42 CFR §84.91

1. (a)Resistance to exhalation airflow will be measured in the facepiece or mouthpiece of open-circuit apparatus with air flowing at a continuous rate of 85 liters per minute.
2. (b)The exhalation resistance of demand apparatus shall not exceed 25 mm. (1 inch) water-column height.
3. (c)The exhalation resistance of pressure-demand apparatus shall not exceed the static pressure in the facepiece by more than 51 mm. (2 inches) water-column height.
4. (d)The static pressure (at zero flow) in the facepiece shall not exceed 38 mm. (1.5 inches) water-column height.
5. (e)Resistance to exhalation airflow will be measured in the facepiece or mouthpiece of closed-circuit apparatus with a breathing machine as described in § 84.88, and the exhalation resistance shall not exceed 51 mm. (2 inches) water-column height.