A World War I British P Helmet c. 1915 Indian muleteers and mule wearing<unk> s, France, February 21, 1940 A Polish MUA<unk> , used in the 1970s and 1980s A<unk>  is a mask used to protect the wearer from inhaling airborne pollutants and toxic<unk> es. 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<unk> s are also respirators, though the word<unk>  is often used to refer to military equipment (such as a field protective mask), the scope used in this article. The<unk>  only protects the user from digesting, inhaling, and contact through the eyes (many agents affect through eye contact). Most combined<unk>  filters will last around 8 hours in a biological or chemical situation. Filters against specific chemical agents can last up to 20 hours. Airborne toxic materials may be<unk> eous (for example, chlorine or mustard<unk> , or particulates (such as biological agents). Many filters provide protection from both types. The first<unk> s mostly used circular lenses made of glass, mica or cellulose acetate to allow vision. Glass and mica were quite brittle and needed frequent replacement. The later Triplex lens style (a cellulose acetate lens sandwiched between glass ones) became more popular, and alongside plain cellulose acetate they became the standard into the 1930s. Panoramic lenses were not popular until the 1930s, but there are some examples of those being used even during the war (Austro-Hungarian 15M). Later, stronger polycarbonate came into use. Some masks have one or two compact air filter containers screwed onto inlets, while others have a large air filtration container connected to the<unk>  via a hose that is sometimes confused with an air-supplied respirator in which an alternate supply of fresh air (oxygen tanks) is delivered.
Gas mask
Venturi mask
Oxygen mask
Oxygen tent
N95 respirator
Air purifier
Pocket mask
Nebulizer