rebreathing

Valves inside these tight-fitting masks control the flow of gases into and out of the masks, so that rebreathing of exhaled gas is minimised.

A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content of each breath.

Continued rebreathing of the same gas will deplete the oxygen to a level which will no longer support consciousness, and eventually life, so gas containing oxygen must be added to the breathing gas to maintain the required concentration of oxygen.

In some devices certain means were provided for the adsorption of carbon dioxide in exhaled air and the rebreathing of the same air many times; in other cases valves were provided for exhalation of used air.