Respirator Exhalation Valve Research
Some types of respirators include an exhalation valve that opens to allow exhaled air to escape through the valve and closes to force inhaled air through the filter. In filtering facepiece respirators (FFRs) and elastomeric half mask respirators (EHMRs), exhalation valves typically include a membrane composed of natural rubber, silicone, or neoprene. This membrane sits atop a support structure and lies beneath a plastic cover.
Respirators with exhalation valves are thought to provide more comfort and may be better suited for longer periods of use. The valve opens and closes based on the wearer’s breathing pattern. During inhalations, the membrane closes against the support structure and blocks the opening, thereby not allowing airflow through the valve opening and thus protecting the wearer. During exhalations, when sufficient positive air pressure is achieved, the membrane lifts and the wearer’s unfiltered breath exhausts (exits) from the valve, with only a portion of the exhaled breath from the wearer passing back through the filter media.
Because an exhalation valve can introduce unfiltered exhaled air into the surroundings, the CDC does not recommend the use of a respirator with an exhalation valve in certain healthcare situations, including but not limited to operating rooms, because the valve may allow contaminants to escape and reach others.
To examine the related knowledge gaps and critical research needs, NIOSH is conducting research to determine the potential for respirators with exhalation valves to contribute to source control—i.e., their ability to filter respiratory secretions to prevent disease transmission to others—as described below.
Source Control: Considerations for Respirators with Exhalation Valves
During the COVID-19 pandemic, medical professionals have expressed concern that healthcare personnel infected with SARS-CoV-2, the virus that causes COVID-19, may spread the disease from unfiltered exhaled air passing through their respirator’s exhalation valve. While preliminary data suggests that the exhaust of particles through respirators with valves is less than or comparable to that of masks without valves (e.g., cloth masks, procedure masks), research gaps remain.
Importantly, the potential for infection from breath exhausted through the respirator’s exhalation valve has not been fully studied. Therefore, NIOSH research is evaluating several areas associated with respiratory disease transmission.
Research Focus Areas
The use of EHMRs, FFRs, other NIOSH-approved respirators, and unregulated masks with exhalation valves as source control needs to be evaluated. Research is needed to understand respiratory droplet size and composition, transmission through the exhalation valve, changes in composition and viability in ambient air, deposition within the human body, and infectious dose for different types of cells.
To investigate these topics, NIOSH has a research portfolio to address respiratory droplet generation, emission through respirators and masks with and without exhalation valves, and transmission through the air. Key research questions currently being studied by NIOSH intramurally and through contracts are detailed below.
- What is the size and composition of human respiratory droplet emissions during various activities (e.g., talking, singing, coughing, and sneezing) and with different kinds of protective masks?
- How do human respiratory droplet emissions compare to mechanically generated aerosols that are typically used in scientific studies?
- What percentage of exhaled particles flow through an FFR exhalation valve (i.e., “outward” filtration efficiency)?
- Which respirator mitigation strategies improve outward filtration efficiency?
- What percentage of exhaled particles flow through the exhalation valve (i.e., exhalation efficiency) for sinusoidal breathing patterns?
- Which size particles do not pass through the exhalation valve, and how does the size distribution change over distance with different respirators and respirator modification strategies?
- How does respirator fit affect exhalation efficiency?
- How effective is filtering exhalation exhaust from EHMRs and how do filters affect breathing with respect to exhalation resistance and CO2 and O2 concentrations?
- How are EHMRs being used in healthcare settings?
- After passing through an exhalation valve, what is the fate of dry particles and droplets with respect to distance and environmental conditions?
Particulate Filter Efficiency of Filtering Facepiece Respirators with Exhalation Valves Under Reverse Airflow Condition
Timeline: 2 months (8/1/2020 to 9/30/2020)
Summary: NIOSH evaluated 13 models of FFRs in two positions: an inward position that corresponds to the direction of inhalation and an outward position that corresponds to the direction of exhalation. Three modifications that inhibit particle penetration by covering the exhalation valve were evaluated: taped (with surgical tape), covered (with an electrocardiogram [ECG] pad), and masked-over (with a surgical mask). These modifications were tested in the outward position in order to compare to the particle penetration through the exhalation valve with no modification.
Learn more in the technical report, Filtering Facepiece Respirators with an Exhalation Valve: Measurements of Filtration Efficiency to Evaluate Their Potential for Source Control.
Evaluation of Elastomeric Half Mask Respirators with Filtered Exhalation
Summary: NIOSH selected four EHMR models for this study based on prevalence in healthcare and use in previous NIOSH healthcare studies. An iterative design approach will be used to redesign the filter housing adapter to minimize any changes in exhalation resistance or CO2 and O2 concentrations inside the EHMRs. Performance changes in the EHMRs that are different from the approval requirements set by NIOSH in TEB-APR-STP-003 would be reported. The final filter adapter housing will be assessed with the suitable exhalation filter media in place and inhalation filters in place to determine the exhalation resistance and CO2 and O2 concentrations inside the EHMRs that would be expected when in use. The resulting filtering adapters will then be subjected to further evaluations to determine their efficacy for use in healthcare settings.
Study of Face Mask and Respirator Exhalation Filtration Efficiency and Exhaled Air Velocity During Wear
Partners: West Virginia University
Timeline: 2 years (9/1/2020 to 8/31/2022)
Summary: The overall objective of this project is to assess the source control characteristics of face masks worn by the public and NIOSH-approved negative-pressure half mask respirators worn by healthcare workers and first responders. NIOSH proposes to evaluate the “exhalation efficiency” of these devices.
Laboratory and Human Subjects Evaluation of Exhaled Aerosols from Respiratory Protection
Partners: University of Utah
Timeline: 1 year (9/30/2020 to 9/29/2021)
Summary: Aerosol emission through respiratory protective devices will be tested in a low-speed wind tunnel. A “source” mannequin will be placed upstream in the wind tunnel and will release simulated respiratory aerosol with and without respiratory protective devices. The source mannequin will generate aerosols, which may be composed of both dry particles and droplets (wet particles), and these aerosols will be measured in the air, on the floor, and by a breathing “receptor” mannequin placed 1 m downstream. Monodisperse charge-neutralized fluorescein aerosols will be generated (FMAG 1250, TSI, Inc.) and delivered through the headform using compressed air. The air velocity will be modulated to attain the target velocity (11 m/s for coughing, 4 m/s for speaking, 0.2 m/s for breathing). An electronic controller will permit continuous or episodic aerosol emission (e.g., breathing, talking, or coughing). Surgical masks will be fitted loosely to the headform, as normally worn, and powered air-purifying respirator (PAPR) hoods will be fitted over the headform. N95 FFRs and elastomeric respirators will be fitted snugly to the headform to achieve a fit factor of 100 or more. The fit factor will be measured during 30 s of normal breathing and 30 s of talking (e.g., with two emission velocities) using a PortaCount Pro+ (TSI, Inc.). Fit factors will be recorded during all experiments in case there is a need to control for this factor in the data analysis. A subset of experiments will establish the “worst-case” emission through the exhalation valve by fully adhering the respirator to the headform to eliminate the potential for air to exit the facepiece through the face seal.
Best Practices and Preferred Uses of Reusable Elastomeric Half Mask Respirators (EHMR) in Healthcare
Partners: University of Maryland School of Medicine
Timeline: 15.1 months (7/27/2020 to 10/29/2021)
Summary: Through this contract with the University of Maryland School of Medicine, NIOSH will support the development of an implementation guide to help inform U.S. health systems about the potential to transition from disposable respirators to elastomeric respirators.
Best Practices and Preferred Uses of Reusable Elastomeric Half Mask Respirators (EHMR) in Healthcare
Partners: Allegheny-Singer Research Institute
Timeline: 16.3 months (8/13/2020 to 12/23/2021)
Summary: Through this contract with Allegheny-Singer Research Institute, NIOSH will support the development of an implementation guide to help inform U.S. health systems about the potential to transition from disposable respirators to elastomeric respirators.
Elastomeric Respirator Exhaled Breath Filter Adaptors – Prototypes
Partners: High Valley Design, INC.
Timeline: 7.6 months (09/27/2020 to 05/14/2021)
Summary: EHMR respirator adaptors will be designed to fit onto the respective respirator encasing the exhalation valve assembly, sealing it from the ambient air, and will incorporate features to accommodate 95% or 100% single-sheet or multi-layer filter media. Four adaptors will be designed, each going through three iterative design steps. Each of the four adaptors will be custom designed to fit commercially available EHMRs. Respirator adaptors will be evaluated for filtration efficiency and pressure buildup within the respirator.
Measurement of Outward Leakage from a Filtering Facepiece Respirator
Summary: The test procedure measures particle numbers and size distributions inside a respirator and just downstream of the exhalation valve. A plastic bag is attached to the exhalation valve shrouds to prevent external air currents from diluting the sample from the exhalation valve while a pressure transducer is used to record the subject’s breathing pattern.