OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at https://www.osha.gov.

May 25, 1994



Acting Director
Directorate of Field Programs
Acting Director
Directorate of Technical Support
SUBJECT: Respirator Exhalation Valve Leakage


We have received inquiries from the health care industry concerning respiratory protection against tuberculosis (TB) exposure. Their concern is that the exhalation valve of respirators may permit inward leakage of air contaminants, and they believe that it is safer to wear a valveless fabric type disposable respirator because such respirator does not have a source of valve leakage. Since this issue is related to proper respirator selection for protection against TB, we need to address the validity of this concern.

There are three potential sources of respirator leakage: filter, faceseal and exhalation valve. The exhalation valve acts like a check valve, permitting air to flow out of the respirator facepiece, and preventing reverse flow through the valve on inhalation. All exhalation valves of approved particulate air-purifying respirators must pass a leakage test as prescribed in Subpart K of [42 CFR 84], the MSHA/NIOSH respirator testing and certification regulation. The test requires the following:

[This document was edited on 03/24/99 to strike information that no longer reflects current OSHA policy.]

1. Dry exhalation valves and valve seats will be subjected to a suction of 25 mm water-column height while in a normal operating position.

2. Leakage between the valve and valve seat shall not exceed 30 millimeters per minute.

Since this is a static test, it does not provide an indication of the valve leakage to be expected from actual use. Furthermore, the test does not indicate whether the exhalation valve would prevent the leakage of aerosols.

The issue of exhalation valve leakage has been examined by several investigators. Their studies measured the leakage rate and/or aerosol penetration under dynamic test conditions. Both new and used exhalation valves were evaluated. The general conclusion was that for a properly maintained exhalation valve of an elastomeric facepiece respirator, the inward leakage of the exhalation valve is less than the allowable leakage of the high-efficiency particulate air (HEPA) filter (0.03%).

To ensure that the exhalation valve is working properly, it must be thoroughly inspected for defects before and after each use, and after cleaning or maintenance. The valve must be properly seated and there must be no foreign material between the valve and the valve seat. The valve cover must be attached properly.

In conclusion, the concern that the exhalation valve would permit inward leakage of air contaminates is unfounded. The wearing of a valveless fabric type disposable respirator has no advantage over a respirator equipped with an exhalation valve.

A summary of the exhalation valve studies is attached for your information. Please contact Ching-tsen Bien of my staff at 202-219-7065 if you need additional information.





There are five published studies that examine the exhalation valve leakage. The first study was conducted by Burgess and Anderson of Harvard University in the late sixties. They developed a dynamic test system which used a submicrometer uranine aerosol to measure valve leakage. The test was performed on a breathing machine with an in-line humidifier to simulate the temperature and humidity of exhaled breath. Tests were performed on mushroom, annular, flap and poppet type exhalation valves. The test results indicated that the mushroom type exhalation valve showed the lowest leakage with a range between 0.002 to 0.008%. The annual and poppet type exhalation valves had leakage similar to the mushroom type valves. The flap type exhalation valves had the highest leakage, between 0.05 to 0.07%.

The next two studies were conducted by the Los Alamos Scientific Laboratory (LASL). LASL developed a system which would be able to perform tests for air as well as aerosol leakage, statically and dynamically, and for instantaneous exhalation resistance during each breathing cycle. The test could be performed with ambient or humid exhalation air and with simulated coughing.

Exhalation valves from six manufacturers were tested. The test results indicated that total leakage under dynamic testing was generally small, less than 5 x 10(-7) m(3)/s (30 ml/min), and met the certification requirement. Aerosol penetration was less than 0.01% (the maximum allowable penetration for the HEPA filter is 0.03%). The authors concluded that exhalation valves from approved respirators contributed very little to the total leakage of a respirator.

Bellin and Hinds of the University of California at Los Angeles developed a system to evaluate the effect of the compromised exhalation valve function on a particle size specific aerosol penetration through exhalation valves. Aerosol penetration was measured on the exhalation valve of the MSA Comfo-II half-mask respirator which was sealed to a mannikin. A breathing machine with work rates of 0, 208, 415, and 622 kg-m/min was used to simulate breathing. Aerosol particles having mass median aerodynamic diameter (MMAD) of 0.5, 2.4, and 8 um were selected for testing. In order to simulate valve defects in a controlled fashion, copper wires having different thicknesses were placed in direct contact with the exhalation valve seat. These wires have the same order of magnitude as human hair. Tests were conducted using valves compromised by paint on their exterior surface. Mask performance was measured by the simultaneous measurement of aerosol concentration inside and outside the facepiece.

The test results indicted that the submicrometer aerosol penetration for normal valves was very low. The penetration was 0.01% at the highest work rate of 622 kg-m/min. The aerosol penetration increased by 100-1,000 fold with the introduction of wires. There was a significant increase in the aerosol penetration of valves which had been coated with paint spray. The authors concluded that the test results agreed with the LASL studies that properly functioning exhalation valves allowed for very little inward leakage. Significant leakage can occur if exhalation valves are dirty or deformed, or if fibers or foreign materials became trapped between the valve and the valve seat.

Brueck, Willeke and co-workers of the University of Cincinnati developed a leakage testing system for exhalation valves which can be used in the laboratory as well as in the field. The system consisted of two components, the Respirator Integrity Tester and the Exhalation Valve Tester. The integrity test involves placing the respirator on a soft, pliable medium which simulates the face of a respirator wearer. The test determines the integrity of the respirator by measuring leakage through all sources, including the exhalation valve. If leakage is found to be significant, the Exhalation Valve Tester would distinguish exhalation valve leakage from the potential leak sources.

In this study, exhalation valve leakage was first evaluated in new, unused mushroom- and flap-type exhalation valves and then in exhalation valves from regularly used respirators which were exposed to dusts and chemical vapors. A total of 54 new respirator valves from five brands of respirators and 67 used respirators were tested. Measurements were repeated five to ten times on randomly selected valves for the determination of leakage variation.

Most of the mushroom type exhalation valves of Respirator A that were tested had leak flow rates of less than 10 cc/min. However, the exhalation valve leak rates were much higher for Respirator B with its flap-type exhalation valves with accordion-like folds. Three of the valves showed leak rates of 100 cc/min or higher. All other valve had leak rates between 10 and 80 cc/min. A leak flow rate in excess of 100 cc/min is considered significant and unsatisfactory. There is a tendency for the leak flow rate through the exhalation valve to increase with an increase in negative pressure.

The test results also indicated that after rinsing and drying, the leakage of exhalation valves was reduced by 40 to 70%. Leak flow through exhalation valves in respirators used in the dusty workplace and the chemical industry showed that greater exhalation valve leakage was found for respirators used in the dusty environment. Upon examination, a light coating of dust was found on the exhalation valve of a majority of the respirators received for testing. Two of the 26 Brand C respirators tested showed valve leakage in excess of 100 cc/min. The remaining 24 respirators showed leak flow between 0.1 and 50 cc/min, with most near 1 cc/min. One of the ten Brand D respirators showed a leak rate in excess of 100 cc/min. The rest had a leak rate between 1 and 20 cc/min. A possible explanation is that dust particles deposited between the valve and the valve seat may prevent the sealing of the valve during inhalation.

Exhalation valves from four Brand E respirators used in the chemical industry showed leak rates between 0.6 and 35 cc/min, and the exhalation valves of Brand F Respirators had a leak rate between 15 and 30 cc/min. The valve leakage for the same respirator was reduced by 45 to 91% after its valves were rinsed with cold water and dried. These exhalation valve leakage studies indicate that proper maintenance and inspection of respirators is an important factor in minimizing exhalation valve leakage.

The authors concluded that exhalation valve leakage in most of the new exhalation valves tested with less than 10 cc/min. Some of the respirators used in dusty environments had significant exhalation valve leakage. Leakage through both new and used exhalation valves decreased after the valves were cleaned with water and dried. Foreign debris between the exhalation valve and valve seat may prevent the valve from closing, causing significant leakage.


1. Burgess, WA and Anderson, DE: Performance of Respirator Exhalation Valves. Amer. Ind. Hyg. Assoc. J. 28:216-223 (1967).

2. Los Alamos National Laboratory: Respirator Studies for the National Institute for Occupational Safety and Health, July 1, 1973 through June 30, 1974. LA 5805-PR, Bruce Held, Project Manager, pp 10-15 (December 1974).

3. Los Alamos National Laboratory: Respirator Studies for the National Institute for Occupational Safety and Health, July 1, 1974 through June 30, 1975. LA 6386-PR, Darrel Douglas, Project Manager, pp 18-25 (August 1976).

4. Bellin, P, and Hinds, WC: Aerosol Penetration Through Respirator Exhalation Valves. Amer. Ind. Hyg. Assoc. J. 51:555-560 (1990).

5. Brueck, S, Lehtimake, M, Krishnan, U, and Willeke, K: Method Development for Measuring Respirator Exhalation Valve Leakage. Appl. Occ. Environ. Hyg. 7:174-179 (1992).