A gas mask is a kind of mask that fits over the face area and nose. It protects from poison gases and many other hazards in the air. During World War II gas masks were sent all over England. They were worn everywhere, even to sleep, in case of surprise attack. However, the Germans didn't attack with gas bombs. Gas masks have a filter, a type of material which lets good air in.
as you can see gas masks were used during WWII but they are still used in today's wars.