42 C.F.R. § 84.300
The closed-circuit escape respirator (CCER), technically a subset of self-contained breathing apparatus (SCBAs) which are otherwise covered under subpart H of this part, is used in certain industrial and other work settings in emergencies to enable users to escape from atmospheres that can be immediately dangerous to life and health. Known in the mining community as self-contained self-rescuers (SCSRs), and in other industries as emergency escape breathing devices (EEBDs) or apparatus (EEBAs), CCERs are relied upon primarily by underground coal miners, sailors in federal service, and railroad workers to escape dangerous atmospheres after a fire, explosion, or chemical release. CCERs are commonly worn on workers' belts or stored in close proximity to be accessible in an emergency. They are relatively small respirators, typically the size of a water canteen, that employ either compressed oxygen with a chemical system for removing exhaled carbon dioxide from the breathing circuit, or a chemical that both provides a source of oxygen and removes exhaled carbon dioxide. Users re-breathe their exhalations after the oxygen and carbon dioxide levels have been restored to suitable levels, which distinguishes these "closed-circuit" self-contained respirators from "open-circuit" self-contained respirators, which vent each exhalation.
42 C.F.R. §84.300