The problem with any kind of face mask, including of course gas masks, is that the wearer’s breath fogs up the lenses very quickly unless some precautions are taken. When a person breathes out, their breath is both warm and contains a high moisture content. When this hits the cold eyepieces of a gas mask this condenses and forms a fog that hinders vision. To counter this World War Two British gas masks came with a number of different anti-dimming kits