Gas masks are breathing devices designed to protect the wearer against harmful substances in the air. A typical gas mask consists of a tight-fitting facepiece that contains filters, respiratory valves, and transparent eyepieces. It is held on the face with a rope and can be used in an 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 use fibre screens (to remove subtlely divided particles) and chemical compounds such as charcoal (to capture or change chemically-toxic gases in the air). Charcoal absorbs and holds a large volume of toxic gas. You can buy gas masks on https://www.gasmaskpro.com/
Gas masks are widely used by the world armed forces. Although it is possible to design a filtering device that will neutralize almost all specific toxic substances in the air, it is not possible to join in one mask protection against all toxic substances. Military gas masks are mated with a view to ward off chemicals that are considered most likely to use in the war.
The gas mask is effective only against a chemical war agent that is spread as a true and dangerous gas when breathing. Agents such as Mustard gas that spread in liquid form and attack the body through the surface of the skin require the use of special protective clothing in addition to gas masks.