Technical Report 

Filtering Facepiece Respirators with an Exhalation Valve: 

Measurements of Filtration Efficiency to Evaluate Their 

Potential for Source Control 

Centers for Disease Control 

and Prevention 

National Institute for Occupational 

Safety and Health 
Technical Report

Filtering Facepiece Respirators with an Exhalation Valve:

Measurements of Filtration Efficiency to Evaluate Their 

Potential for Source Control

DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

National Personal Protective Technology Laboratory
This document is in the public domain and may be freely 

copied or reprinted.

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Suggested Citation

NIOSH [2020]. Filtering facepiece respirators with an exhalation valve: measurements 

of filtration efficiency to evaluate their potential for source control. By Portnoff L,

Schall J, Brannen J, Suhon N, Strickland K, Meyers J. U.S. Department of Health and 

Human Services, Centers for Disease Control and Prevention, National Institute for 

Occupational Safety and Health, DHHS (NIOSH) Publication No. 2021-107.

https://doi.org/10.26616/NIOSHPUB2021107

DOI: https://doi.org/10.26616/NIOSHPUB2021107

DHHS (NIOSH) Publication No. 2021-107

December 2020

Cover photo by Shutterstock
iii

CONTENTS

List of Figures ...............................................................................................................iv

List of Tables .................................................................................................................iv

Acronyms and Abbreviations ...................................................................................... v

Unit of Measure Abbreviations .................................................................................... v

Executive Summary ...................................................................................................... 1

Introduction ................................................................................................................... 3

Exhalation Valves............................................................................................................ 4

Objective ......................................................................................................................... 5

Background ................................................................................................................... 5

Methods ......................................................................................................................... 6

Test Equipment ............................................................................................................... 7

Effect of Flowrate ............................................................................................................ 8

Control Group.................................................................................................................. 8

Inward Position......................................................................................................... 9

Outward Position...................................................................................................... 9

Experimental Group ........................................................................................................ 9

Mitigation Strategies................................................................................................. 9

Mitigation Strategy Selection.................................................................................. 10

Non-FFR Protective Devices......................................................................................... 10

Filtration Test Findings............................................................................................... 11

Discussion................................................................................................................... 13

Limitations ..................................................................................................................... 15

Conclusions................................................................................................................. 16

For Further Information ................................................................................................. 17

References................................................................................................................... 18

Appendix...................................................................................................................... 21
iv

LIST OF FIGURES

Figure 1. Simulations of particle exhalation through an FFR without an

exhalation valve, an FFR with an exhalation valve, and a barrier face covering ............. 6

Figure 2. The two positions used for the 13 FFR models tested ..................................... 8

Figure 3. Three mitigations used on FFRs to measure the reduction of

particle penetration.......................................................................................................... 9

Figure 4. Filtration efficiency by testing position at flowrates of 25, 55, and 85 lpm, 

reported as percentage of penetration .......................................................................... 11

Figure 5. Filtration efficiency of each FFR tested in the outward position

control—i.e., with no mitigation measure....................................................................... 12

Figure 6. Filtration efficiency of non-FFR protective devices by type,

reported as percentage of penetration .......................................................................... 13

LIST OF TABLES

Table 1. FFRs with an exhalation valve tested for this study........................................... 7

Table A1. Particle penetration descriptive statistics by mitigation and flowrate

among the 13 FFR models............................................................................................ 21

Table A2. Particle penetration descriptive statistics by model for the

outward and inward position controls ............................................................................ 22

Table A3. Particle penetration descriptive statistics by flowrate

among each of the four non-FFR protective devices..................................................... 23
v

ACRONYMS AND ABBREVIATIONS

CDC Centers for Disease Control and Prevention

CMD count median diameter

FFR filtering facepiece respirator

GSD geometric standard deviation

HCP healthcare personnel

MMAD mass median aerodynamic diameter

NaCl sodium chloride

NIOSH National Institute for Occupational Safety and Health

NPPTL National Personal Protective Technology Laboratory

PAPR powered air-purifying respirator

UNIT OF MEASURE ABBREVIATIONS

cm centimeter

g gram

g/cc grams per cubic centimeter

in inches

lpm liters per minute

mg/m3 milligrams per cubic meter

mm millimeter

mmH2O millimeters of water column

nm nanometer

μm micrometer
EXECUTIVE SUMMARY

Filtering facepiece respirators (FFRs) are used extensively by healthcare personnel (HCP) during

a pandemic. FFRs are primarily reserved for those personnel who have a greater risk and longer 

duration of exposure compared with other workers and the general public. Some FFR models

contain an exhalation valve, which is a device that closes to allow inhaled breath to be pulled

through the filter media and opens in order to allow exhaled breath to be expelled from the 

respirator through the exhalation valve as well as the filter media. These FFR models provide the 

wearer with a level of protection similar to that of an FFR without an exhalation valve, and they

are thought to increase the wearer’s comfort at high work rates and be suitable for longer periods 

of use. However, respiratory secretions expelled by wearers may exit along with air through the 

exhalation valve. A concern with FFRs with an exhalation valve is that individuals may spread

disease if unfiltered, virus-laden aerosols pass through the valve. 

Current guidance from the Centers for Disease Control and Prevention (CDC) does not

recommend using an FFR with an exhalation valve for source control (i.e., to filter respiratory 

secretions to prevent disease transmission to others) and advises that if only this option is 

available and source control is needed, then the valve should be covered with a surgical mask,

procedure mask, or a cloth face covering that does not interfere with the respirator fit. The CDC

has requested research in order to provide improved science-based recommendations on the use of

exhalation valves. The current study was designed to respond to this request and to evaluate 

particle penetration in FFRs with an exhalation valve. Further, this research compares mitigation 

strategies, including investigating the practice of covering the FFR exhalation valve with a mask. 

Finally, some publications have criticized the use of FFRs with an exhalation valve, and airline 

industry leaders have banned their use on flights. Given the above, more research is needed to 

evaluate particle emission through exhalation valves so that a better understanding of risk can be 

conveyed to the public.

To address these issues, the National Personal Protective Technology Laboratory at the National

Institute for Occupational Safety and Health (NIOSH), CDC, developed a study with three aims:

(1) to measure the filtration efficiency provided by FFRs with an exhalation valve under

conditions of inward airflow (i.e., in the direction of inhalation) and outward airflow (i.e., in the

direction of exhalation); (2) to evaluate how particle penetration in FFRs with an exhalation valve

compares to particle penetration in surgical masks, procedure masks, cloth face coverings, and

fabric from cotton t-shirts; and (3) to determine the filtration efficiency of three modifications to

the exhalation valve in FFRs with the goal of mitigating the emission of unfiltered particles.

To accomplish these three aims, thirteen FFR models were each tested in two positions: 

inward position, which is used by the NIOSH Respirator Approval Program when testing N-type 

respirators, and outward position, which was used experimentally to channel airflow in the

direction of exhalation. For the inward position, three mitigation strategies were used:

(1) covering the valve on the interior of the FFR with commonly available surgical tape,

(2) covering the valve on the interior of the FFR with an electrocardiogram (ECG) pad; and

(3) stretching a surgical mask over the exterior of the FFR. 

The purpose of these three strategies was to measure the varying filtration efficiencies to 

determine their contribution toward source control. Both positions and all mitigation strategies 

were tested at three airflow rates: 25, 55, and 85 lpm (liters per minute). In addition to the FFR 

evaluations, researchers evaluated a selection of surgical masks, procedure masks, cloth face 

coverings, and fabric from cotton t-shirts using the outward position and flowrates detailed above. 

1
Findings for the 13 FFR models were derived from a total of 1,125 tests. In the inward position, 

particle penetration was less than 5%, which is the target value for NIOSH Respirator Approval 

Program tests. For all three mitigation strategies in the outward position where air flowed out 

through the valve, particle penetration ranged from <1% to 55%. The variance in particle 

penetration for the outward position is likely explained by design differences between FFR 

models and variances in how the exhalation valve opens. For all models, at all flowrates, the best 

mitigation strategy was to cover the valve from inside the FFR with an ECG pad. Securing 

surgical tape over the valve from inside the FFR performed nearly as well. Covering the FFR with 

a surgical mask was the least effective mitigation strategy. 

For the small sample of non-FFR devices (surgical masks, procedure masks, cloth face coverings, 

and fabric from cotton t-shirts) evaluated using the outward position, results are included to 

verify that the equipment and test methods employed in the current study produced comparable 

results to previous NIOSH research that explored the particle penetration of select types of nonFFR devices. 

FFRs with an exhalation valve provide respiratory protection to the wearer and—according to the 

findings from this study—can also reduce particle emissions to levels similar to or better than 

those provided by surgical masks and unregulated barrier face coverings. This study shows that 

modifications to these respirators can further reduce particle emissions. The use of an ECG pad or 

surgical tape secured over the valve from the inside of the FFR can provide source control similar 

to that of an FFR with no exhalation valve. 

These results represent one of the first measurements of particle penetration through FFRs with 

an exhalation valve that are tested in an outward position, and the findings have important 

implications for guidance on source control and mitigation. 

2
3

INTRODUCTION

Masks and respiratory protective devices may protect the wearer to varying degrees from 

exposure to potentially infectious droplets, and these devices have varying degrees of protection. 

These devices can be classified into three categories: (1) respirators (e.g., those that are NIOSHapproved respirators or authorized for emergency use); (2) masks intended for medical purposes 

(e.g., FDA-cleared surgical masks including those labeled as procedure masks); and

(3) masks intended for a non-medical purpose.1

Respirators are designed to filter the most penetrating particles, which are <1 micrometer (μm), 

and respirators are fit-tested to try to minimize inward leakage of particles around the face seal.

Surgical masks are evaluated to less stringent filtration efficiency requirements [ASTM 2019] 

than NIOSH-approved respirators [CFR 42 Part 84]. However, some may have filtration 

efficiency that performs similar to that found in respirators. Importantly, surgical masks do not 

have a fit requirement and therefore are more susceptible to inward leakage when compared to 

respirators.

Unregulated barrier face coverings typically lack electrostatically charged fibers, multi-layer

filter media, and other properties that provide the ability for respirators to filter submicron 

particles (<1 μm). Although barrier face coverings are not recommended to filter submicron 

particles, they have demonstrated the ability to filter large particles and diffuse airflow

[Clase et al. 2020]. In addition to providing varying degrees of protection to the wearer, the 

above protective devices have the potential to filter respiratory secretions from the wearer to help 

prevent disease transmission to others (i.e., they may provide source control).

Filtering facepiece respirators (FFRs) are a type of respirator that is approved by NIOSH and 

designed to protect the wearer by filtering the smallest and most penetrating particles—0.35-μm

mass median aerodynamic diameter (MMAD). FFRs provide protection from large droplets that 

are found in coughs and sneezes and also fine droplets that are emitted by talking. Surgical N95 

FFRs are a subcategory of NIOSH-approved respirators and have the same level of filtration as 

N95 respirators. Surgical N95 FFRs do not have exhalation valves. Because they are used 

extensively by healthcare personnel (HCP) during a pandemic, surgical N95 FFRs and all 

unvalved NIOSH-approved FFRs are primarily reserved for those personnel with a greater 

potential for exposure compared with other workers and the general public.

A pandemic strains the availability of respiratory protection. During a pandemic, HCP, 

emergency responders, and other critical infrastructure workers may struggle to acquire 

respiratory protection. For this reason, unregulated barrier face coverings are recommended for 

use by the general public and non-HCP to reduce airborne transmission of virus during a 

pandemic while preserving critical supplies of respirators and masks needed by HCP.

While HCP primarily rely on FFRs, elastomeric (full facepiece and half-mask) respirators and 

powered air-purifying respirators (PAPRs) are alternatives. However, some FFRs, elastomeric 

respirators, and PAPRs allow exhaled breath to be expelled from the respirator without passing 

through the filter media. Particles are emitted with this exhaled breath, which may impact the 

level of source control provided by these types of respirators.

1 Masks that are not intended for a medical purpose are not medical devices and are not regulated by the FDA.
4

Some FFRs have a device called an exhalation valve that bypasses the filter media to exhaust 

exhalation directly to the surroundings. However, the effectiveness of FFRs with exhalation 

valves as a potential contributor to source control has not been evaluated before now.

Exhalation Valves

Exhalation valves in respirators are devices that close to allow inhaled breath to be pulled 

through the filter media and open to allow exhaled breath to be expelled from the respirator 

through the exhalation valve as well as the filter media. In FFRs, an exhalation valve typically 

has a membrane composed of natural rubber, silicone, or neoprene. The membrane sits atop a 

support structure and lies beneath a plastic cover. During inhalation, the membrane closes 

against the support structure and blocks the valve hole, thereby providing maximum air filtration 

through the filter material. During exhalation, if there is adequate air pressure, the air lifts the 

membrane and allows a passageway through which unfiltered exhaled breath can escape. These 

valves are included in some FFR models and are thought to increase the wearer’s comfort at high 

work rates and be suitable for longer periods of use. A respirator with an exhalation valve 

provides the wearer with a level of protection similar to that of one without an exhalation valve. 

However, the exhalation valve is likely to affect the level of source control provided by a 

respirator as some particles expelled by wearers may exit through the exhalation valve.

If an infected individual is wearing a respirator with an exhalation valve, then the amount of viral 

emissions relates to the individual’s respiratory secretions that would be associated with 

breathing, talking, coughing, and sneezing. These activities result in expelling droplets of 

different sizes containing varying amounts of virus. Large droplets quickly fall to the ground, but 

small droplets remain airborne, accumulate, and may be inhaled by others. Once droplets become 

airborne, their transport and viability depend upon such factors as travel distance, length of time 

airborne, fresh air exchange, and sunlight [Chin et al. 2020; Ratnesar-Shumate et al. 2020; 

Augenbraun et al. 2020]. Finally, once virus-laden droplets are inhaled, their ability to infect a 

person also depends on the droplets making contact with mucosal cells in the respiratory tract,

attachment to a virus receptor, and a dose sufficient to cause infection [Perrotta et al. 2020; 

Schröder 2020].

Respirators are highly effective at filtering fine particulates (i.e. <2.5 μm) and virus to protect the 

wearer [Zhou et al. 2018; Dau et al. 2020]. However, respirators with an exhalation valve that 

release respiratory emissions directly to the surroundings may not be suitable when a sterile field 

must be maintained [DHHS 2015]. On its “Considerations for Wearing Masks” page, the CDC 

advises the general public about all types of masks and states that the CDC does not recommend 

using masks with exhalation valves for source control [CDC 2020a]. On its “Personal Protective 

Equipment: Questions and Answers” page, whose primary audience is healthcare personnel, the 

CDC provides advice on the use of protective equipment and states that if only a respirator with 

an exhalation valve is available and source control is needed, then “the exhalation valve should 

be covered with a surgical mask, procedure mask, or a cloth face covering that does not interfere 

with the respirator fit” [CDC 2020b].

2

In recent publications, the above CDC statements have been interpreted to mean that “masks 

with exhalation valves can do more harm than good” [Jones 2020], that exhalation valves pose 

2

In some sources, “cloth face coverings” are referred to as “cloth masks.”
5

major concerns about outward protection [Ippolito et al. 2020], and that a “mask with a vent is 

useless in protecting others” and is the equivalent of “wearing nothing” [Pallini 2020].

Some municipalities have banned the use of masks with exhalation valves [Department of Public 

Health Order of the Health Officer 2020], and airline industry leaders also prohibit their use 

[Alaska Airlines 2020; American Airlines 2020; Delta Airlines 2020; Southwest Airlines 2020].3

Given the above, more research is needed to evaluate particle emission through exhalation valves 

so that a better understanding of risk can be conveyed to the public. The CDC has requested 

research in order to provide improved science-based recommendations on the use of exhalation 

valves. The current study was designed to respond to this request and to evaluate particle 

penetration in FFRs with an exhalation valve.

Objective

To provide data to address the issue of the extent to which FFRs with an exhalation valve may 

provide source control, the objective of this study is threefold:

(1) to measure the filtration efficiency provided by FFRs with an exhalation valve under 

inward and outward airflow conditions (12 N-95 FFRs and 1 N-99 FFR);

(2) to evaluate how particle penetration in FFRs with an exhalation valve compares to 

particle penetration in surgical masks, procedure masks, cloth face coverings, and fabric 

from cotton t-shirts; and

(3) to determine the filtration efficiency of three modifications to the exhalation valve in 

FFRs with the goal of mitigating the emission of unfiltered particles.

BACKGROUND

Visualizations of mannequins emitting a simulated cough and sneeze are helpful tools to 

demonstrate how different types of masks provide source control. As examples, at Florida 

Atlantic University, researchers use green laser light to visualize droplet transmission

[Verma et al. 2020]. Other studies employ Schlieren imaging, a tool used to photograph the flow 

of fluids of varying densities, which also helps to visualize movements of aerosols such as those

generated by a cough [Tang et al. 2008; Staymates 2020].

The above studies used visualizations of humans emitting respiratory secretions. To help readers 

visualize particle transport in the context of the current study, models were employed to generate 

two-dimensional illustrations. Figure 1, left, shows how an FFR without an exhalation valve 

provides a tight fit and filters particles. Figure 1, center, shows how an FFR with an exhalation 

valve provides a tight fit, filters particles, and releases particles through the valve. Figure 1, right,

shows how a barrier face covering does not provide a tight fit; it filters fewer particles, allows 

them to be released through the gaps around the covering, and pushes some particles backwards.

3

In the cited sources, other terminologies used for “exhalation valve” include “one-way vent,” “exhaust valve,” and 

“vent.”
6

Illustrations by NIOSH

Figure 1. Simulations of particle exhalation through an FFR without an exhalation valve

(left), an FFR with an exhalation valve (center), and a barrier face covering (right).

Density of particles near the face represents particles remaining inside the mask.

Illustrations for Figure 1 were created using Blender software, a three-dimensional modelling 

and rendering package (Stichting Blender Foundation, Amsterdam). Model settings were chosen 

to represent particle penetration efficiency and the presence or absence of face sealing based on 

mask type, as follows: left—the FFR without an exhalation valve provides a tight seal around the 

face and permits <5% of particles to escape the respirator; center—the FFR permits <5% of 

particles to escape through the filter media and also has an exhalation valve that provides an 

opening that allows 20% of the particles to escape through the valve; right—the barrier face 

covering does not have a seal around the face, which causes particles to move behind the head 

and also releases 50% of particles through the covering.

METHODS

The NIOSH National Personal Protective Technology Laboratory (NPPTL) evaluated 13 FFRs 

from 10 manufacturers to determine the inward and outward filtration efficiency of particles 

through the exhalation valves. A convenience sample of models was composed from all available 

excess NPPTL stock of FFRs with exhalation valves that had four or more specimens. Because 

of a limited supply of product, researchers tested FFRs that were beyond their “use-by” date and 

were taken from an open box; however, researchers did not expect that this would affect the 

study results. The above constraints are identified in Table 1 and acknowledged as a limitation of 

the study.

Each model was evaluated using a TSI 8130 filtration efficiency tester with a sodium chloride 

(NaCl) 2% solution in distilled water.

4 Six replicates were tested for each model, unless 

otherwise indicated in Table 1.

4 The sodium chloride (NaCl) aerosol has a count median diameter (CMD) of 75 ± 20 nanometer (nm) and a 

geometric standard deviation (GSD) of ≤1.86. With a density of 2.13, the MMAD is 0.347 μm [Eninger et al. 2008].
7

Table 1. FFRs with an exhalation valve tested for this study. The table

includes two classes of respirators, variable sample sizes, and

non-NIOSH-approved respirators, due to a limited supply of product.

Supplier

Manufacturer

Model 

Number Class Sample Size

Beyond

“Use-by” Date

Open 

Box

Approval 

Number

Dräger Safety AG & 

Company, KGaA

X-plore 

1750 N95 6 No Yes 84A-4396

Jinhua Meixin 

Protective Equipment

Factory

2001V N95 6 No Yes

NonNIOSHapproved***

3M Company 9211 N95 5** Unknown Yes 84A-2668

3M Company 8511CN N95 6 No Yes 84A-5402

Visca Safety 

Comercial Limitada

Visca 

2740V N95 6 No Yes 84A-4486

AirGas Inc., Radnor 65059520A N95 6 No Yes 84A-6250

Willson Dalloz Safety 

Products NBW95V N95 6 Yes—2017 Yes 84A-4378

Makrite Industries, 

Inc. 710VOV N95 6 No Yes 84A-9219

Makrite Industries, 

Inc. 9800V N95 6 No Yes 84A-9055

ATEM Company, Ltd. 4030 N95 4** No Yes 84A-7720

Moldex-Metric, Inc. 2310 N99* 6 Yes—2012 No 84A-1459

Uline S-10479 N95 6 Unknown Yes 84A-3714

Makrite Industries, 

Inc. 2201V N95 6 No Yes 84A-9231

 *The N99 FFR is designed to have greater filtration than an N95 FFR.

 **For these two models, 6 specimens were not available.

 ***This respirator model met NIOSH testing requirements but does not have a NIOSH approval.

Test Equipment

The following test equipment was used for these evaluations:

• A TSI Model 8130 filtration efficiency tester.

• A microbalance accurate to 0.0001 g.

• Type A/E glass filters, 102-mm-diameter, with a 1-μm pore size.

• Sodium chloride (NaCl), a 2% solution in distilled water.

• A temperature and humidity chamber.

• A test fixture (rectangular tube of 6.4-mm polycarbonate [20.3 x 20.3 x 11.4 cm]).

• Aluminum plates with a centered 7.6-cm-diameter hole on the top and bottom of the test 

fixture.

Pressure drop across the respirator—which is an indicator of breathing resistance—was 

measured by the TSI 8130.

For each FFR, NIOSH researchers examined two positions and three mitigations at three 

flowrates. The total number of possible tests was 1,170 (13 models × [2 positions + 3 

mitigations] × 3 flowrates × 6 replicates); however, six replicates were not available for every 

model, and therefore the dataset includes 1,125 tests.
8

Effect of Flowrate

With the NIOSH Respirator Approval Program standard test procedure, a flowrate of 85 lpm is 

specified for FFRs. This flowrate is intended to correspond to breathing that would occur during 

moderate exercise. When following this test procedure, airflow travels in one direction and with 

a velocity that corresponds to the maximum flow in the breathing cycle (i.e., instantaneous peak 

flowrate). Lower airflow rates of 25 lpm and 55 lpm were also evaluated to compare the effect 

that lower breathing rates may have on filtration efficiency.

Control Group

The control group evaluated FFRs in two positions (see Figure 2). The inward position

corresponds to the direction of inhalation; the outward position corresponds to the direction of 

exhalation. For both positions, the valve in the FFR was not altered in any way. The inward 

position is a control to validate the filtration efficiency of the FFRs. The outward position is a 

control that measures the filtration efficiency without any mitigation strategy.

Photos by NIOSH

Figure 2. The two positions used for the 13 FFR models tested. The

inward position (left) is used by the NIOSH Respirator Approval Program

when testing N-type respirators. The outward position (right) was used

experimentally to channel airflow in the direction of exhalation.

Inward position Outward position Airflow Airflow
9

Inward Position

The inward position control measures the inward filtration efficiency of the FFR. This is the 

same position that the NIOSH Respirator Approval Program uses to approve N-type respirators.

For the 12 N95 FFRs tested, the efficiency should be >95%, which is equivalent to <5% 

penetration (penetration = 1 - filtration). For the one N99 FFR tested, the efficiency should be 

>99%, which is equivalent to <1% penetration (penetration = 1 - filtration). The respirators were 

sealed with beeswax, as shown in Figure 2.

Outward Position

The outward position control measures the outward filtration efficiency through the filter media

and includes additional unfiltered particles passing through the exhalation valve, if open. This 

position is not used by the NIOSH Respirator Approval Program to approve N-type respirators.

For the 13 FFRs tested, the efficiency should be a combination of particles that pass through the 

filter media and the exhalation valve. The respirators were sealed with beeswax.

Experimental Group

Three mitigations to inhibit particle penetration by covering the exhalation valve (see Figure 3) 

were chosen as the experimental group: taped (with surgical tape), covered (with an 

electrocardiogram [ECG] pad), and masked-over (with a surgical mask). The mitigations were 

tested in the outward position in order to compare particle penetration in relation to the outward 

position control.

Photos by NIOSH

Figure 3. Three mitigations used on FFRs to measure the reduction of particle

penetration. Dotted lines represent tape edges (left) and ECG pad edges (center).

Mitigation Strategies

Taped Mitigation

The taped testing mitigation had a 2" x 2" swatch gently pressed onto the interior of the FFR. As 

with the inward position sealing approach, the respirator was sealed with beeswax. A Nexcare 

Interior Interior Exterior

Taped

(surgical tape pressed into interior)

Covered

(ECG pad pressed into interior)

Masked-over

(surgical mask stretched over exterior)
10

gentle paper tape, with medium hold, was used (hospital name: 3M Micropore surgical tape). 

The taped test used the outward position but with the surgical tape covering the exhalation valve.

Covered Mitigation

The covered testing mitigation had an ECG pad (3M Red Dot) gently pressed onto the interior of 

the respirator. As with the inward position sealing approach, the respirator was sealed with 

beeswax. The covered test used the outward position, but with the ECG pad covering the 

exhalation valve.

Masked-over Mitigation

The masked-over testing mitigation used a surgical mask stretched over the exterior of the 

respirator. As with the inward position sealing approach, the respirator was sealed with beeswax. 

The surgical mask was then stretched over the FFR to simulate a reasonably tight fit and was 

secured with hot glue. The nose wire of the surgical mask was pinched around the respirator, and 

the four corners where the elastic band attaches were sealed with beeswax, as shown in Figure 3, 

right. For the masked-over mitigation, the test results depend upon the FFR/surgical mask 

interface, so great care was taken to simulate a realistic, snug fit.

Mitigation Strategy Selection

Taped and covered mitigations were selected because the materials are available in a hospital, 

are nontoxic, and provide good adherence to moist surfaces. Two concerns are that the adhesive 

could pull away from the surface, thereby not blocking airflow to the same degree over time, and 

that these adhesives could contain chemicals that have toxicological effects. In light of these 

concerns, the surgical tape and ECG pads used in this study both have no expected toxicological 

effects in relation to skin contact, inhalation, and ingestion, and they provide greater adherence 

in moist conditions. The masked-over mitigation was selected because this aligns with the 

current CDC recommendation [CDC 2020b] if source control is needed and only an FFR with an 

exhalation valve is available.

Non-FFR Protective Devices

In addition to the FFR evaluations, researchers also evaluated a small selection of masks 

intended for medical purposes and unregulated barrier face coverings made up of four models of 

surgical masks, seven models of procedure masks, six models of cloth face coverings, and two 

types of fabric from cotton t-shirts.

For the surgical masks, two models were used for the masked-over mitigation strategy. For the 

cloth face coverings, two had filter inserts and three had exhalation valves. The sample size of 

the non-FFR devices evaluated in this current study was too small to represent the population of 

those devices but is included to provide confidence that the equipment and test methods 

employed in the current study produced results comparable to previous NIOSH research that 

explored the particle penetration of select types of non-FFR devices [Rengasamy et al. 2009; 

Rengasamy et al. 2010].
11

FILTRATION TEST FINDINGS

The findings for the 1,125 tests performed on the 13 FFRs with an exhalation valve are 

summarized in Figure 4.

Figure 4. Filtration efficiency by testing position at flowrates of 25, 55, and 85 lpm,

reported as percentage of penetration. The boxes show median, 25% quantile, and

75% quantile of all 13 FFRs tested (n=1,125). Three mitigations were performed in the

outward position. For related descriptive statistics, see Table A1 in the Appendix.

The penetration with the inward position control was <5% for all but one test. Results for the

outward position control (in which the airflow goes the same direction as exhaled breath) ranged 

from <1% to 55%. The covered mitigation performed nearly as well as the inward position 

control. Considering the median penetration at 85 lpm, penetration was 31% for the outward 

position control; penetration was 23% for the masked-over mitigation; penetration was 5% for 

the taped mitigation; penetration was 2% for the covered mitigation. This high-to-low order 

(outward position control, masked-over, taped, and covered) was the same for every flowrate. 

The penetration was lowest at 25 lpm and highest at 55 lpm and 85 lpm.

Figure 5 shows test results of the outward position control for each FFR model. Flowrates of 25, 

55, and 85 lpm are indicated by color and shape.
12

Figure 5. Filtration efficiency of each FFR tested in the outward position

control—i.e., with no mitigation measure. Results reported as percentage

of penetration (n=1,125). Boxes show median, 25% quantile, and 75% quantile

with icons that indicate the range of flowrates (25, 55, and 85 lpm).

For related descriptive statistics, see Table A2 in the Appendix.

For the outward position controls, some models (65059520A, 2201V, and 4030) had less than 

20% penetration even without any mitigation. Other models (S-10479, 2310, 2001V, X-Plore 

1750, VICSA 2740V, 9800V) had much greater penetration with a median penetration above 

40%. Some models showed large variability in penetration, with model 2001V ranging from 

<1% to 44% penetration.

The least particle penetration occurred at 25 lpm across all models with one exception—model 

2310, which had a minimum penetration of 8% at 85 lpm. The maximum penetration occurred at 

55 lpm for most models. For the models that had less than 20% penetration (65059520A, 2201V, 

and 4030), the maximum penetration occurred at 85 lpm.

Figure 6 shows results of surgical masks, procedure masks, cloth face coverings, and fabric from 

cotton t-shirts evaluated for submicron particle penetration using the outward position.

 

 

 

 

 

 

 

 

 

 

 

 

 
13

Figure 6. Filtration efficiency of non-FFR protective devices by type, reported

as percentage of penetration. The dotted line shows results found by

Rengasamy et al. [2009] for surgical masks when fitted over a mannequin.

Importantly, when interpreting these findings, note that unregulated devices

(procedure masks, cloth face coverings, and fabric from cotton t-shirts)

are not expected to efficiently filter submicron particles. The penetration

for these devices would be lower as particle size increases. For related

descriptive statistics, see Table A3 in the Appendix.

Particle penetration through the surgical mask ranged from 2% to 17%. Procedure masks were 

most variable and ranged from 1% to 85%. Cloth face coverings ranged from 45% to 91%, and 

fabric from cotton t-shirts tested ranged from 24% to 92%.

For the two cloth face coverings that had a filter insert, the inserted filter improved performance 

by 6%, 11%, and 14% at 25 lpm, 55 lpm, and 85 lpm, respectively, in one face covering. In the 

other face covering, the filter improved performance by 39%, 22%, and 20%, respectively. For

the three cloth face coverings with exhalation valves, the outward percent penetration efficiency 

ranged from 67% to 91%, while for the three cloth face coverings without exhalation valves, the 

outward percent penetration ranged from 45% to 87%.

DISCUSSION

This study evaluated filtration efficiency of small particles with a 0.35-μm MMAD through 

FFRs with an exhalation valve. Measurements were obtained with a TSI 8130 filtration 

efficiency tester by positioning the FFRs in the inward position and also by placing the FFRs in 

the outward position to reverse the airflow to be the same direction of exhaled breath. NIOSH 

evaluated three mitigation strategies: covered, taped, and masked-over. Tests were conducted at 

three flowrates that correspond to low and moderate peak inhalation flowrates: 25, 55, and 85 

lpm.

In the inward position, the penetration was less than 5%, which is the target value in the NIOSH 

Respirator Approval Program tests.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
14

With air flowing out through the exhalation valve, penetration ranged from <1% to 55%. The 

penetration was mostly dependent upon the FFR model and indicative of models having different 

exhalation valve designs and materials. Although the functioning of the exhalation valve was not 

evaluated in this study, NIOSH researchers inferred that the valves on some models may have 

remained mostly closed; this inference is based on low penetration findings such as 

measurements of <1% that would not be possible with an open exhalation valve (as demonstrated 

by model 65059520A in Figure 5). As an explanation, those FFRs whose valves did not open 

may have been designed to open at higher flowrates. The variability within each FFR model 

could also be explained by how the exhalation valve opens, with some models consistently

opening to the same position and other models having variations with the valve even fluttering in 

the air currents.

For all models of FFRs, at all flowrates, the best mitigation strategy was to cover the exhalation 

valve inside the FFR with an ECG pad. With this mitigation, the penetration was below 5% 

except for one model. Unlike the other FFRs where the tape/ECG pad could make contact with a

hard, plastic ring of the valve, the rim of this model was covered with filter media; thereby the 

adhesive bonded to the fabric and not the plastic, not allowing a complete seal. Securing surgical 

tape over the exhalation valve inside the FFR performed nearly as well as using the ECG pad.

Covering the FFR with a surgical mask was the least effective mitigation strategy. This may be 

explained by the principle that contaminants are more easily controlled at their source before 

they are released, spread, or diluted. A further explanation as to why the covered mitigation 

strategy was not effective could be that after hitting the exhalation valve, the flow of air travels

parallel with the surgical mask covering. A poorly fitting surgical mask with an exhalation valve 

that protrudes from the FFR provides a clear exhaust channel for the test aerosol.

For the non-FFR protective devices, the surgical mask had the least penetration (2% to 17%); 

however, the current study methods sealed the mask with beeswax and did not account for fit. 

When accounting for fit, the outward leakage of submicron particles in surgical masks in a 

previous NIOSH study was found to be much greater (7% to 76%) [Rengasamy et al. 2009].

Procedure masks showed the greatest range of particle penetration (1% to 85%). This is 

expected, because the term “procedure mask” may not denote a medical device and would 

therefore not be regulated by the FDA unless the manufacturer claims the mask to be for medical 

use. The unregulated barrier face coverings were not expected to perform well with the small 

particles generated for the filtration efficiency tests [Clase et al. 2020]. However, these study 

results for unregulated barrier face coverings do not imply that they cannot efficiently filter large 

particles.

For the six cloth face coverings, the submicron particle penetration ranged from 24% to 92%. 

For the two types of fabric from cotton t-shirts, submicron particle penetration ranged from 45% 

to 91%. Although this sample size was small, these penetration percentages for the cloth face 

coverings and fabric from cotton t-shirts are comparable to the 40% to 90% penetration found by 

previous NIOSH testing of cloth face coverings and fabrics [Rengasamy et al. 2010]. The results 

for the six face coverings tested in the current study showed a minimum penetration (24%) that 

was slightly lower than in the previous NIOSH testing, while the maximum penetration (92%) 

was nearly the same.

Based on the study results, an important question emerges: Does breathing out through an 

unmitigated FFR with an exhalation valve provide greater source control than that provided by a 
15

surgical mask? Although penetration depends on which model of FFR with exhalation valve and 

which model of surgical mask is considered, the maximum particle penetration through the 

unmitigated FFRs evaluated in this study was 55%, which occurred at 55 lpm, while the 

maximum penetration for surgical masks was 17% for the filter media and 76% when 

considering fit [Rengasamy et al. 2009]. Increasing the flowrate did not increase pressure 

because the airflow through the exhalation valve is finite; the increased pressure at 85 lpm 

channeled air through the filter media when the exhalation valve approached saturation. This 

phenomenon could make the contribution towards source control provided by FFRs with an 

exhalation valve more consistent than that provided by surgical masks, which can have openings 

on the perimeter of the mask that have more variability in surface area than the opening of an

exhalation valve.

This study evaluated the filtration efficiency of FFRs with an exhalation valve to consider their 

potential for source control. Even without mitigation, every FFR studied here allowed

penetration of <55% of the submicron particles, meaning that nearly half of the most penetrating 

particles were filtered regardless of respirator model. With mitigation measures such as surgical 

tape or an ECG pad secured inside, the FFRs can perform nearly as well as an FFR with no 

exhalation valve.

This study, using the most penetrable particle size, indicates that even unmitigated FFRs with an 

exhalation valve can help to provide some degree of source control. A similar evaluation of 

PAPR and elastomeric respirator mitigation strategies would be an important continuation of this 

research, since a surgical mask over the exhalation valve of an elastomeric respirator is a

mitigation currently being used by some healthcare institutions to help provide source control.

Limitations

The primary limitation of this study is that only one particle size range was used for all the 

testing (0.35-μm MMAD with a ≤1.86 GSD). These particles have two routes of escape—

through the filter media and through the exhalation valve. For the particles that escape through 

the filter media, these submicron particles are considered to be the maximum penetrating particle 

size, and when neutralized these particles are expected to more easily penetrate through the filter 

media [Eshbaugh et al. 2008]. For those respiratory secretions that escape through the exhalation 

valve, the larger droplets have more inertial energy and are therefore more likely to impact onto 

surfaces rather than follow the air currents [Fuchs 1986]. For these reasons, the particles studied 

here should represent the “worst-case” particle penetration, and larger particle sizes are expected 

to have less penetration.

Other limitations are as follows:

• The TSI 8130 test requires an airtight seal and does not address FFR fit.

• The study did not account for conditions where an FFR adds outward leakage due to 

improper fit when comparing these respirators to surgical masks.

• The effect of humidity/wetness, which can cause the exhalation valve to stick, was not 

evaluated, and this study used machine-generated aerosol that lacks the moisture content 

• that would be found in human breath.

• The study did not evaluate the length of time and conditions for adherence of a surgical 

tape or an ECG pad to the inside of an FFR.
16

• The TSI 8130 test does not represent particles that are emitted by way of coughing or 

sneezing.

• The study did not evaluate aerosol concentrations that are typically emitted by humans.

• FFR models and specimen selection were not completely randomized.

• The effects of “use-by” date and “open box” of FFRs listed in Table 1 were not 

evaluated.

• The study evaluated three flowrates that correspond to moderate worker breathing rates; 

these were continuous flowrates to represent the instantaneous peak flowrate associated 

with sinusoidal breathing.

• The 25 lpm flowrate may not have reached equilibrium during the sample time

(38 seconds) and this may affect the accuracy and precision of the measurements at this 

flowrate.

• The conclusions are generalized from the models tested. Researchers assess that 13 

models from 10 manufacturers is a good effort to survey the population; however, 

additional models may exceed the bounds of the models selected for testing.

CONCLUSIONS

Based on a sample size of 13 models, this study found that unmitigated FFRs with an exhalation 

valve that were tested in an outward position (with particles traveling in the direction of 

exhalation) have a wide range of penetration, emitting between <1% and 55%. Further testing 

could measure greater particle penetration.

Even without mitigation, FFRs with exhalation valves can reduce 0.35-μm MMAD particle 

emissions more consistently than surgical masks, procedure masks, cloth face coverings, or 

fabric from cotton t-shirts; however, the 0.35-μm MMAD particle emissions are not expected to 

be lower for every model.

With mitigations that cover the exhalation valve, 0.35-μm MMAD particle emissions can be 

limited to 5% if the filter media does not interfere with providing a good seal to the exhalation 

valve. The exhalation valves could be improved by establishing criteria for valve performance. 

Such criteria may be to maintain a plastic surface that is not obstructed with filter media or to 

have minimum and maximum outward leakage at given airflow conditions.

Covering an FFR with a surgical mask was not the most effective mitigation strategy evaluated 

here. The best mitigation strategy evaluated was to cover the exhalation valve inside the FFR 

with an ECG pad. Securing surgical tape over the exhalation valve inside the FFR performed 

nearly as well as using the ECG pad. Researchers intentionally selected products with no 

expected toxicological effects in relation to skin contact, inhalation, and ingestion.

FFRs with an exhalation valve provide respiratory protection to the wearer, and this study

demonstrates that they can also reduce 0.35-μm MMAD particle emissions to levels similar to or 

better than those provided by surgical masks and unregulated barrier face coverings. Based on 

these study results, modification of FFRs with an exhalation valve can further limit the emission 

of particles by the wearer, although such modifications should be carefully evaluated to conform 

to medical protocols and NIOSH Respirator Approval Program standards. During shortages of 

respiratory protection, FFRs with an exhalation valve provide an additional source of equipment 

for workers who need respiratory protection and are concerned about source control.
17

These results represent one of the first measurements of particle penetration through FFRs with 

an exhalation valve that are tested in an outward position, and the findings have important 

implications for guidance on source control and mitigation.

For Further Information

For further information on the use of personal protective equipment, please visit NPPTL’s 

“Contact Us” page.
18

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21

APPENDIX

Table A1. Particle penetration descriptive statistics

by mitigation and flowrate among the 13 FFR models

Mitigations Flowrate

Mean 

Particle 

Penetration 

(%)

Standard 

Deviation N

Minimum 

Mean 

Penetration 

Variability 

Among the 

13 Models

Maximum 

Mean 

Penetration 

Variability 

Among the 

13 Models

Outward 

position

25 lpm 16.03 10.22 75 1.36 27.92

55 lpm 31.03 14.77 75 5.33 53.16

85 lpm 31.45 12.05 75 8.43 44.68

Covered

25 lpm 0.83 1.80 75 0.03 6.54

55 lpm 1.58 2.96 75 0.06 10.73

85 lpm 1.80 2.82 75 0.11 10.43

Taped

25 lpm 2.98 2.58 75 0.26 9.13

55 lpm 4.69 4.18 75 0.88 16.25

85 lpm 4.74 4.03 75 1.10 15.96

Masked-over

25 lpm 11.91 8.00 75 0.85 25.48

55 lpm 24.21 11.18 75 4.85 40.53

85 lpm 23.42 8.54 75 7.07 34.74
22

Table A2. Particle penetration descriptive statistics

by model for the outward and inward position controls

Model Position N

Mean 

Particle 

Penetration

Standard 

Deviation

p-value 

difference

2001V

Outward 18 0.26 0.24

<0.001

Inward 18 24.39 16.93

2201V

Outward 18 0.06 0.05

<0.001

Inward 18 8.67 5.96

2310

Outward 18 0.27 0.14

<0.001

Inward 18 24.39 6.52

4030

Outward 12 0.08 0.07

<0.001

Inward 12 9.56 6.36

65059520A

Outward 18 1.27 1.17

0.09

Inward 18 5.14 4.04

710VOV

Outward 18 0.37 0.31

<0.001

Inward 18 38.58 9.25

8511

Outward 18 0.70 0.43

<0.001

Inward 18 35.75 9.83

9211

Outward 15 0.23 0.05

<0.001

Inward 15 40.60 12.98

9800V

Outward 18 1.95 1.44

<0.001

Inward 18 31.87 10.35

ONE-Fit NBW95V

Outward 18 1.53 1.13

<0.001

Inward 18 34.46 12.62

S-10479

Outward 18 0.70 0.44

<0.001

Inward 18 23.60 7.09

VICSA 2740V

Outward 18 0.56 0.19

<0.001

Inward 18 31.34 6.52

X-Plore 1750

Outward 18 0.63 0.78

<0.001

Inward 18 28.73 6.49
23

Table A3. Particle penetration descriptive statistics by flowrate

among each of the four non-FFR protective devices

Type Flowrate N

Mean 

Penetration

Standard 

Deviation Minimum Maximum 

Surgical mask

25 lpm 4 2.83 1.70 1.50 5.30

55 lpm 4 5.18 3.66 2.80 10.60

85 lpm 4 8.98 5.41 5.50 17.00

Procedure mask

25 lpm 7 19.86 24.45 1.00 64.90

55 lpm 7 26.87 26.00 2.50 62.60

85 lpm 7 38.40 35.05 4.80 84.80

Cloth face 

covering

25 lpm 8 69.25 12.20 45.00 84.10

55 lpm 8 74.36 7.86 56.80 81.10

85 lpm 8 83.15 7.83 67.10 91.30

Fabric from

cotton t-shirts

25 lpm 2 40.15 23.41 23.60 56.70

55 lpm 2 58.25 23.41 41.70 74.80

85 lpm 2 65.90 36.77 39.90 91.90
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DOI: https://doi.org/10.26616/NIOSHPUB2021107