A gas mask is a mask worn over the face to protect the wearer from inhaling airborne pollutants and toxic materials. 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. Some gas masks are also respirators, though the word gas mask is often used to refer to military equipment (e.g. Field Protective Mask, etc.) Airborne toxic materials may be gaseous (for example the chlorine gas used in World War I) or particulate (such as many biological agents developed for weapons such as bacteria, viruses and toxins). Many gas masks include protection from both types. Aside from serving their functional purposes, gas masks are also used as accessories in heavy metal music; (such an example would be members of Slipknot; or the character in the Frgt/10 video of Linkin Park). Another band that uses gas masks is the Norwegian Kaizers Orchestra. The gas mask is also the logo for the Los Angeles based underground hip-hop/rap group, Psycho Realm. Band gas masks are also used by graffiti taggers, due to the fact that the mask protects them from the graffiti canister's noxious fumes; in addition to industrial music fashion and imagery. Also closely related is the eroticization of gas masks as a sexual fetish. During riots where teargas or CS-gas is employed by riot police, gasmask are commonly used by both police and rioters alike.