Half face respirators cover half of the face, from nose to under the chin. They are the most common respirators. The filters and/or cartridges remove aerosols, vapors, gases, or a combination of these contaminants. The face pieces are usually made of rubber, silicone, or neoprene. Additional resistance from the filters/cartridges as well as the inhalation/exhalation valves increase breathing work with a half face respirator. As with full face pieces, half face respirators operate by the worker’s respiratory effort drawing workplace air through filters and cartridges to purify the air. Any break in the respirator’s facial seal causes the surrounding atmosphere-- including any contaminants, to flow into the respirator face piece, where it is inhaled by the wearer. Therefore, protective or corrective eyewear worn with half masks must not interfere with respirator fit, and must be fit tested with the respirator to ensure proper fit.
Reference:
Navy and Marine Corps Public Health Center