42 C.F.R. § 84.77 – Inhalation and exhalation valves; minimum requirements.

(a) Inhalation and exhalation valves shall be provided where necessary and protected against damage and distortion.
(b) Exhalation valves shall be—
1. (1) Protected against external influence; and
2. (2) Designed and constructed to prevent inward leakage of contaminated air.