42 CFR § 84.91 - Breathing resistance test; exhalation.

---
identifier: "/us/cfr/t42/s84.91"
source: "ecfr"
legal_status: "authoritative_unofficial"
title: "42 CFR § 84.91 - Breathing resistance test; exhalation."
title_number: 42
title_name: "Public Health"
section_number: "84.91"
section_name: "Breathing resistance test; exhalation."
chapter_name: "PUBLIC HEALTH SERVICE, DEPARTMENT OF HEALTH AND HUMAN SERVICES"
subchapter_number: "G"
subchapter_name: "OCCUPATIONAL SAFETY AND HEALTH RESEARCH AND RELATED ACTIVITIES"
part_number: "84"
part_name: "APPROVAL OF RESPIRATORY PROTECTIVE DEVICES"
positive_law: false
currency: "2026-04-05"
last_updated: "2026-04-05"
format_version: "1.1.0"
generator: "[email protected]"
authority: "29 U.S.C. 651  30 U.S.C. 3, 5, 7, 811, 842(h), 844."
regulatory_source: "60 FR 30355, June 8, 1995, unless otherwise noted."
cfr_part: "84"
---


# 84.91 Breathing resistance test; exhalation.

(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.

(b) The exhalation resistance of demand apparatus shall not exceed 25 mm. (1 inch) water-column height.

(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.

(d) The static pressure (at zero flow) in the facepiece shall not exceed 38 mm. (1.5 inches) water-column height.

(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.