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SUMMARY 1 Summary I N ACCORDANCE WITH the Montreal Protocol on Substances That Deplete the Ozone Layer, which calls for phasing out the use of chlorofluorocarbons (CFCs), the U.S. Navy proposes to replace CFC refrigerants aboard its submarines with hydrofluorocarbons HFC-236fa and HFC-404a. To protect submariners from adverse health effects resulting from exposure to accidental releases of those compounds, the Navy plans to set emergency exposure guidance levels (EEGLs) and continuous exposure guidance levels (CEGLs) for them. An EEGL is defined as a concentration of a substance in air that is judged to be acceptable for the performance of specific tasks during rare emergency conditions lasting for periods of 1-24 hr. EEGLs are intended to prevent irreversible harm and degradation in crew performance. Temporary discomfort, such as eye or upper-respiratory-tract irritation, is permissible as long as there is no effect on judgment, performance, or ability to respond to an emergency. To protect submariners from exposures that are longer than 24 hr, CEGLs are set to provide a ceiling guidance level for up to 90 days of continuous exposure to a substance. The intent of a CEGL is to avoid any adverse health effects, either immediate or delayed, associated with prolonged exposures and to avoid any degradation in performance. Some conditions, such as a slight headache, which might be acceptable for short periods in emergency situations, are not permissible for longer-term exposures. The Navy proposes to use the same exposure guidance levels for HFC-