A gas mask is a close-fitting cover for part or all of the head, but almost always covers the eyes, nose and mouth. It is designed to filter out poisonous gases from the air.

The reason that mouth, nose and eyes need protecting is that they all have very sensitive moist tissues. Gases are easily absorbed by moisture, where they turn into poisonous liquids. Many people in the First World War suffered blindness from poison gases because their eyes were not covered.

Gas masks are not only used in war, as may commonly be thought. Firemen wear gas masks when they have to deal with a fire where the burning materials give off poisonous fumes.