CFR /  Title 42  /  Part 84  /  Sec. 84.77 Inhalation and exhalation valves; minimum

(a) Inhalation and exhalation valves shall be provided where necessary and protected against damage and distortion.

(b) Exhalation valves shall be--

(1) Protected against external influence; and

(2) Designed and constructed to prevent inward leakage of contaminated air.