42 CFR 84.90 - Breathing resistance test; inhalation.
(b) The inhalation resistance of open-circuit apparatus shall not exceed 32 mm. (1.25 inch) water-column height (at a flow rate of 120 liters per minute).
(c) The inhalation resistance of closed-circuit apparatus shall not exceed the difference between exhalation resistance ( § 84.91(e)) and 10 cm. (4 inches) water-column height.