Abstract: A self-contained pendulum breathing main unit includes a first canister containing a bed of oxygen generating and carbon dioxide removing chemical, a breathing hose connected with the canister inlet at one side of the bed and a breathing bag connected with an opening to the opposite side of the bed. Tubular connections are associated with the cannister inlet and outlet for connecting a supplemental unit thereto. There also is a normally open valve for the inlet. Attachable to the main unit is a supplemental breathing unit provided with a canister containing the same kind of chemical as the first canister. The supplemental unit has a tubular inlet which closes the normally open valve and connects the second canister with the breathing hose when the two units are attached to each other. The second unit also is provided with a tubular outlet which connects the second canister with said opening in the first canister to put the second canister in communication with the breathing bag.