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.
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.
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.
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%
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.
Three categories of fabric masks were evaluated: “basic fabric masks”, “fitted fabric masks”, and “N95 + Cover Combinations.”
Fabric Mask Categories:
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%).
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).
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.
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.
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
- Basic Fabric Mask Breathability Report
- Fitted Fabric Mask Breathability Report
- N95 + NWPP Cover Breathability Report
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