Technical Report

Evaluation of Exhalation Resistance and Inspired 

Carbon Dioxide Concentration in Elastomeric Half-Mask 

Respirators with Modified or Covered Exhalation Valves

Centers for Disease Control

and Prevention

National Institute for Occupational 

Safety and Health
Technical Report

Evaluation of Exhalation Resistance and Inspired Carbon 

Dioxide Concentration in Elastomeric Half-Mask 

Respirators with Modified or Covered Exhalation Valves

DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health
ii

This document is in the public domain and may be freely 

copied or reprinted.

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Suggested Citation 

NIOSH [2022]. Evaluation of exhalation resistance and inspired carbon dioxide 

concentration in elastomeric half-mask respirators with modified or covered exhalation

valves. By Strickland KT, Fernando R, Schall J, Walbert G, Brannen 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. 2022-109. 

https://doi.org/10.26616/NIOSHPUB2022109

DOI: https://doi.org/10.26616/NIOSHPUB2022109

DHHS (NIOSH) Publication No. 2022-109

January 2022 

Cover photos by NIOSH
iii

TABLE OF CONTENTS

LIST OF FIGURES..........................................................................................................iv

LIST OF TABLES............................................................................................................iv

ACRONYMS AND ABBREVIATIONS ............................................................................. v

UNIT OF MEASURE ABBREVIATIONS ......................................................................... v

EXECUTIVE SUMMARY................................................................................................. 1

INTRODUCTION............................................................................................................. 3

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

Objective ......................................................................................................................... 7

METHODS ...................................................................................................................... 8

Elastomeric Half-Mask Respirator Configurations........................................................... 8

Respirator Test Conditions............................................................................................ 11

Control.................................................................................................................................. 12

Modified to Filter Exhaled Breath .................................................................................... 12

Covered with Level 1 Surgical Mask............................................................................... 13

Covered with Level 3 Surgical Mask............................................................................... 14

Exhalation Resistance................................................................................................... 15

Carbon Dioxide Concentration ...................................................................................... 15

RESULTS...................................................................................................................... 16

Exhalation Resistance................................................................................................... 16

Carbon Dioxide Concentration ...................................................................................... 18

DISCUSSION................................................................................................................ 20

Exhalation Resistance................................................................................................... 21

Carbon Dioxide Concentration ...................................................................................... 22

Limitations ..................................................................................................................... 24

CONCLUSION .............................................................................................................. 25

For Further Information ................................................................................................. 26

REFERENCES.............................................................................................................. 26
iv

LIST OF FIGURES

Figure 1. Contrasting interior designs of an elastomeric half-mask respirator and a 

valved filtering facepiece respirator................................................................................. 6

Figure 2. Interior of the 3M 6200 EHMR control and the 3M 6200 EHMR modified to 

filter exhaled breath....................................................................................................... 12

Figure 3. Inhalation and exhalation airflow directions as represented by

colored arrows in the 3M 6200 EHMR control and the 3M 6200 EMHR modified

to filter exhaled breath................................................................................................... 13

Figure 4. MSA Advantage 200 LS covered with Level 1 surgical mask donned

on a headform ............................................................................................................... 14

Figure 5. Honeywell RU85004M covered with Level 3 surgical mask donned

on a headform ............................................................................................................... 14

Figure 6. Plot comparing exhalation resistance of EHMR controls to EHMRs modified

to filter exhaled breath by removing the inhalation valve membrane and blocking the 

exhalation valve, directing the exhaled breath through the respirator filters.................. 17

Figure 7. Plot comparing exhalation resistance of control EHMRs, EHMRs covered

with Level 1 surgical mask, and EHMRs covered with Level 3 surgical mask............... 18

Figure 8. Plot comparing inspired CO2 levels of control EHMRs and EHMRs modified

to filter exhaled breath by removing the inhalation valve membrane and blocking the 

exhalation valve, directing the exhaled breath through the respirator filters.................. 19

Figure 9. Plot comparing inspired CO2 levels of control EHMRs, EHMRs covered

with Level 1 surgical mask, and EHMRs covered with Level 3 surgical mask............... 20

LIST OF TABLES

Table 1. Nine elastomeric half-mask respirator configurations used in this study ........... 9
v 

ACRONYMS AND ABBREVIATIONS

APR air-purifying respirator

CDC Centers for Disease Control and Prevention

CO2 carbon dioxide

EHMR elastomeric half-mask respirator

FFR filtering facepiece respirator

NIOSH National Institute for Occupational Safety and Health

OSHA Occupational Safety and Health Administration

PAPR powered air-purifying respirator

STP Standard Testing Procedure

UNIT OF MEASURE ABBREVIATIONS

hertz Hz

L/min liters per minute

μm micrometers

mg/m3 milligrams per cubic meter

mm millimeters

mmH2O millimeters of water

mmHg millimeters of mercury
1 

EXECUTIVE SUMMARY

Elastomeric half-mask respirators (EHMRs) and filtering facepiece respirators (FFRs), the most 

common filter class being “N95,” are both classified as air-purifying respirators. Unlike FFRs, 

which are not designed to be cleaned or disinfected, EHMRs are designed to be routinely 

cleaned, which could include disinfection, and maintained for longer-term use. Their reusability 

and durability to withstand disinfection makes EHMRs appealing for use in the healthcare 

industry. 

One design feature found in most commercially available EHMRs is an exhalation valve. 

Exhalation valves are intended to increase user comfort by reducing the breathing resistance 

during exhalation, and they may also reduce moisture and heat buildup in the respirator 

facepiece. During exhalation, when subjected to the positive pressure created in the respirator 

facepiece by the user’s exhaled breath, the exhalation valve opens and allows the exhaled breath 

to exhaust, unfiltered, into the environment. 

The degree of source control—i.e., the use of well-fitting cloth masks, facemasks, or respirators 

to cover a person’s mouth and nose to prevent the spread of respiratory secretions when they are 

breathing, talking, sneezing, or coughing—provided by EHMRs with exhalation valves is not 

known. As a result, healthcare workers who are wearing an EHMR with an exhalation valve and

infected with a contagious respiratory pathogen may unknowingly spread the pathogen to those 

around them via unfiltered breath exhausting from the exhalation valve. Current guidance from 

the Centers for Disease Control and Prevention (CDC) does not recommend using EHMRs with 

exhalation valves for source control. However, at the time of this study, the CDC recommended 

that if only a respirator with an exhalation valve was available, then the exhalation valve should 

be covered with a surgical mask, procedure mask, or a cloth face covering as long as it did not 

interfere with respirator fit. CDC guidance on covering exhalation valves continues to evolve as 

new information becomes available.

Modifying or covering the exhalation valve of an EHMR to improve its potential efficacy for 

source control may result in higher exhalation resistance or elevated concentrations of carbon 

dioxide gas (CO2) in the respirator facepiece, which may subsequently increase user discomfort 

and reduce user acceptance. This practice also voids the National Institute for Occupational 

Safety and Health (NIOSH) approval for the device, which does not permit the substitution, 

modification, addition, or omission of parts by the user. NIOSH, the federal institute that 

approves respirators in the United States, requires that exhalation resistance not exceed 25 

millimeters of water (mmH2O). While there is no respirator performance requirement related to 

inspired CO2 level for EHMRs, NIOSH has established a limit for other respirator types such as 

tight-fitting, powered air-purifying respirators with the blower unit off, for which the lowest of 

three donnings is not to exceed 2% inspired CO2 if intended use is for escape purposes. 

This study explores the impact of modifying or covering the exhalation valves of an EHMR on 

exhaled breathing resistance and inspired CO2. Two approaches were considered: (1) modifying 

EHMRs to filter the exhaled breath through the inhalation filters; (2) covering EHMR exhalation 

valves with surgical masks. Nine EHMR configurations approved by NIOSH were included in 

this study, where the “configurations” were the combination of a facepiece, filter, and other 

required components. These nine EHMR configurations were selected from three different 

respirator manufacturers and five different facepiece models. Four of the five facepiece models 
2

were assessed in both N95 and P100 filter configurations, and one facepiece model was assessed 

in a P100 filter configuration only. 

Exhalation resistance and inspired CO2 level were measured in each of the nine EHMR 

configurations under four study conditions, for a total of 72 measurements: 

(1) No EHMR modifications or coverings—experimental control.

(2) EHMR modified to filter exhaled breath through inhalation filters.

(3) EHMR exhalation valves covered with Level 1 surgical mask (per ASTM F2100).

(4) EHMR exhalation valves covered with Level 3 surgical mask (per ASTM F2100).

Both exhalation resistance and inspired CO2 level were measured in accordance with NIOSH 

Standard Testing Procedures. For the exhalation resistance measurement, three samples were 

each donned one time (n=3) on a manikin, which was made to simulate exhalation at a constant 

airflow rate of 85 L/min. The initial resistance was then measured. For the inspired CO2 level 

measurement, one sample was donned three times each (n=3) on a manikin, which was made to 

simulate breathing at a rate of 14.5 breaths/min and a tidal volume of 0.724 L. The simulated 

exhaled breath consisted of 5% CO2, and the inspired CO2 concentration was measured at 40 Hz 

and averaged over three respiratory cycles. 

Test results showed that all EHMR configurations under all test conditions met the NIOSH 

exhalation resistance performance requirement of not exceeding 25 mmH2O. Eight of nine 

configurations modified to filter the exhaled breath showed measurable increases in mean 

exhalation resistance compared to their respective controls, increasing from between 4.40 and 

9.48 mmH2O in the controls to between 8.21 and 18.5 mmH2O in the modified configurations. 

All configurations covered with Level 1 or Level 3 surgical masks also showed measurable 

increases in mean exhalation resistance compared to their respective controls, increasing from 

between 4.40 and 9.48 mmH2O in the controls to between 7.11 and 17.6 mmH2O in the covered

configurations. 

The lowest of the three inspired CO2 level measurements for all EHMR configurations under all 

test conditions was less than 2%, with one observation of one configuration modified to filter the 

exhaled breath exceeding 2%. All nine EHMR configurations modified to filter the exhaled 

breath showed measurable increases in mean inspired CO2 levels compared to their respective 

controls, increasing from between 0.47% and 1.03% in the controls to between 1.25% and 1.94% 

in the modified configurations, while the inspired CO2 levels of EHMRs covered with Level 1 or 

Level 3 surgical masks did not consistently increase across all configurations, changing from 

between 0.47% and 1.03% in the controls to 0.80% and 1.26% in the covered configurations. 

Findings indicate that EHMRs can meet the NIOSH exhalation resistance performance 

requirement after being (1) modified to filter the exhaled breath, (2) covered with a Level 1 

surgical mask, or (3) covered with a Level 3 surgical mask. Findings indicate that modifying 

EHMRs to filter the exhaled breath increases inspired CO2 levels, potentially increasing user 

discomfort but unlikely to cause serious physiological symptoms in healthy users, as all but one 

measurement was below 2%. Covering EHMR exhalation valves with surgical masks is unlikely 

to increase discomfort due to elevated inspired CO2 levels nor to cause serious physiological 

symptoms, as all measurements were well below 2%. 
3

NIOSH’s performance requirements provide an approval pathway for new EHMR designs 

without exhalation valves, which may be desirable when considering source control. In fact, 

while this study was in progress, several EHMR models without exhalation valves and an EHMR 

with a filtered exhalation valve accessory were approved. Since this study showed that 

modifying the EHMRs to filter the exhaled breath generally increased both exhalation resistance 

and inspired CO2 levels, and the magnitude of those increases varied among EHMR 

configurations, future research on the comfort and tolerability of new and modified EHMRs

without exhalation valves needs to inform and improve designs appropriate for use in healthcare. 

INTRODUCTION 

Elastomeric half-mask respirators (EHMRs) and filtering facepiece respirators (FFRs), the most 

common filter class being “N95,” are classified as air-purifying respirators (APRs) [Respiratory 

protection 2004], meaning that they remove contaminants from the air via an air-purifying 

element such as a filter, cartridge, or canister, providing filtered air to the user. The National 

Institute for Occupational Safety and Health (NIOSH) approves both FFRs and EHMRs and

evaluates each using similar tests, in accordance with minimum performance requirements

[NIOSH 1995]. The Occupational Safety and Health Administration (OSHA) considers the 

expected level of protection provided by both FFRs and EHMRs to be the same, in that it 

designates all half-mask (i.e., covering the nose and mouth) air-purifying respirators an Assigned 

Protection Factor1 of 10 [Respiratory protection 2004]. 

One characteristic that distinguishes EHMRs from FFRs is their reuse capability. Unlike FFRs, 

which are not designed to be cleaned or disinfected, EHMRs are designed to be routinely 

cleaned, which could include disinfection, and maintained for longer-term use. The reusability of 

EHMRs makes them advantageous for use in the healthcare industry, particularly as a tool for 

mitigating single-use respirator supply shortages created by public health emergencies [Patel et 

al. 2017]. The ability to withstand surface disinfection (i.e., removal and inactivation of many or 

all infectious materials) is beneficial for reducing surface transmission of infectious disease to 

healthcare workers from their personal protective equipment, especially during outbreaks. A U.S. 

federal government interagency report on improving protective equipment in healthcare 

identified these two factors—reusability and repeated disinfection durability—as high-priority, 

recommended features of respirators for healthcare workers [U.S. Department of Veterans 

Affairs 2009]. 

In addition to the advantages of reusability and repeated disinfection durability of EHMRs, users 

may also perceive a higher sense of protection against communicable diseases provided by 

EHMRs compared to FFRs [Hines et al. 2019]. Many EHMRs can also be equipped with airpurifying elements having greater filtration efficiency than N95s, such as P100 filters (≥ 99.97% 

versus ≥ 95%, respectively). Disadvantages of EHMR use in healthcare as compared to FFRs 

include attributes such as communication and comfort. Communication, both with patients and 

coworkers, was consistently rated more negatively in EHMRs compared to FFRs, as was 

comfort. Neither comfort nor communication issues were barriers, however, to EHMR 

1 OSHA defines the Assigned Protection Factor as “the workplace level of respiratory protection that a respirator or 

class of respirators is expected to provide employees when the employer implements a continuing, effective 

respiratory protection program.” Employers must use the Assigned Protection Factors in the OSHA standard 

1910.134 to select a respirator that “meets or exceeds the required level of employee protection.” 
4

acceptance among experienced EHMR users when perceived risk and protectiveness were 

included [Hines et al. 2019]. 

One comfort-related design feature prevalent in EHMRs is an exhalation valve. An exhalation 

valve is a mechanical assembly housed in the body of a respirator that opens when subjected to 

positive pressure, allowing the user’s exhaled breath to escape the respirator. An exhalation 

valve consists of a holder (frame), a membrane (a disc or flap) that sits on top of the holder, and 

a protective cover. An exhalation valve is intended to reduce the resistance that users experience 

when exhaling, and it may also reduce facial temperature and the buildup of moisture inside the 

respirator—three additional recommended characteristics of respirators intended for use in 

healthcare [U.S. Department of Veterans Affairs 2009]. Since a properly functioning exhalation 

valve remains closed under the negative pressure experienced during inhalation, its sole purpose 

is to improve wearability, and it does not contribute to the protection of the user against airborne 

contaminants. EHMRs and FFRs with exhalation valves provide the same level of protection to 

the user as those that do not have a valve [CDC 2020a]. In terms of regulatory requirements, 

NIOSH requires exhalation valves be provided where necessary, protected against damage and 

external influence, and designed and constructed to prevent inward leakage of contaminated air 

[Approval of respiratory protective devices 1995]. Though the improved comfort provided by 

EHMRs with exhalation valves may be important to users, their efficacy for source control—

which is the use of well-fitting cloth masks, facemasks, or respirators to cover a person’s mouth 

and nose to prevent the spread of respiratory secretions when they are breathing, talking, 

sneezing, or coughing [CDC 2021b]—is not known, nor does NIOSH evaluate or approve 

respirators for the purposes of providing source control. 

Healthcare workers who are wearing an EHMR with an exhalation valve, and who may be 

infected with a contagious pathogen, may unknowingly spread the pathogen to those around 

them via unfiltered breath exhausting from the exhalation valve. The level of spread depends on 

the disease transmission routes [Jones and Brosseau 2015] as well as factors such as the 

concentration of infectious materials on particles of different sizes; proximity between people; 

amount of time spent among people in close proximity; and environmental conditions such as 

temperature, humidity, and air circulation. Particles of all sizes may be emitted by both 

symptomatic and asymptomatic people through actions such as sneezing, coughing, singing, 

talking, and breathing [CDC 2020b].

In response to the concerns regarding asymptomatic and pre-symptomatic disease transmission, 

the Centers for Disease Control and Prevention (CDC) does not recommend wearing EHMRs

with exhalation valves when source control is desired [CDC 2021c] and specifies that “until 

more is understood on exhalation valves, elastomeric respirators with unfiltered exhalation 

valves should not be used as source control in surgical and other healthcare settings due to 

concerns that air coming out of the exhalation valve may contaminate the sterile field.”2

However, at the time of this study, the CDC recommendation on using respirators with 

exhalation valves was that if they were the only available option and source control was needed, 

then the exhalation valve should be covered with a surgical mask, procedure mask, or a cloth 

mask without interfering with the respirator fit. This recommendation drove the practice among 

2 Although “air” was used in the cited source (CDC 2021c), in this report “breath” will be used with the 

understanding that it is synonymous with “air” in relation to the context of it being expelled by a wearer through an 

exhalation valve.
5

healthcare workers in the workplace to cover respirators with exhalation valves by using the 

above recommended mask types when source control was desired. 

Background

In a previous study, NIOSH evaluated the potential source control, by measuring outward 

filtration efficiency, provided by FFRs with exhalation valves (1) covered with a surgical mask, 

(2) blocked with an electrocardiogram (ECG) pad, or (3) blocked with surgical tape. In the same

study, NIOSH also evaluated the outward particle penetration of FFRs with exhalation valves

without implementing any mitigation strategies (i.e., the exhalation valve was not blocked or

covered) [NIOSH 2020b].

Findings from this NIOSH study indicate that FFRs with exhalation valves allow between <1% 

and 55% of particles to penetrate out of the respirator, with a median penetration value of 31% at 

a challenge aerosol flow rate of 85 L/min [NIOSH 2020b]. This study resulted in updated 

guidance for FFRs with exhalation valves, posted on April 9, 2021, suggesting that, in most nonmedical settings, wearing a NIOSH-approved N95 FFR with an exhalation valve to provide 

source control is acceptable [CDC 2021c]. 

NIOSH found that covering the outside of the FFR exhalation valve with a surgical mask, per the 

previous CDC recommendation, reduced the median penetration value from 31% to 23%, which 

was the least effective of three mitigation strategies assessed. Blocking the exhalation valve from 

inside the respirator, either with micropore surgical tape (5% median outward penetration) or 

non-porous ECG pads (2% median outward penetration) was more effective at mitigating 

outward penetration of particles. This study was conducted on a limited selection of 13 FFR 

models, not EHMRs, with a charge neutralized sodium chloride aerosol with a mass median 

aerodynamic diameter of 0.35 μm, which are considered the most penetrating particle 

characteristics for typical filter media. It was not conducted on droplets or large particles, for 

which penetration values would likely be lower. Additionally, the penetration values were 

measured using an automated filter test instrument at constant flow rates and high aerosol 

concentrations (up to 200 mg/m3

), unlike the cyclical breathing patterns—for which penetration 

values would generally be lower—and likely lower concentrations of particles in the exhaled 

breath expelled by actual respirator users. 

Due to design differences between EHMRs and FFRs, exhaled particle penetration out of 

EHMRs may be higher in comparison to exhaled particle penetration out of FFRs with 

exhalation valves. Figure 1 depicts the facepiece interior of an example EHMR (Figure 1, left) 

and an example valved FFR (Figure 1, right). The facepiece forms a protective barrier between 

the user’s respiratory tract and the external, potentially contaminated, environment. In the case of 

both EHMRs and FFRs, the facepiece is a half-mask facepiece, meaning it covers the user’s nose 

and mouth and seals under the user’s chin. While the FFR facepiece is entirely composed of 

filter media and without replaceable parts, the EHMR facepiece is composed of silicon or other 

elastomeric material and configurable with replaceable air-purifying elements (e.g., filters or 

cartridges) that affix onto the facepiece to form a complete respirator. Most commercially 

available EHMRs have inhalation valves in addition to exhalation valves. The inhalation valves 

open when subjected to negative pressure (inhalation) and remain closed under positive pressure 

(exhalation), while the exhalation valve remains closed under negative pressure and opens under 

positive pressure. Consequently, during the exhalation phase of the respiratory cycle, the 
6 

exhalation valve is the pathway of least resistance for the particle-laden secretions in the exhaled 

breath to travel, and the inhalation valves prevent the particle-laden secretions in the exhaled 

breath from flowing through the filters. 

This EHMR design contrasts with the design of valved FFRs, which have a single exhalation 

valve surrounded by filter media, resulting in a portion of the exhaled breath exhausting through 

the filter media in parallel with the exhalation valve. This FFR design characteristic explains the

unmitigated median outward penetration value found by NIOSH of 31%, equating to a filtration 

efficiency of 69% [NIOSH 2020b], indicating that the tested FFRs with exhalation valves 

reduced particle emissions out of the respirator by a median value of 69%. One other factor, 

present in both FFRs and EHMRs, is the exhalation valves themselves, which may block or 

deflect particles via the valve membrane or the protective covering. The contribution of the 

exhalation valves alone to providing source control is not known but is likely to be dependent on 

particle size.

Photos by NIOSH

Figure 1. Contrasting interior designs of an elastomeric half-mask respirator (left) and a

valved filtering facepiece respirator (right). The elastomeric half-mask respirator is the

MSA Advantage 200 LS with two inhalation valves and one exhalation valve. The filtering

facepiece respirator is the 3M 8511 with a single exhalation valve embedded in the filter media.

When blocking the exhalation valve from inside the FFR with a non-porous ECG pad, the 

challenge aerosol generated from inside the respirator was prevented from exhausting unfiltered 

out of the exhalation valve. It was instead redirected through the filter media, reducing particle 

emissions out of the FFRs by a median value of 98%. A measured filtration efficiency of 98% is

comparable to the 95% NIOSH minimum inward filtration efficiency requirement for N95 FFRs 

[Approval of respiratory protective devices 1995]. 

Due to the aforementioned design differences between EHMRs and FFRs, blocking an EHMR 

exhalation valve may negatively affect the comfort of respirator users to a greater extent than 

blocking the exhalation valve of an FFR. Moreover, modifying any type of respirator, including 

the addition of an external covering, may impact the respirator’s NIOSH approval. 
7

By definition, NIOSH approval is issued for individual respirator assemblies that have been 

examined, inspected, tested, and determined to have met general construction and minimum 

performance requirements [Approval of respiratory protective devices 1995]; therefore, any 

substitution, modification, addition, or omission of parts by the user [NIOSH 2020c] voids the 

NIOSH approval. Because OSHA requires that NIOSH-approved respirators be used in 

workplaces that are part of an OSHA respiratory protection program, and that these respirators 

be used in accordance with the conditions of its approval, modifying respirators may also result 

in a noncompliance with OSHA’s respiratory protection standard [Respiratory protection 2004]. 

An approval holder, or entity that designs, manufactures, assembles, or controls a NIOSHapproved respirator, may modify or change features of existing respirators with NIOSH

approval, which involves meeting a set of minimum performance requirements. For EHMRs, the 

NIOSH minimum performance requirements include filtration efficiency, inhalation and 

exhalation resistance, and exhalation valve leakage [Approval of respiratory protective devices 

1995]. Of these performance requirements, exhalation resistance may be most affected when 

modifying EHMRs to improve potential source control. Higher exhalation resistance may result 

in higher discomfort for the user and subsequent lower acceptance, as well as increased 

physiological burden [NIOSH 1995]. 

During normal EHMR usage, most of the carbon dioxide gas (CO2) contained in the user’s 

exhaled breath exits through the exhalation valve into the external environment. The residual 

exhaled CO2 remains within the facepiece and is rebreathed during inhalation. Attempting to 

modify EHMRs to filter the exhaled breath may result in increased “dead space,” or stagnant air 

where gas exchange is not taking place, in the facepiece. Increased dead space may lead to 

higher concentrations of CO2 lingering in the facepiece and being rebreathed by the user. The 

inhalation toxicity level of CO2 is minimal, with typically no effects at less than or equal to 1% 

inspired CO2 [ESHG n.d]. However, elevated levels cause increases in respiration rate and tidal 

volume, and concentrations greater than 2% can lead to potential discomfort for users and 

possibly physiological symptoms such as fatigue, dizziness, headache, and shortness of breath, 

among others [Kloos and Lamonica 1966; Roberge et al. 2010]. The accumulation of CO2 inside 

the respirator facepiece is typically not a NIOSH performance requirement for half-mask 

respirators. However, the accumulation of CO2 may become a concern when modifying or 

obstructing the exhalation valve, and respirators submitted for NIOSH approval must be 

designed on sound engineering and scientific principles and not create safety issues for the user. 

Additionally, respirator approval schemes in other countries and regions (e.g., China, Europe) 

assess inspired CO2 with varying test conditions and permit a maximum of 1% inspired CO2. 

Objective

This study explores two approaches for modifying or covering the exhalation valves of EHMRs:

(1) modifying EHMRs to filter the exhaled breath by removing the inhalation valve

membranes and blocking the exhalation valve;

(2) covering EHMR exhalation valves with surgical masks, per the CDC recommendation at

the time of this study.

This study evaluated the effects of these strategies on two measures of respirator performance in 

nine NIOSH-approved EHMR configurations, as follows: 
8 

• Measuring the exhalation resistance of EHMRs covered with surgical masks and

EHMRs modified to filter exhaled breath.

• Measuring inspired CO2 levels of EHMRs covered with surgical masks and EHMRs

modified to filter the exhaled breath.

METHODS 

Elastomeric Half-Mask Respirator Configurations

Table 1 lists the manufacturers, model/part numbers, and NIOSH approval numbers for the nine 

EHMR configurations chosen for this study, based on their reported use by large healthcare

systems and variability between exhalation valve designs.3

Often, the same facepiece component of a NIOSH-approved EHMR is approved for use with 

different types of air-purifying elements, including chemical cartridges or canisters, particulate 

filters, and combination cartridges (i.e., gas/vapor and particulate). This study focused on two 

levels of particulate protection—N95 and P100—as these are commonly used in healthcare [The 

National Academies of Sciences, Engineering, and Medicine Consensus Study Report 2018]. 

Together, a facepiece, a filter or filters, and other required components constitute an EHMR 

“configuration.” All EHMRs evaluated in this study were designated as medium-sized by the 

respective manufacturers. All EHMRs evaluated in this study were new (unused). The nine 

configurations in this study were selected from three different respirator manufacturers and five 

different facepiece models, as follows: 

• The 3M 6200 and 3M 7502 (3M Company, St. Paul, MN) were tested with both N95 and

P100 filter configurations.

• The MSA Advantage 200 LS (Mine Safety Appliances, Pittsburgh, PA) was tested only

in the P100 configuration. This approach was taken because the MSA Advantage 200 LS

did not have a NIOSH-approved N95 filter configuration without additional chemical

protections at the time of this testing.

• The Honeywell North RU85001M and RU85004M (Honeywell International Inc.,

Charlotte, NC) were tested in both N95 and P100 filter configurations.

3 Note that none of the EHMR configurations in this study are cleared by the U.S. Food and Drug Administration for 

marketing as medical devices.
9 

Table 1. Nine elastomeric half-mask respirator configurations used in this study

Manufacturer

and part (part #)

NIOSH testing 

and certification 

(TC-) approval 

number

Photos of respirator configurations tested, 

showing the front view (left) and side view 

(right). Photos by NIOSH.

3M 6200 with N95 

filter (#5N11), filter 

adapter (#603), 

and filter retainer 

(#501) 

84A-0376

3M 6200 with P100 

filter (#7093) 84A-0071

3M 7502 with N95 

filter (#5N11), filter 

adapter (#603), 

and filter retainer 

(#501)

84A-0376

3M 7502 with P100 

filter (#7093) 84A-0071

Honeywell North 

RU85001M* with 

P100 filter 

cartridges 

(#7580P100) and 

filter cover 

(#N750029)

84A-7529
10 

Table 1 (continued). Nine elastomeric half-mask respirator configurations used in this study

Manufacturer 

and part (part #)

NIOSH testing 

and certification 

(TC-) approval 

number

Photos of respirator configurations tested, 

showing the front view (left) and side view 

(right). Photos by NIOSH.

Honeywell North 

RU85001M* with 

N95 filter 

(#7506N95),

filter holder 

(#N750015),

and filter cover 

(#N750027)

84A-7542

Honeywell North 

RU85004M* with 

P100 filter 

cartridges 

(#7580P100)

and filter cover 

(#N750029)

84A-7529

Honeywell North 

RU85004M* with 

N95 filter 

(#7506N95),

filter holder 

(#N750015), and 

filter cover 

(#N750027)

84A-7542

MSA Advantage 

200 LS 

(#815444) with

low-profile P100 

cartridges 

(#815369)

84A-5420

* The holed black piece positioned between the filters of the Honeywell EHMRs is a speech diaphragm, which is a

device used to improve speech intelligibility. It is not an exhalation valve and was not modified for this study.
11

Respirator Test Conditions

Performance of all nine EHMR configurations was assessed under one control and three

experimental conditions, designated and discussed below as follows: (1) control, (2) modified to 

filter exhaled breath through inhalation filters, (3) covered with Level 1 surgical mask, and (4) 

covered with Level 3 surgical mask. 

The combination of the nine EHMR configurations, four test conditions, and two measurements 

of exhalation resistance and inspired CO2 levels (with three replicates per measurement) resulted 

in 216 observations. For each configuration, one control (or set of controls, depending on the test 

requirements) served as the control for all three experimental conditions. 

Considering the CDC recommendation at the time of this study to cover respirators with 

exhalation valves with a surgical mask, procedure mask, or a cloth face covering when source 

control was desired, surgical masks were ultimately chosen for this study to represent the 

covering that may create more exhalation resistance than other types of coverings, resulting from 

their higher filtration efficiency. In a previous study, NIOSH found particle penetration ranges of 

2% to 17% for surgical masks, 1% to 85% for procedure masks, and 24% to 92% for cloth face 

coverings when tested against a sodium chloride aerosol with a mass median aerodynamic 

diameter of 0.35 μm, which is the most penetrating particle size for filters, at a flow rate of 85 

L/min [NIOSH 2020b]. Additionally, surgical masks were selected for this study due to their 

behind-the-head ties, which allow for more precise positioning over the EHMR exhalation valves 

and can be tightened for better fit; by comparison, procedure masks typically have ear loops for 

easier donning and doffing but there is less control of positioning over the EHMR exhalation 

valve and an inability to tighten for fit [ASTM 2021]. Cloth face coverings were not included in 

this study because of their wide range of materials, varying head suspension designs, and lack of 

applicable regulatory standards. 

Both Level 1 and Level 3 masks were chosen for study due to the different levels of resistance to 

penetration of synthetic blood, filtration efficiencies, and differential pressures, as specified by 

ASTM International [ASTM 2021]4

. It was expected that the higher liquid resistance, filtration 

efficiency, and differential pressure characteristics of Level 3 masks compared to Level 1 masks

might result in higher exhalation resistance when applied as a covering over an EHMR 

exhalation valve. Level 1 masks are resistant to the penetration synthetic blood at 80 millimeters 

of mercury (mmHg) (~1088 mmH2O), and Level 3 masks are resistant at 160 mmHg (~2175 

mmH2O) [ASTM 2017]. Level 1 masks also have both a bacterial filtration efficiency and latex 

sphere-based particulate filtration efficiency of ≥ 95%, whereas both filtration efficiency 

requirements are ≥ 98% for Level 3 masks. Finally, ASTM F2100 specifies slightly lower

differential pressure requirements for Level 1 than Level 3 masks, limiting the breathing 

resistance of Level 1 masks to 5.0 mm H2O/cm2 for 4.9 cm2 swatches subjected to 8 L/min 

airflow (corresponding to 24.5 mm H2O at a face velocity of 27.2 cm/s) and the resistance of 

Level 3 masks to 6.0 mm H2O/cm2

 (corresponding to 29.4 mm H2O) under the same swatch size 

and airflow conditions.

4 ASTM F2100 classifies all medical masks that are used in providing healthcare services such as surgery and 

patient care, which includes medical masks with ties (surgical masks) and ear loops (procedure masks or isolation 

masks).
12

Control

The control respirators were unmodified (Figure 2, left) NIOSH-approved configurations. 

Controls were commercially available respirators purchased off-the-shelf and assembled per the 

manufacturers’ user instructions. In the controls, during inhalation, the inhalation valves open 

and allow the ambient air to enter through the respirator filters, while the exhalation valve 

remains closed (Figure 3, left). During exhalation, the exhalation valve opens to allow the 

exhaled breath to escape, unfiltered, into the environment, while the inhalation valves remain 

closed, preventing the exhaled breath from flowing through the respirator filters.

Modified to Filter Exhaled Breath

As shown in Figure 2, right, EHMRs were modified to filter the exhaled breath. The inhalation 

valve membranes, or flaps, were removed, and the exhalation valve was blocked with a single 

layer of off-the-shelf duct tape from the inside, sealing along the edges of the valve port, 

blocking air discharge through the exhalation valve. The respirator filters themselves were 

unmodified. This modification allowed the inhaled breath to enter the facepiece through the 

respirator filters and the now-empty inhalation valve port from the environment (Figure 3, right), 

providing filtered air to the user, while directing the exhaled breath back through the empty port, 

then through the same filters that were used during inhalation, and out into the environment, 

again filtered. 

Photos by NIOSH

Figure 2. Interior of the 3M 6200 EHMR control (left) and the 3M 6200 EHMR modified to filter

exhaled breath (right). Modifications involved removing the inhalation valve membranes and

blocking the exhalation valve with duct tape. In the photos, the facepiece sealing areas of

the respirators were folded back to show full interior views, including the inhalation valves.
13

Figure 3. Inhalation and exhalation airflow directions as represented by colored

arrows in the 3M 6200 EHMR control (left) and the 3M 6200 EMHR modified to filter 

exhaled breath (right). Airflow modifications were achieved by removing the

inhalation valves and blocking the exhalation valve as shown in Figure 2.

Covered with Level 1 Surgical Mask

As shown in Figure 4 as one example, the nine EHMRs with exhalation valves were covered by 

a Halyard Fluidshield Level 1 surgical mask (#28806) (per ASTM F2100), but otherwise the 

respirators were unmodified from their control condition. 

The inhalation valve membranes remained in place and the exhalation valve was not blocked. 

After the EHMRs were donned, the surgical mask was donned over the EHMR without 

interfering with respirator fit. The center of the mask was aligned with the exhalation valve, and

the surgical mask ties were secured with the top tie behind the head and the bottom tie behind the 

neck. The ties were secured tight enough so that the surgical mask snugly covered the exhalation 

valve.

When covering the EHMR exhalation valve with a Level 1 surgical mask, the inhalation airflow 

entered through the inhalation valves, which could open normally. The exhalation airflow exited 

the respirator through the open exhalation valve and out of the respirator before being deflected 

by the surgical mask or flowing through it. 

Because of differences in exhalation valve positioning and exhalation valve protective covering 

design among the respirator models, surgical masks were positioned differently on each model, 

but were consistently “centered” over each model’s exhalation valve. Due to the loose fit of the 

surgical masks and the at-most partial coverage of the inhalation filters, it was not anticipated 

that the inhalation airflow directions or resistance was impacted by the surgical mask.
14

Covered with Level 3 Surgical Mask

As shown in Figure 5 as one example, the nine EHMRs with exhalation valves were covered by 

a Halyard Fluidshield Level 3 surgical mask (#48207) (per ASTM F2100), but otherwise the 

respirators were unmodified from their control condition. 

The inhalation valve membranes remained intact and the exhalation valve was not blocked. The 

procedure for donning Level 3 surgical masks was the same as the procedure for Level 1 surgical 

masks, described above, as were the inhalation and exhalation airflow directions. 

Photos by NIOSH

Figure 4. MSA Advantage 200 LS covered with Level 1 surgical mask donned on a

headform. The MSA Advantage 200 LS exhalation valve is positioned on the front of

the facepiece and frontward facing. The center of the surgical mask is aligned over

the exhalation valve and results in partial coverage of the filters, but this partial

coverage was assumed not to affect inhalation resistance or airflow direction. 

Photos by NIOSH

Figure 5. Honeywell RU85004M covered with Level 3 surgical mask donned on

a headform. The Honeywell RU85004M exhalation valve is positioned on the lower

part of the facepiece and is downward facing. The center of the surgical mask

is aligned over the exhalation valve and does not result in coverage of the filters.
15

Exhalation Resistance

The exhalation resistance test was conducted per NIOSH Standard Testing Procedure (STP) 

TEB-APR-STP-0003 [NIOSH 2019]. A source of compressed air was set to a volumetric flow 

rate of 85 L/min using a Brooks Instrument Co. 5853S mass flow controller (Brooks Instrument, 

Hatfield, PA) with a Brooks Control and Read-out Unit model 0154. The metered compressed air 

source was supplied to an anthropometric headform so that the headform simulated exhalation at 

a constant flow rate of 85 L/min, comparable to a high exertion level in a healthy individual. The 

resistance was measured using a Setra Datum 2000 Model 239 digital manometer 

(Setra Systems, Inc, Boxborough, MA), which was connected to a pressure tap on a line between 

the mass flow controller and the headform. 

Respirators were donned on the headform and the straps were tightened enough to ensure the 

respirator was maintaining contact around the sealing area but not tight enough to deform 

volume inside the respirator. The 85 L/min of compressed air was supplied to the headform and 

the resistance was measured and recorded. This process was repeated for three respirator samples

of each configuration, per STP TEB-APR-STP-003, for a total of three replicates. Each of the 

three samples were donned and doffed once consecutively to account for variability in exhalation 

resistance measurements due to donning. Further details, including specific test equipment, can 

be found in the STP. The pass-fail requirements of STP TEB-APR-STP-003 specify that, upon 

initial exhalation, the exhalation resistance of EHMRs should not exceed 25 mmH2O for any of 

the three samples.

Carbon Dioxide Concentration

The Determination of Facepiece Carbon-Dioxide and Oxygen Concentration Levels— 

Tight-Fitting, Powered Air-Purifying Respirators, With the Blower Unit Off test was conducted 

per STP RCT-APR-STP-0064 [NIOSH 2020a], but only CO2 data was collected. This test

typically assesses other types of APRs such as tight-fitting powered air-purifying respirators 

(PAPRs) with the blower unit off to allow for escape from a hazardous atmosphere during 

blower failure. STP RCT-APR-STP-0064 was used for this study as the most appropriate STP 

for assessing inspired CO2 level in non-powered, half-mask APRs. Breathing was simulated in a 

headform using a NIOSH mechanical breathing machine. The breathing frequency was set to 

14.5 breaths/min and the tidal volume to 0.724 L, equivalent to a minute ventilation of 10.5 

L/min, a sedentary exertion level in healthy individuals. During the exhalation phase of the 

respiratory cycle, the headform was made to simulate exhalation of non-humidified, breathing 

quality air containing 5% CO2 using a series of supplied gas sources, mass flow controllers, a 

mixing chamber, one-way valves, and a solenoid valve. 

Prior to a respirator being donned on the headform, the test was run to determine a “blank” 

(i.e., inspired CO2 levels without a respirator donned), and the concentration of CO2 was 

measured on the inhalation phase of the respiratory cycle. A minimum of three respiratory cycles 

were run, which was previously determined to be the minimum number of cycles required to 

provide a valid CO2 level for each donning based on the frequency and accuracy of the CO2

analyzer. Inspired CO2 concentration was measured at the mouth of the headform using a breathresponse CO2 analyzer recording at a frequency of 40 Hz. The inspired CO2 profile was allowed 

to come to a steady state, and then mean percent inspired CO2 was arithmetically averaged over 

the course of the three respiratory cycles.
16

For this test, the respirators were donned and the straps were tightened enough to ensure the 

respirator was maintaining contact around the sealing area but not tight enough to deform the 

volume inside the respirator. Again, the inspired CO2 profile was allowed to come to a steady 

state, and then the inspired CO2 concentration was measured over the course of a minimum of 

three respiratory cycles. The CO2 concentrations of the “blank” were subtracted from the results 

with the respirator donned, and the result was the inspired CO2 level. This test was repeated three 

times with one respirator sample of each configuration being donned, doffed, and re-donned 

three times in succession, for a total of three replicates, to account for variability in inspired CO2

concentrations due to donning. Further details, including specific test equipment, can be found in 

the STP. Since EHMRs are not typically assessed with this test during the NIOSH approval 

process, there is no pass-fail requirement for this respirator class. However, for other types of 

APRs such as tight-fitting PAPRs with the blower unit off used for escape purposes, the lowest 

of the three inspired CO2 level measurements should not exceed 2% [NIOSH 2020a]. This 2% 

pass/fail performance requirement was used in the context of this study as well. 

RESULTS 

Exhalation Resistance

Figure 6 displays exhalation resistance results for the following two test conditions: (1) control 

and (2) modified to filter exhaled breath. The following results are noteworthy from the figure:

• All nine EHMR configurations modified to filter the exhaled breath met the NIOSH

pass/fail performance requirement of not exceeding 25 mmH2O exhalation resistance at

85 L/min.

• Eight of the nine EHMR configurations modified to filter the exhaled breath displayed

increases in mean exhalation resistance values when compared to their respective control

configurations.

• When considering the eight EHMR configurations tested in both N95 and P100 filter

configurations, increases in mean exhalation resistance varied by respirator model and

filter type, with the P100 filter configurations increasing from between 4.49 and 8.13

mmH2O in the controls to between 13.4 and 18.5 mmH2O in the modified configurations.

The mean exhalation resistance of the N95 filter configurations increased from between

4.40 and 8.30 mmH2O in the controls to between 9.14 and 11.2 mmH2O in the modified

configurations.

• Of the four facepiece models tested in both N95 and P100 filter configurations, all of the

P100 filter configurations modified to filter exhaled breath measured higher exhalation

resistance than their modified N95 filter counterparts. The mean exhalation resistance of

these four modified EHMRs with P100 filters increased by at least 8 mmH2O, while the

resistance of the modified EHMRs with N95 filters increased by less than 5 mmH2O.
17 

Figure 6. Plot comparing exhalation resistance of EHMR controls to EHMRs modified to 

filter exhaled breath by removing the inhalation valve membrane and blocking the exhalation

valve, directing the exhaled breath through the respirator filters. Vertical bars represent the 

range of observations (n=3) and the circles or X’s within the vertical bars represent the

means for the control and modified to filter exhaled breath test conditions, respectively. 

Figure 7 displays the exhalation resistance results for the following three test conditions: (1) 

control, (2) covered with Level 1 surgical mask, and (3) covered with Level 3 surgical mask. The 

following results are noteworthy from the figure: 

• All EHMR configurations covered with Level 1 or Level 3 surgical masks met the

NIOSH pass/fail performance requirement of not exceeding 25 mmH2O exhalation

resistance at 85 L/min.

• All EHMR configurations covered with Level 1 or Level 3 surgical masks displayed

increases in mean exhalation resistance values when compared to the control

configurations.

• Increases in mean exhalation resistance varied by respirator configuration, with EHMRs

covered with surgical masks increasing from between 4.40 and 9.48 mmH2O in the

controls to between 7.11 and 17.6 mmH2O in the covered with Level 1 surgical mask

configurations and to between 7.37 and 16.2 mmH2O in the covered with Level 3 surgical

mask configurations.

Control Modified to filter exhaled breath

Pass

Elastomeric half-mask respirator - filter type

Exhalation resistance, mmH20

Fail

3M 6200 - 3M 6200 - 3M 7502 - 3M 7502 - Honeywell Honeywell Honeywell Honeywell MSA

N95 P100 N95 P100 RU85001 - RU85001 - RU85004 - RU85004 - Advantage

N95 P100 N95 P100 200 LS -

P100

30

25

20

15

10

5

0

30

25

20

15

10

5

0

×
18 

Figure 7. Plot comparing exhalation resistance of control EHMRs, EHMRs covered with Level 1 surgical mask,

and EHMRs covered with Level 3 surgical mask. The vertical bars represent the range of observations (n=3)

and the circles, triangles, or plus signs within the vertical bars represent the means for the control, covered

with Level 1 surgical mask, and covered with Level 3 surgical mask test conditions, respectively. 

Carbon Dioxide Concentration

Figure 8 displays inspired CO2 levels for the following two test conditions: (1) control and (2) 

modified to filter exhaled breath. The following results are noteworthy from the figure:

• The lowest of the three inspired CO2 level measurements of all EHMR configurations

modified to filter the exhaled breath were less than 2%, meaning all EHMR

configurations modified to filter the exhaled breath met the pass/fail performance

requirement for this study.

• All EHMR configurations modified to filter exhaled breath showed increases in mean

inspired CO2 values when compared to the controls.

• The degree of increase in mean inspired CO2 levels varied by model but did not appear to

vary by filter type, with N95 filter configurations increasing from between 0.65% and

0.97% inspired CO2 in the controls to between 1.25% and 1.94% in the modified

configurations. The mean inspired CO2 levels of the P100 filter configurations increased

from between 0.47% and 1.03% inspired CO2 in the controls to between 1.36% and

1.77% inspired CO in the modified configurations.

Control Covered with Level 1 surgical mask

Pass

Elastomeric half-mask respirator - filter type

Exhalation resistance, mmH20

Fail

3M 6200 - 3M 6200 - 3M 7502 - 3M 7502 - Honeywell Honeywell Honeywell Honeywell MSA

N95 P100 N95 P100 RU85001 - RU85001 - RU85004 - RU85004 - Advantage

N95 P100 N95 P100 200 LS -

P100

Covered with Level 3 surgical mask

30

25

20

15

10

5

0

30

25

20

15

10

5

0

+
19 

Figure 8. Plot comparing inspired CO2 levels of control EHMRs and EHMRs modified to filter exhaled breath

by removing the inhalation valve membrane and blocking the exhalation valve, directing the exhaled breath

through the respirator filters. The vertical bars represent the range of observations (n=3) and the circles or X’s

within the vertical bars represent the means for the control and modified to filter the exhaled breath test

conditions, respectively. The dotted reference line at 2% inspired CO2 represents the limit for the evaluation, 

and only one of the three donnings must measure an inspired CO2 level below 2% to “pass” in the context

of this study. As shown in this figure, the Honeywell RU85001 with P100 filters modified to filter the exhaled

breath would be considered “passing,” since the bottom of the vertical bar is below 2%.

Figure 9 displays the inspired CO2 levels for the following three test conditions: (1) control, 

(2) covered with Level 1 surgical mask, and (3) covered with Level 3 surgical mask. The

following results are noteworthy from the figure:

• The lowest of the three inspired CO2 level measurements of all EHMR configurations

covered with a Level 1 or Level 3 surgical mask were less than 2%, meaning all EHMR

configurations covered with a Level 1 or Level 3 surgical mask met the pass/fail

performance requirement for this study.

• Six of the nine EHMR configurations covered with Level 1 surgical masks showed

increases in mean inspired CO2 levels values when compared to the controls.

• Six of the nine EHMR configurations covered with Level 3 surgical masks showed

increases in mean inspired CO2 levels values when compared to the controls.

• Differences in inspired CO2 levels varied by respirator configuration but did not appear to

vary by filter type nor surgical mask type, changing from between 0.47% and 1.03%

inspired CO2 in the controls to between 0.80% and 1.26% inspired CO2 in the covered

Inspired CO2, %

Elastomeric half-mask respirator – filter type

3M 6200 - 3M 6200 - 3M 7502 - 3M 7502 - Honeywell Honeywell Honeywell Honeywell MSA

N95 P100 N95 P100 RU85001 - RU85001 - RU85004 - RU85004 - Advantage

N95 P100 N95 P100 200 LS -

P100

Control Modified to filter exhaled breath

Fail

Pass

2.5

2.0

1.5

1.0

0.5

0.0

×
20 

with Level 1 surgical mask configurations and to between 0.83% and 1.18% in the 

covered with Level 3 surgical mask configurations. 

Figure 9. Plot comparing inspired CO2 levels of control EHMRs, EHMRs covered with Level 1 surgical mask,

and EHMRs covered with Level 3 surgical mask. The vertical bars represent the range of observations (n=3)

and the circles, triangles, or plus signs within the vertical bars represent the means for the control, covered

with Level 1 surgical mask, and covered with Level 3 surgical mask test conditions, respectively. The dotted

reference line at 2% inspired CO2 represents the limit for the evaluation in the context of this study.

DISCUSSION 

Previous research conducted on FFRs with exhalation valves reported the median outward 

particle penetration of 31% when tested against particles with most penetrating size and charge 

characteristics [NIOSH 2020b]. This research assessed mitigation strategies of covering the 

exhalation valve with a surgical mask and blocking the exhalation valve from the inside with a 

non-porous ECG pad, reducing these median penetration values to 23% and 2%, respectively. 

Due to the design differences between FFRs and EHMRs, particle penetration out of EHMRs 

may be higher than for FFRs. The presence of inhalation valves in EHMRs prevents exhaled 

breath from exiting through the respirator filters and instead forces the exhaled breath through 

the exhalation valve, unfiltered, into the environment—this as opposed to FFRs, where the 

exhalation valve is surrounded by filter media, allowing for filtration of a portion (median value 

of 69% as found in the above study) of the exhaled particles without the implementation of any 

mitigation strategies. The presence of inhalation valves in EHMRs precludes the filtration of 

Elastomeric half-mask respirator ̶ filter type

Inspired CO2, %

3M 6200 - 3M 6200 - 3M 7502 - 3M 7502 - Honeywell Honeywell Honeywell Honeywell MSA

 N95 P100 N95 P100 RU85001 - RU85001 - RU85004 - RU85004 - Advantage

 N95 P100 N95 P100 200 LS -

 P100

Control Covered with Level 1 surgical mask Covered with Level 3 surgical mask

Fail

Pass

2.5

2.0

1.5

1.0

0.5

0.0

+
21

particles by the respirator filters, making covering or modifying EHMRs desirable when source 

control is needed. 

Modifying or covering EHMRs with exhalation valves to improve potential efficacy for source 

control may result in increased exhalation breathing resistance and inspired CO2 levels, which 

may subsequently decrease user comfort and reduce user acceptance. This study assessed the 

effect of modifying EHMRs to filter the exhaled breath using the inhalation filters or covering 

the EHMR exhalation valves with a surgical mask, per the CDC recommendation at the time of 

this study, on the exhalation resistance and inspired CO2 level. Although the respirator 

modifications or added surgical mask coverings in this study were assumed to improve source 

control in EHMRs with exhalation valves, researchers did not evaluate the effectiveness of using 

these respirators for source control. The level of respiratory protection provided to the user by 

the modified and covered EHMRs was assumed to have been maintained. 

Exhalation Resistance

To assess exhalation resistance, EHMRs were donned on a headform that was made to simulate 

exhalation at a flow rate of 85 L/min, providing positive pressure to the facepiece. Applying 

positive pressure to the control EHMRs closes the inhalation valves and opens the exhalation 

valve, allowing the simulated exhaled breath to exit through the exhalation valve and bypass the 

respirator filters. As a result, the type of filter did not affect the exhalation resistance in the 

control condition. Four of the five EHMRs in this study were assessed in both N95 and P100 

filter configurations, and results show that the mean exhalation resistance of the controls with 

N95 filters and controls with P100 filters were within 1 mmH2O of each other for each of these 

four models. 

By removing the inhalation valve membranes and blocking the exhalation valve, the simulated 

exhaled breath was directed back through the respirator filters typically used for inhalation only. 

As a result of this modification, the filter type becomes a significant factor in the exhalation 

resistance measurements of the EHMRs modified to filter exhaled breath. Of the four EHMRs

tested with both filter types, all of the P100 filter configurations modified to filter exhaled breath

measured higher mean exhalation resistance than their N95 filter modified counterparts—on 

average about 7 mmH2O higher. This was expected due to the higher filtration efficiency

provided by P100 filters (99.97% of airborne particles) compared to N95 filters (95% of airborne 

particles) [Approval of respiratory protective devices 1995]. Higher filtration efficiencies tend to 

result in higher resistances due to differences in filter media material, filter surface area, or basis 

weight of the filter media.5

The exhalation resistance of one modified EHMR model, which was only studied in a P100 filter

configuration, resulted in a decrease in mean exhalation resistance compared to the controls and 

also measured one of the highest variabilities in this study, yielding a 9-mmH2O difference 

between the highest and lowest observations. Larger sample sizes would be needed to determine 

why an EHMR modified to filter the exhaled breath would measure less exhalation resistance 

5 Note that the exhalation resistance performance requirement of not exceeding 25 mmH2O applies to both N95 and 

P100 air-purifying particulate respirators.
22

than an unmodified EHMR with an exhalation valve, but possible reasons may be attributed to 

the respirator design, the modifications made, or the interaction of these two variables.

Overall, the degree to which surgical masks caused increases in exhalation resistance varied 

greatly by respirator configuration, with increases of less than 1 mmH2O for one configuration to 

greater than 10 mmH2O for another. When covering the EHMRs with surgical masks, no 

modifications were made to the respirators themselves, meaning the inhalation valve membranes 

were left in and the exhalation valve was not blocked. Due to the inhalation valves remaining 

closed under positive pressure, not allowing the simulated exhaled breath to flow through the 

respirator filters, theoretically the filter type should not impact the exhalation resistance when 

covering EHMRs with surgical masks. Despite this reasoning, findings indicate that some 

EHMRs with different filter types, but covered with the same surgical mask, measured 

differences greater than 5 mmH2O. This finding may be attributed to differences in how the 

respirators, surgical masks, or both were donned. Thus, the level of exhalation resistance is 

dependent not only on the respirator design, but also on how the surgical mask covers the 

respirator. A tighter seal over the valve could result in higher exhalation resistance.

When comparing the effects of the two types of surgical masks on exhalation resistance, the 

exhalation resistance of EHMRs covered with Level 1 masks were within 2 mmH2O of those 

covered with Level 3 masks—with the exception of one model with P100 filters, for which the 

EHMR covered with a Level 3 surgical mask measured a mean resistance over 4 mmH2O higher 

than when covered with a Level 1 surgical mask. These findings indicate little difference 

between the exhalation resistance of EHMRs covered with Level 1 surgical masks and those 

covered with Level 3 surgical masks, despite the potentially different design characteristics to 

meet ASTM F2100 requirements—though the differential pressure limit for Level 3 masks is 

only 1 mmH2O/cm2 higher than the limit for Level 1 masks.

In this study, the exhalation resistance was measured at a continuous airflow rate of 85 L/min, 

per NIOSH TEB-APR-STP-0003 [NIOSH 2019], which is comparable to the mean expiration 

flow rate experienced by healthy adult males exercising at high work rates [Silverman et al. 

1951; Janssen et al. 2005]. Adult females and unhealthy people would likely experience lower 

expiration flow rates at high work rates [Janssen et al. 2005; Coyne et al. 2009]. This test 

condition represents a rigorous test for assessing exhalation resistance, since higher expiratory 

airflow rates experienced during hard work result in higher breathing resistances—though it is 

possible that certain workplace activities in healthcare that require high exertion may create 

higher expiratory flow rates and subsequently result in higher breathing resistances. 

Carbon Dioxide Concentration

To create the simulated breath during the test for inspired CO2 levels, a breathing machine cam 

was rotated using the linear movement of two pistons with pre-determined endpoints, creating a 

repeatable, fixed breathing pattern. This breathing simulation is unresponsive to downstream 

resistance, including resistance induced by the respirator, its filters, and surgical masks. 

Therefore, inspired CO2 levels were dependent on only two factors: the breathing rate and the 

facepiece volume. For this study, and in the STP, the breathing rate was held constant at a minute 

ventilation of 10.5 L/min, meaning differences in inspired CO2 levels between the different test 

conditions imposed can be attributed solely to facepiece volume. 
23

EHMRs modified to filter the exhaled breath by removing the inhalation valve membranes and 

blocking the exhalation valve displayed increases between 0.33% and 0.98% mean inspired CO2

levels when compared to the controls. Removing the inhalation valve membranes introduces the 

respirator filters into the facepiece volume, thereby increasing the dead space and subsequent 

inspired CO2 levels. Differences between the N95 filter configurations and P100 filter 

configurations of the modified EHMRs did not exceed 0.3%, indicating that the volumes (sizes) 

of the N95 and P100 filters are similar for each EHMR model. 

Applying a Level 1 surgical mask over the exhalation valves increased the mean inspired CO2

levels in six of the nine configurations tested. Likewise, applying a Level 3 surgical mask over 

the exhalation valves increased the mean inspired CO2 levels in six of the nine configurations 

tested. However, neither approach of applying a Level 1 or a Level 3 surgical mask caused 

increases greater than 0.5% inspired CO2, indicating that the surgical masks did not increase the 

dead space in the facepiece. Since the two inhalation valve membranes and one exhalation valve 

membrane remained in place when the EHMRs were covered, the space between the exhalation 

valve and the surgical masks was not part of the respirator dead space. As with the exhalation 

resistance measurements, differences between the inspired CO2 levels of EHMRs covered with 

Level 1 or Level 3 surgical masks were small, with each configuration measuring within about 

0.1% inspired CO2 for both levels. 

Though there is no inspired CO2 level performance requirement for half-mask APRs, OSHA has 

a permissible exposure limit of 5,000 parts per million CO2 time-weighted average, or 0.5% for 

an 8-hour workday [CDC 1994]. At CO2 concentrations less than or equal to 2.0%, discomfort 

may be perceived, dependent on factors such as the exposure time and concentration and the 

physical fitness and work rate of the user. CO2 concentrations of greater than 2.0% may result in 

more significant discomfort for the user and physiological symptoms such as fatigue, dizziness, 

headache, or shortness of breath [Kloos and Lamonica 1966; Roberge et al. 2010].

The mean inspired CO2 levels of all the controls assessed in this study measured less than or 

equal to 1%. Additionally, applying a surgical mask over the exhalation valves generally caused 

little to no increase in inspired CO2 levels, with mean inspired CO2 levels measuring less than 

1% and up to about 1.25% across all configurations. At these levels, it is not expected that users 

would experience discomfort or physiological symptoms due to the buildup of CO2 in the 

facepiece. However, modifying the EHMRs to filter the user’s exhaled breath caused the mean 

inspired CO2 values to increase to at least 1.25% inspired CO2 and nearly up to 2% in certain 

models, with one measurement of one configuration exceeding 2%. At these levels, it is possible 

that users may be affected by the higher levels of inspired CO2, particularly when wearing these 

modified devices for longer periods of time in sedentary settings. 

The breathing rate used in this study was a sedentary breathing rate [Silverman et al. 1945], 

equivalent to that found in healthy adult males, ages 19–35, sitting still. In the cited study, upon 

which the NIOSH STP for determining CO2 concentrations in respirators was based, this 

breathing rate and these healthy subjects were chosen to provide the greatest concentration of 

inspired CO2 likely to be experienced in a workplace setting. A higher work rate or activity level 

would decrease the concentration of CO2, and the sedentary breathing rate used in the current 

study is likely lower than what healthcare workers would experience when wearing EHMRs, 

making this a rigorous test for determining the buildup of CO2. 
24

Limitations

High variability in the data, seen in select datasets in Figures 6–9, are likely due to the variability 

between respirator donnings and how tightly the respirator was secured to the headforms, as each 

observation was a separate donning. Unlike FFRs, EHMRs are not sealed to the headform with 

glue or wax during the exhalation resistance test [NIOSH 2019] and rely solely on the respirator 

straps to secure it to the headform. Likewise, EHMRs (in the case of this study) and other 

respirator classes (in the case of approval testing) are not sealed to the headform during the 

inspired CO2 level test and are mounted using the respirator’s own straps or head harness 

[NIOSH 2020a]. Additionally, donning a surgical mask over the EHMRs introduced further 

variability due to variability between donnings for both the exhalation resistance and inspired 

CO2 level tests. 

Because only nine EHMR configurations were assessed and sample sizes were small (n=3), the 

conclusions that can be drawn and analyses that can be performed are limited. A larger sample 

size and larger selection of models is needed to classify and assess the relationships of respirator 

design features on exhalation resistance and inspired CO2 levels. Potential variables of interest 

include facepiece design and volume (including respirator size, in that only medium sizes were 

assessed in this study), respirator fit, exhalation valve positioning, exhalation valve protective 

cover design, and filter materials and design. 

Despite CDC’s broad recommendation at the time of this study to cover respirators with 

exhalation valves with surgical masks, procedure masks, or cloth face coverings, this study only 

evaluated surgical masks as a covering over an EHMR. As described in the Respirator Test 

Conditions subsection of Methods, this decision was made because surgical masks have behindthe-head ties, which allowed for more precise positioning over the exhalation valve and offered 

the capability to be tightened for fit [ASTM 2021]. However, it is possible that applying different 

types of coverings over an EHMR could create different or increased exhalation resistances 

compared to those measured in this study. 

The method in this study of modifying EHMRs to filter the exhaled breath involved those filters 

typically used in one direction only for the purpose of filtering inhaled breath. Although common 

filters are believed to provide similar filtration regardless of the direction, the filtration 

efficiencies of the EHMR filters used in this study were not evaluated for directionality, nor does 

NIOSH assess filtration efficiency in the exhalation airflow direction as part of respirator 

approval testing. 

The filters used in this study are disposable filters that are coupled with reusable facepieces and 

other required components to provide a complete respirator assembly. Typically, respirator 

manufacturers recommend replacing EHMR filters when soiled or clogged with particulate, 

when the respirator becomes difficult to breathe through, or after a specified time frame, such as 

after an 8-hour work shift when used in industrial settings [CDC 2020a]. For canisters and 

cartridges that provide protection against gases and vapors, OSHA requires the employer to 

implement a canister or cartridge change schedule [Respiratory protection 2004]. Changing of 

the EHMR filters may be needed more frequently when modifying the respirators to filter the 

user’s exhaled breath, as filtration of the heated and humidified exhaled breath could impact 

filter performance and life.
25

CONCLUSION 

This study demonstrates that EHMRs can be modified by blocking the exhalation valve and 

removing the inhalation valve membranes and still meet the NIOSH exhalation resistance 

minimum performance requirement. Although the end user cannot make modifications to the 

design of a NIOSH-approved respirator, as this voids the NIOSH approval [NIOSH 2020c], the 

results of this study should provide respirator manufacturers with confidence that new or 

redesigned EHMRs without an exhalation valve, which may be desirable to healthcare workers 

when source control is needed, can still meet the exhalation resistance performance requirement. 

Regardless, for new EHMR designs without an exhalation valve or for modifications to existing 

EHMR designs to obtain NIOSH approval, the applicant must go through the standard approval 

process and meet all necessary requirements.

While this study was in progress, several EHMR models without exhalation valves were 

approved by NIOSH [CDC 2021a]. Another existing EHMR has been approved with an 

exhalation valve filtering adaptor accessory that attaches to the outside of the exhalation valve, 

providing another approach to filtering the exhaled breath different from that used in this study. 

The approval process involved meeting the exhalation resistance performance requirement, but 

the filtration efficiency of the exhalation valve filter accessory was not measured. The most 

effective EHMR design characteristics for improving potential source control while meeting

current performance requirements will be assessed in future studies. Future research is also 

needed to address the potential source control efficacy, in the form of outward filtration 

efficiency, of the recently approved non-valved EHMR models and EHMR model with an 

exhalation valve filter accessory.

The CDC recommendation at the time of this study was to cover EHMRs with exhalation valves 

with a surgical mask, procedure mask, or a cloth face covering when both source control and 

respiratory protection were required. This study demonstrates that EHMR exhalation valves can 

be covered with a surgical mask and meet the exhalation resistance performance requirement 

required for NIOSH approval. Further, covering the EHMRs with surgical masks was not found 

to significantly increase inspired CO2 levels compared to uncovered EHMR exhalation valves. 

Though results varied significantly among respirator models and seemed to be heavily dependent 

on how the surgical mask and respirator were donned, the measured exhalation resistance 

generally did not approach the 25 mmH2O NIOSH limit, indicating that covering the exhalation 

valve with other types of loose-fitting coverings may also meet the NIOSH exhalation resistance 

performance requirement. 

The results of this study demonstrate promise that EHMRs with exhalation valves can be 

modified or covered to help improve potential source control efficacy with minimal impact to

comfort and acceptance due to increased exhalation resistance and inspired CO2 levels. However, 

due to the increases in both exhalation resistance and inspired CO2 levels created by modifying 

EHMRs to filter the exhaled breath, as well as variability between respirator configurations, 

additional studies may need to be completed on the physiological and/or comfort-related impacts 

on users, such as changes in concentrations of CO2 in the blood or how respirator time-of-use 

impacts comfort. Other measures of comfort such as moisture and heat buildup inside the 

facepiece also need to be studied. This future research can be in the form of laboratory and/or 

human subject tests [Shaffer et al. 2014] and can inform design features of EHMRs to optimize 

user comfort while also providing improved source control. As a result, EHMRs may be 
26

designed or include additional accessories to help provide source control as well as respiratory 

protection. This will further the integration of reusable respirators into healthcare to mitigate 

disposable respirator shortages and help reduce the potential for expelled particles to reach the 

environment. 

For Further Information

For further information on elastomeric half-mask respirators, please visit the NIOSH Elastomeric 

Half Mask Respirator Resources page or contact the NIOSH National Personal Protective 

Technology Laboratory. 

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DHHS (NIOSH) Publication No. 2018–123

Promoting productive workplaces through safety and health 

research DHHS (NIOSH) Publication No. 2022-109

DOI: https://doi.org/10.26616/NIOSHPUB2022109