Gas masks are used to protect users from inhaling pollutants and toxic gases in the air. The mask forms a sealed covering 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, although the term gas mask is often used to refer to military equipment (such as field masks). The user of the gas mask is not protected by the gas absorbed by the skin. In the case of NBC, most respirator filters will last for 24 hours.

Toxic substances in the air may be gaseous (e.g., mustard gas and chlorine) or particulate (e.g., biological agents). Many gas masks include two types of protection. Gas masks are used in buildings to prevent welding fumes, in demolition to prevent asbestos or other harmful particles, and in the use of harmful substances in the chemical industry, such as repairing leaking equipment or cleaning after leakage; gas masks are usually provided for workers to prevent leakage.

During demonstrations and protests, riot police use tear gas or CS gas, and gas masks are commonly used by police and protesters. In addition to its functional use, gas masks are also used as symbols in industrial music. The most famous example is the sub genre of drums and bass, called neurofunk. The signs are used by graffiti journalists because the masks protect them from toxic fumes from paint cans, while urban explorers venture into environments where harmful substances such as asbestos may be present.

The traditional gas mask style with two small round eyelets originated when the only suitable material for these eyelets was glass or acrylic. As the glass is very fragile, the glass eyelet must be kept small and thick. Later, it was found that polycarbonate made the gas mask have a large full open window. Some connect one or two filters to the mask, while others use hoses to connect a larger filter to the mask, which is sometimes confused with respirators