorocarbons: HFC-134a and HCFC-123 (NRC 1996), which provides a toxicity assessment and establishes EEGLs and CEGLs for HFC-134a, a component of HFC-404a. EXPOSURE GUIDANCE LEVELS Criteria and methods used to determine EEGLs and CEGLs are detailed in