Gas Mask, a device that covers the face to protect the wearer from poisonous gases. A canister attached to the mask contains substances that filter out poison gases but permit air to enter. The gas-filtering material in the canister may be activated charcoal, or a mixture of charcoal and chemicals, such as sodium carbonate.

Gas masks were devised during World War I to protect troops against poison gas attacks, but were unnecessary in World War II because gas was not used. Now gas masks are used chiefly for rescue work in fires, mines, and chemical plants. Police wear gas masks when they use tear gas.