42 CFR 84.177 - Inhalation and exhalation valves; minimum requirements.
(a) Inhalation and exhalation valves shall be protected against distortion.
(b) Inhalation valves shall be designed and constructed and provided where necessary to prevent excessive exhaled air from adversely affecting filters, except where filters are specifically designed to resist moisture.
(c) Exhalation valves shall be:
(1) Provided where necessary;
(2) Protected against damage and external influence; and
(3) Designed and constructed to prevent inward leakage of contaminated air.