Full face respirators provide the most reliable fit and protection, as they enclose the entire face, including the eyes. They also contain the greatest amount of dead air space, increasing the required breathing effort. This respirator provides protection from aerosols, vapors, gases, or a combination of these contaminants with its filters and/or cartridges. During inhalation, air is drawn in through the air-purifying filters/cartridges. Additional resistance from the filters/cartridges as well as the inhalation/exhalation valves increase breathing work with a full face respirator. If the respirator facial seal is not intact (or perfectly fitted), the surrounding air-- including any contaminants, can flow into the respirator face piece where it is inhaled by the wearer. Because of this, ordinary corrective glasses cannot be worn with a full face respirator. Instead, NIOSH requires every manufacturer of approved full face piece respirators to provide spectacle kits, which mount inside the mask for vision correction. The elastomeric face pieces are usually made of rubber, silicone, or neoprene.
Reference:
Navy and Marine Corps Public Health Center