The Momsen lung contains a canister of soda lime, which removes poisonous carbon dioxide from the exhaled air and then replenishes the air with oxygen. Two tubes lead from the bag to a mouthpiece: one with which to inhale air and the other to with which to exhale spent air. The device hangs around the wearer's neck and is strapped around the waist. Besides providing oxygen for the ascent, it also allows a submariner to rise slowly to the surface, thus avoiding embolisms.