A gas mask is a mask used to protect the wearer from inhaling airborne pollutants and toxic gases. The mask forms a sealed cover over the nose and mouth, but may also cover the eyes and other vulnerable soft tissues of the face. Most gas masks are also respirators, though the word gas mask is often used to refer to military equipment (such as a field protective mask), the scope used in this article. The gas mask only protects the user from digesting, inhaling and contact through the eyes (many agents affect through eye contact). 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. Airborne toxic materials may be gaseous (for example, mustard gas and chlorine gas) or particulates (such as biological agents). Many filters include protection from both types. The first gas masks mostly used circular lenses made either of glass, mica or cellulose acetate. With the exception of the latter, these materials were quite brittle and needed frequent replacement.