42 C.F.R. § 84.120 – Inhalation and exhalation valves; minimum requirements.

(a) Inhalation and exhalation valves shall be provided where necessary and protected against damage and distortion.
(b) Inhalation valves shall be designed and constructed to prevent excessive exhaled air from adversely affecting cartridges, canisters, and filters.
(c) Exhalation valves shall be protected against external influence, and designed and constructed to prevent inward leakage of contaminated air.