Gas mask is a protective device which protects the wearer from breathing in harmful gases. As the polluted air enters the mask, the harmful gases, smoke, dust or chemicals from the air are removed by adsorption. The commonly used adsorbent — a mixture of activated charcoal with reactive chemicals, caustic soda fused on pumice stone, fused calcium chloride and silica gel can remove most kinds of fumes and toxic gases from air. The exhaled air is expelled through a one-way valve in the mask.