Breathable Masks: Testing Results

Key result: Quantitative testing shows that fabric masks constructed from spunbond nonwoven polypropylene (NWPP) and/or quilting cotton are breathable and can satisfy NIOSH criteria for breathability.

Ready for more testing? Visit our MakerMask Lab Testing Campaign for details.

Introduction: Breathable Masks

Breathing isn’t optional, and as COVID continues and mask wearing becomes commonplace, we are becoming more aware of the air around us and every breath we take (12 to 15 breaths/minute, ~20,000 breaths/day) [EPA, 2014]. As such, any mask that covers both the mouth and nose MUST be breathable. The challenge for mask makers is that in addition to being breathable, masks for COVID should also act as barriers to droplets and to provide filtration. Unfortunately, there is usually a trade-off between breathability and filtration: as filtration increases, breathability generally decreases. As a result, it is important to keep breathability in mind when selecting mask materials and layering combinations. In this post we share quantitative breathability data for masks including inhalation resistance, exhalation resistance, and CO₂ levels.

Click to expand more information about inhalation, exhalation, and CO₂ levels

Breathing consists of two distinct phases – inhalation and exhalation.

Inhalation, or breathing in, draws air into the lungs allowing atmospheric oxygen (O₂) to pass through the lungs (the alveoli) and into the bloodstream to our cells.
Exhalation, or breathing out, expels air from the lungs allowing carbon dioxide (CO₂) and water (H₂O) generated by our cells to pass from the bloodstream into the lungs and out into the air around us.

Mask Breathability

Both inhalation and exhalation require the lungs to do work. Depending on the type of masks worn, and the duration of time that masks are worn for, the amount of work that the lungs are doing increases. Just like the lungs do increasing amounts of work as you transition from sitting on the couch to walking to jogging, the amount of work the lungs do increases as you transition from no mask to masks with increasing levels of filtration. For example:

Work of breathing is not increased with 4 hours of continuous use of disposable medical masks [Ciocan et al, 2020].
Work of breathing is increased with continuous use of NIOSH approved N95s [Davis and Chen, 2020; Sinkule et al, 2013]

How difficult it is to breathe through a mask depends on the breathability of the mask fabrics used, the fit of the mask, and in tightly fitted masks, the volume of air enclosed between the mask and the face.

Breathing Resistance

Mask breathability is a measure of how breathable a mask is, and can be quantified in terms of breathing resistance. Masks that are the hardest to breathe through have the highest breathing resistances. According to NIOSH criteria for masks and respirators, two types of breathing resistance are measured, inhalation resistance and exhalation resistance, and the results are presented as pressure drops across the mask with units of millimeters of water (mm H₂O). The specific tests performed included:

Inhalation Resistance (TEB- APR – STP – 0007), which should be less than 35 mm H₂O
Exhalation Resistance (TEB- APR – STP – 0003), which should be less than 25 mm H₂O

Mask CO₂ Levels

According to OSHA, fabric masks, medical masks, and surgical masks DO NOT accumulate unsafe levels of carbon dioxide [OSHA 2020]. These types of masks are designed to block small particles and droplets (≥ 1000 nanometers), but allow gases like oxygen (O₂; 0.12 nanometers) and carbon dioxide (CO₂; 0.23 nanometers) to pass freely both through (and around) the masks. Although NIOSH doesn’t yet require CO₂ testing for medical masks or medical respirators, they do offer guidelines for some types of tightly sealing respirators and gas masks. Guidance for these masks requires CO₂ levels of less than 2% [NIOSH 2020; NIOSH Breath; RCT-APR-STP-0064]. The specific tests performed included:

Carbon Dioxide Concentration (RCT-APR-STP-0064), which should be less than 2%

Click to expand OSHA's answer to questions about CO₂ accumulation in cloth masks

Does wearing a medical/surgical mask or cloth face covering cause unsafe oxygen levels or harmful carbon dioxide levels to the wearer?

“No. Medical masks, including surgical masks, are routinely worn by healthcare workers throughout the day as part of their personal protective equipment (PPE) ensembles and do not compromise their oxygen levels or cause carbon dioxide buildup. They are designed to be breathed through and can protect against respiratory droplets, which are typically much larger than tiny carbon dioxide particles. Consequently, most carbon dioxide particles will either go through the mask or escape along the mask’s loose-fitting perimeter. Some carbon dioxide might collect between the mask and the wearer’s face, but not at unsafe levels… workers may easily remove their medical masks or cloth face coverings periodically (and when not in close proximity with others) to eliminate any negligible build-up of carbon dioxide that might occur.” – OSHA 2020

Testing: Breathable Masks

With the help of community mask makers, MakerMask was able to send fourteen masks to ATOR Labs for formal breathability testing of inhalation and exhalation resistances and was able to evaluate CO₂ levels in 10 of these masks. Testing used ISO16900 and NIOSH Testing methodologies as adapted for use on the lab’s Automated Breathing and Metabolic Simulator [ATOR Labs, 2020; Eshbaugh, 2008]. It is important to note that analysis was performed on full masks and not on fabric samples.

The specific tests performed included: Inhalation Resistance (TEB- APR – STP – 0007), Exhalation Resistance (TEB- APR – STP – 0003), and Carbon Dioxide Concentration (RCT-APR-STP-0064).

Three categories of fabric masks were evaluated: “basic fabric masks”, “fitted fabric masks”, and “N95 + Cover Combinations.”

Fabric Mask Categories:

Photo of the seven “basic fabric masks” sent to ATOR labs for breathability testing. (Photo courtesy of J. from Florida) — The seven “basic fabric masks” sent to ATOR labs for breathability testing (April 2020) were constructed by the same sewist using different combinations of fabrics (click on image for details). (Photo courtesy of J. from Florida)

Basic Fabric Masks (N=7) were constructed using either the MakerMask: Surge pattern (a pleated rectangular mask design with a nose wire and head ties) or the Craft Passion pattern (a contoured mask constructed from two shaped pieces with ear loops). These masks were selected because they were easy to sew and representative of mask patterns in common usage by sewists in March/April 2020. These masks were constructed from between one and three layers of spunbond nonwoven polypropylene (NWPP) and/or cotton. NWPP was selected based on the recommendations posted by MakerMask.org, and tightly woven cottons (quilting cotton/flannel) were selected based on general guidance available to sewists at the time.

Fitted Fabric Masks (N=4) were MakerMask: Fit prototypes that sealed tightly to the face, passed informal fit tests (DIY smoke tests), and included extra filtration layers. These masks were constructed using three or more layers of commonly available fabrics, required more skill to sew, and were designed to maximize particle filtration while maintaining breathability standards similar to those of commercially available N95s.

N95 + Cover Combinations (N=3) were tested to determine if a mask cover constructed from a single layer of NWPP fabric (the MakerMask: Cover) was breathable when used in combination with a NIOSH approved N95. Breathability results for the N95+ Cover were published previously by MakerMask at, “A Breathable Combination: An N95 Plus a NWPP Mask Cover.”

Results: Breathability and CO₂ Levels

The mask designs, materials, layering combinations, and test results for each of the masks tested is shown in Table 1 (below), and the results are summarized in Table 2. All of the fabric masks tested passed NIOSH criteria for inhalation resistance (< 35 mm H₂0), exhalation resistance (< 25 mm H₂0) and CO₂ levels (< 2%).

Breathable Mask Results Summary Table Showing the mask materials and combinations tested. Html version for easier readability is in clickable box below — **Table 1.** Breathability and CO₂ data for masks tested at ATOR Labs using ISO16900 and NIOSH Testing Methodologies

Discussion: Breathable Masks

Although individual differences between mask materials and layering combinations were not conclusively different, significant (p<0.05) differences were seen between the basic fabric masks and the other mask types/categories tested (table 2).

Table including MakerMask Breathability Results showing that basic fabric masks are the most breathable of the masks tested — **Table 2.** Results Overview: Inhalation Resistance, Exhalation Resistance, and CO2 levels.

Deeper Dive – Mask Breathability

All fourteen of the fabric masks tested passed NIOSH criteria for both inhalation resistance (< 35 mm H₂0) and exhalation resistance (< 25 mm H₂0). However, basic fabric masks were the easiest to breathe through. In fact, the inhalation resistance for these masks (5 ± 1 mm H₂O) was significantly lower than masks with extra filtration layers (14 ± 3 mm H₂O; P < 0.001) or combinations including N95s (14 ± 1 mm H₂O; P < 0.001). The exhalation resistance was similar for both the basic fabric masks (8.1 ± 0.7 mm H₂O) and the fitted fabric masks (8.3 ± 1.7 mm H₂O). Fitted Fabric Masks had inhalation resistances (14 ± 3 mm H₂O) similar to those observed in NIOSH certified N95s by Roberge et al, 2010 (14 ± 2 mm H₂O; N=3) and Sinkule et al, 2013 (12 ± 2 mm H₂O; N=18). The N95 + Cover Combinations we tested had inhalation resistances (14 ± 1 mm H₂O) that were similar to the fitted fabric masks and slightly better than those reported by Sinkule et al, 2013 for N95s covered with surgical masks (15 ± 3 mm H₂O; n=18).

Deeper Dive – Mask CO₂ Levels

All ten of the fabric masks tested passed the NIOSH criteria for CO₂ levels (< 2%). For basic fabric masks (n=7), the mean CO₂ level was 0.7 ± 0.2 %, which is significantly below the NIOSH criteria level. In the literature, N95s test with a typical CO₂ level of 1.6 ± 0.5% (N=18) whereas N95s used with surgical mask covers have CO₂ levels of 1.98 ± 0.39%, which is much closer to the limit at 2% [Sinkule et al, 2013]. The N95 + NWPP mask cover combinations tested by MakerMask (N=3, 1.7 ± 0.1% CO₂) had CO₂ levels similar to those reported by Sinkule, et al. for N95s alone.

Click here for a deeper dive into CO₂ levels associated with changes in human health

CO₂ levels associated with changes in human health [USDA 2020; ISO/TS 16976-3:2019(E)]:

Deeper Dive – Mask Materials, Layers, and Designs.

Our experiences wearing these masks suggest that there is significant variability in how it feels to breathe through cotton vs NWPP, one-layer vs three, and across the various sewing patterns. But surprisingly, we saw few differences in inhalation and exhalation resistance across our wide variety of basic fabric masks. One possibility is that the formal test results are being dominated by leakage of air between the masks and the head form since basic fabric masks do not to seal to the face. If so, we may be observing the resistance at which air begins leaking around the edges of the mask instead of passing through it.

Conclusion

All of the masks tested were within the acceptable range for inhalation resistance, exhalation resistance, and CO₂ levels. Basic fabric masks were more breathable than masks designed with enhanced filtration. In addition, CO₂ levels in fabric masks (0.7%) were consistent with NIOSH and OSHA statements that CO₂ levels in cloth masks are generally considered safe for most users.

Although breathability is an important factor in choosing which mask materials and designs to use, it is critical to also consider other mask properties such as filtration, water resistance, comfort, fit, and cleaning and disinfection options. (A breathable mask that doesn’t provide filtration or droplet protection likely doesn’t meet your needs, and N95 levels of filtration inherently come with some reduction in breathability.) For an overview on balancing these considerations, check out “The Big 4: Criteria for Community Mask Materials.” For additional quantitative data on NWPP performance, see “Fluid Resistance: Mask Testing Results”.

Links to Formal Lab Reports for Mask Breathability and CO₂ Test Results

Special Thanks

MakerMask testing is donor-supported. To contribute to our ongoing efforts at mask testing and validation, tax-deductible donations can be made via Athena Response. Thank you and stay tuned for more science!

Special thanks to: ATOR Labs who donated time and resources to perform these tests for MakerMask free of charge and Nicole Paul, who did the first round of technical writing, and to the volunteer sewists that created the masks for testing:

Julie M. from Florida who made the MakerMask: Surge and Craft Passion prototypes for breathability testing and Linda S. of Massachusetts who made a duplicate set for particle filtration testing
John R. and Courtney L from California who made the MakerMask: Covers for testing
Lucille S. from Massachusetts who made the MakerMask: Fit prototypes for testing

REFERENCES

References

ATOR Labs (2020), Letter on file. Tested by using ISO16900 and NIOSH testing methodologies adapted for use on the ATOR Labs Automated Breathing and Metabolic Simulator; values above reflect breathing pressures at 40 liters per minute (LPM) respiratory minute volume (RMV); which is it is comparable to the 85 LPM continuous flow (Eshbaugh, 2008). For context, 40 RMV is roughly equivalent to a person that is moving around in an occupational setting. NOTE: Minute, inhalation mean, and inhalation peak flows from 40 RMV cyclic flow conditions match those from 85 LPM continuous flow conditions. Note: Testing procedures for respirators and masks are typically conducted with either continuous flow rates of 85-95 liters per minute (LPM) or sinusoidal respiratory minute volume (RMV) values of 30-64 L/min (CEN, 2000a; CEN, 2000b; CFR, 1995) [CDC PPE Conformity (2020), PPE Conformity Assessment Studies and Evaluations Inhalation and Exhalation Resistance and Filtration Performance of Stockpiled Air-Purifying Respirators; Janssen (2005), Interpretation of Inhalation Airflow Measurements for Respirator Design and Testing; Nelson Labs Respirators, RESPIRATOR TESTS AND REQUIREMENTS; Wikipedia (2020). Respiratory Minute Volume and Respiratory Minute Ventilation are both referred to as RMV]
3M (2020), Comparison of FFP2, KN95, and N95 and Other Filtering Facepiece Respirator Classes
Chandrasekaran B, Fernandes S. (2020) “Exercise with facemask; Are we handling a devil’s sword?” – A physiological hypothesis [published online ahead of print, 2020 Jun 22]. Med Hypotheses. 2020;144:110002. doi:10.1016/j.mehy.2020.110002
Eninger (2008), WHAT DOES RESPIRATOR CERTIFICATION TELL US ABOUT FILTRATION OF ULTRAFINE PARTICLES?
Epstein et al. (2020), Return to training in the COVID-19 era: The physiological effects of face masks during exercise [published online ahead of print, 2020 Sep 24]. Scand J Med Sci Sports. 2020;10.1111/sms.13832. doi:10.1111/sms.13832
Eshbaugh (2008), N95 and P100 Respirator Filter Efficiency Under High Constant and Cyclic Flow.
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Kim et al (2015). Pressure drop of filtering facepiece respirators: How low should we go?. International journal of occupational medicine and environmental health, 28(1), 71–80.
NIOSH (2019) Determination of Inhalation Resistance Test, Air Purifying Respirator Test, Air-Purifying Respirators Standard Testing Procedure (TEB-APR-STP-0007-508)
NIOSH (2020) RCT-APR-STP-0064 DETERMINATION OF FACEPIECE CARBONDIOXIDE AND OXYGEN CONCENTRATION LEVELS OF TIGHT FITTING, POWERED AIR PURIFYING RESPIRATORS WITH THE BLOWER UNIT OFF OR NON-POWERED RESPIRATORS STANDARD TESTING PROCEDURE
Rengasamy S, Eimer B, Shaffer RE. (2010) Simple respiratory protection–evaluation of the filtration performance of cloth masks and common fabric materials against 20-1000 nm size particles. Ann Occup Hyg. 2010;54(7):789–798. doi:10.1093/annhyg/meq044. Accessed at https://academic.oup.com/annweh/article/54/7/789/202744 on March 12, 2020.
Samannan et al, 2020. Effect of Face Masks on Gas Exchange in Healthy Persons and Patients with COPD. Ann Am Thorac Soc. 2020 Oct 2. doi: 10.1513/AnnalsATS.202007-812RL. Epub ahead of print. PMID: 33003954.
Sinkule (2013), “Evaluation of N95 Respirator Use with a Surgical Mask Cover: Effects on Breathing Resistance and Inhaled Carbon Dioxide”
Tellier (2019) “Recognition of aerosol transmission of infectious agents: a commentary“

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Breathable Masks: Testing Results

Breathable Masks: Testing Results