42 CFR 84.200 - Inhalation and exhalation valves; minimum requirements.
(a) Inhalation and exhalation valves shall be provided where necessary and protected against distortion.
(b) Inhalation valves shall be designed and constructed to prevent excessive exhaled air from entering cartridges or adversely affecting canisters.
(c) Exhalation valves shall be -
(1) Protected against damage and external influence; and
(2) Designed and constructed to prevent inward leakage of contaminated air.