42 CFR 84.137 - Inhalation and exhalation valves; check valves; minimum requirements.
(a) Inhalation and exhalation valves shall be provided where necessary and protected against distortion.
(b) Exhalation valves shall be:
(1) Protected against damage and external influence; and
(2) Designed and constructed to prevent inward leakage of contaminated air.
(c) Check valves designed and constructed to allow airflow toward the facepiece only shall be provided in the connections to the facepiece or in the hose fitting near the facepiece of all Type A, AE, B, and BE supplied-air respirators.