The gas mask is a mask used to protect the wearer from inhaling air pollutants and toxic gases. The mask forms a sealed cover on the nose and mouth, but can also close the eyes and other vulnerable soft tissues of the face. Most gas masks are also respirators, though the word gas masking is often used to refer to military equipment.
The gas mask only protects the user from digesting, inhaling and contact through the eyes. Most combined gas mask filters will last around 8 hours in a nuclear biological chemical (NBC) situation. Chemical-specific filters can last up to 20 hours in an NBC situation.
Toxic material in the air can be gas or particles. Many filters include protection from both types. The first gas mask is mostly used circular lenses made from glass, mica or cellulose acetate. With the last exception, these ingredients are quite fragile and frequently need replacement.
Then, the triplex lens style i.e. two layers of glass and one layer of cellulose acetate became more popular. Modern gas masks are quite safe and do not use asbestos, but it is still important to be careful when using a modern gas mask. Gas masks have a limited useful lifespan.