Abstract: A method and system for closed-circuit combined unit respiration, is disclosed. More specifically, an enriched air, multi-use closed-circuit self-contained breathing apparatus for on-demand transition between a powered air purifying respirator (PAPR) mode and a closed-circuit rebreather mode, is disclosed. The system may comprise a carbon dioxide scrubbing canister configured to receive an exhaled gas and scrub carbon dioxide from the exhaled gas when in a closed-circuit mode. A breathing bag may receive the scrubbed gas when in the closed-circuit mode. The scrubbed gas may be infused with oxygen, resulting in an enriched gas when in the closed-circuit mode. A cooler comprising a plurality of heatsink fins may cool the enriched gas and a breathing hose may transmit the enriched gas for inhalation when in the closed-circuit mode. The system may further comprise a PAPR to intake and filter external air for inhalation when in the PAPR mode.