gas mask, face covering or device used to protect the wearer from injurious gases and other noxious materials by filtering and purifying inhaled air. In addition to military use (see chemical warfare), gas masks are employed in mining, in industrial chemistry, and by firemen and rescue squads. The gas mask consists essentially of a face cover with two eyepieces and a mouthpiece that communicates with a canister containing a filter. The filter absorbs noxious gases as they pass through the canister to the mouth. The face cover also has a one-way outlet valve for exhaled air. See poison gas.