A gas mask is a mask worn on the face to protect the body from airborne pollutants and toxins. The mask may cover the eyes and other vulnerable soft tissues of the face, and will always form a sealed cover over the nose and mouth.
Toxins may be gaseous (for example the chlorine used in WWI), or particulate (such as many biological agents developed for weapons). Many gas masks include protection from both types of toxin. The advantage of a gas mask over other breathing devices is that it does not require the user to carry an air supply (as in the use of scuba gear). However, this means that the user is dependent on the air in the atmosphere, the very medium in which toxins may be present. Thus, the mask must remove the toxins, and relay cleaned air to the user.
There are three main ways of achieving this: filtration, absorption and adsorption, and reaction and exchange.