A gas mask is a device to protect the respiratory tract, eyes, and face from noxious aerosols or gases. It is also called a respirator, and, in military terminology, a protective mask. The mask may purify the air passed through it; supply pure air or oxygen from a remote source or a cylinder under pressure; or regenerate exhaled air to provide oxygen from the moisture while carbon dioxide is absorbed.
The types of masks that purify the air generally consist of a facepiece and a canister. The facepiece is of impervious material, held tightly to the face, and includes clear lenses to permit vision. Well-designed facepieces have a nose cup that deflects the incoming air over the inside of the eyepieces to prevent their fogging and reduces the amount of exhaled air available for rebreathing. They also have an exhaust valve through which the exhaled air escapes. Masks designed specifically to protect against dust, paints, or other aerosols not harmful to the eyes may cover only the nose and mouth.
The canister, which purifies the incoming air, may be attached directly to the facepiece or connected to it by a flexible tube. A mask for general purposes, such as the military protective mask, contains a filter to remove particulate matter and a bed of granular material to eliminate dangerous gases. The filter is a pad of fibrous material. Gases are removed by several different actions. The noxious vapors may be retained in the canister by adsorption on charcoal or reaction with a chemical, or they may be converted to harmless gases by oxidants or catalysts deposited on the charcoal. Some materials, such as tear gas, are a mixture of aerosol and vapor, so the filter must either precede the charcoal or be combined with it. In the modern U. S. military mask the canister is eliminated by combining filter and charcoal into a single pad mounted in the facepiece. This mask also has drinking tubes and a means for mouth-to-mouth resuscitation with both persons masked.
It is impossible to design a canister of a reasonable size that will protect against all gases. Military masks protect against expected military agents, which normally consist of heavy molecules. They do not protect against carbon monoxide, ammonia, acid, or organic vapors. They are ineffective in confined spaces where the oxygen content is too low to sustain life, as in a fire-filled room, or where the concentration of agent is too high. Special masks and canisters are required for these situations.
After the first large-scale gas attack in World War I, in which the Germans used chlorine, protective devices made of gauze pads impregnated with a chemical were improvised. Protection then progressed through crude masks having tubes in the mouth and noseclips and heavy canisters, to much more comfortable masks.