The typical gas mask consists of a tight-fitting facepiece that contains filters, an exhalation valve, and transparent eyepieces. It is held to the face by straps and can be worn in association with a protective hood. The filter elements in the cheeks of the mask remove contaminants from the air that is drawn through the mask by the wearer’s inhaling. The filters, which can be replaced, clean the air but do not add oxygen to it (some masks are connected by a hose to a separate tank of oxygen). The most common filters employ fibre screens (to strain out finely divided solid particles) and  chemical compounds such as charcoal (to capture or chemically alter poisonous gases in the air). Charcoal absorbs and holds a fairly large volume of poisonous gases.