Two years into the COVID-19 pandemic, a wide variety of mask options are available. How do they compare?
Figure 1 shows masks arranged into a spectrum, roughly in order of their filtration efficiency.
Different masks serve different tasks. A mask can function as source control (my mask protects you) and/or personal protective equipment (PPE) (my mask protects me).
Let’s see how each mask group fares on three important factors—fit, filtration, and breathability.
- Respirators: N95, FFP2, KF94, KN95, P2 (blue y purple in figure)
- Hybrid Fabric Masks: Commercial, DIY (green in figure)
- Disposable Masks (orange an yellow in figure)
- Generic Fabric Masks (red in figure)
Respirators: N95, FFP2, KF94, KN95, P2
An N95 is a particulate filtering facepiece respirator approved by NIOSH, and is considered the gold standard for respiratory PPE. Other countries have similar respirator standards. The following table provides an overview of the respirator standards for filtration and breathability.
- Filtration: All respirators in this table filter 94% or more of submicron particles. This high filtration is typically achieved by an electrostatically charged meltblown nonwoven polypropylene layer, which is usually too fragile to maintain filtration after washing.
- Breathability: The inhalation resistance of these respirators can be up to 35 mm H2O @ 85 lpm. However, most commonly accessible respirators are well below that maximum limit, making them easier to breath through and more comfortable as shown in Armbrust’s mask testing database (Note: the database expresses breathability in Pascals where 35 mm H2O equals 343 Pascals).
- Fit: Respirators are designed for a tight fit. Generally, behind-the-head straps (N95) provide a more secure fit than ear loops (most KF94 and KN95). When properly fitted and donned, there is minimum leakage of air around the edges.
- Reuse: Respirators are designed for single-use only. However, during times of supply shortage, the CDC devised a limited reuse strategy for respirators in healthcare settings by the rotation method, suggesting reuse up to 5 times. Limited data suggests that respirators retain adequate filtration after reuse. For example, Du et al, 2020 observed that N95s maintained a filtration of 97% after 3 days. “Mask Nerd” mechanical engineer Aaron Collins observed that most respirators can be reused for 40 hours with minimal drop in filtration efficiency. Furthermore, manufacturer 3M noted in their FAQ that N95s can be reused until they are dirty, damaged, or difficult to breathe through. Discard the mask when the straps or nose piece no longer provide a good fit.
In the beginning of the pandemic, due to a shortage of respirators, the public was urged to reserve them for healthcare or other frontline workers. As supply increased, many countries updated their guidance to remove supply concerns. Furthermore, during periods of high community transmission, health authorities are encouraging the public to upgrade to N95 or equivalent to provide the best protection to the wearer.
Among all the masks discussed in this post, respirators (e.g., N95, FFP2, KF94, KN95, P2) are the most protective.
|Where to buy respirators and medical masks
Beware of counterfeits when shopping for respirators. Here are some trusted sources:
Canada: See this site for Canadian sources.
Hybrid Fabric Masks
Hybrid Fabric Masks are fabric masks that incorporate one or more layers of high filtration material to offer better protection to the wearer. This group includes commercially available masks (e.g., Vogmask, Enro Mask, Happy Mask, Halo Mask) and some Do-It-Yourself (DIY) reusable masks. The performance of Hybrid Fabric Masks varies widely.
To help consumers and manufacturers, ASTM International established a standard for Barrier Face Coverings (ASTM F3502) that can be applied to this group of masks, as detailed in this blog post. The ASTM F3502 ratings can help consumers compare masks, and can apply to both disposable and reusable masks. Currently few masks in the market are tested to this fairly new standard. See examples in this CDC list y this blog post. Table 2 compares the US face covering standard to its European counterpart (CWA 17553).
Commercially Available Hybrid Fabric Masks
- Filtration: The ASTM F3502 Barrier Face Covering standard classifies masks into two levels of filtration of 0.3 μm particles: Level 1 (lower performance: 20%) and Level 2 (higher performance: 50%). Filtration varies depending on the fit and the filtration of the fabric employed. Aerosol scientist Dr. Linsey Marr tested some commercially available Hybrid Fabric Masks and found that their filtration efficiencies as worn, and after washing, were lower than advertised. Typically, the filtration of Hybrid Fabric Masks is lower than N95.
- Breathability: The ASTM F3502 Barrier Face Covering standard classifies masks into two levels of breathability: Level 1 (lower performance: 15 mmH2O) and Level 2 (higher performance: 5 mmH2O). Both are more breathable than the N95 standard, though many N95s would meet Level 1 breathability.
- Fit: Different designs fit different face shapes. For optimal performance, ensure all gaps are removed to minimize leakage around the edges. See this blog post for fit improvement ideas.
- Reuse: Many Hybrid Fabric Masks boast reusability. Adhere to the manufacturer’s washing instructions.
|Deeper Dive: Filtration Efficiency and Particle Size
Filtration efficiency is a function of particle size. Both the ASTM F3502 and the NIOSH N95 standards measure filtration efficiency at 0.3 um, the most penetrating particle size (MPPS) for most filter materials. Testing at the MPPS simulates a worst case filtration test. Consequently, even though an ASTM F3502 level 1 mask only filters 20% of 0.3 um particles, it could perform much better against larger particles. For example, the 2-ply quilt cotton in this CSU study filters sub-micron particles very poorly, but improves to 50% at 5 um and 90% at 10 um. See this blog post for insights on interpreting mask filtration.
DIY Hybrid Fabric Masks
- Filtration: Without access to testing, most DIY Hybrid Fabric Masks have unknown filtration characteristics. Unlike manufacturers, DIY mask makers often lack access to specialized high filtration materials and testing resources. Published filtration data are available from a number of sources that can serve as references for sewists. However, it is important to pay attention to the test conditions since many parameters (e.g. particle size and flow rate) vary from lab to lab and influence test results.
- Maker Mask tested two 3-layer fabric mask combinations that are consistent with the ASTM F3502 Barrier Face Covering standard.
- Multi-layer fabric testing from Colorado State University. Test conditions differ from the ASTM F3502.
- This University of British Columbia database includes filtration and breathability data for a variety of fabrics, and a calculator to estimate filtration and breathability of multi-layer fabric combinations. Test conditions differ from the ASTM F3502.
- Breathability: Breathability is essential to performance since a mask with poor breathability is likely to leak around the edges. Sewists may employ materials that offer better breathability than cotton with similar filtration, for example, spunbond nonwoven polypropylene.
- Fit: DIY masks can be custom made to fit a particular face shape to reduce leakage around the edges.
- Reuse: Most DIY masks are made from washable materials. Always follow the mask maker’s washing instructions. This post provides general information on cleaning and disinfecting masks.
Disposable masks can be separated into two categories: medical masks, which have been tested and meet appropriate standards for breathability and filtration, and generic disposable masks, which have unverified performance characteristics. Since these two types of masks look similar but may have vastly different performance characteristics, pay close attention to the labeling.
Medical Masks (ASTM F2100, EN14683)
A medical mask is a disposable mask that complies with the ASTM F2100 standard or similar EU standard EN14683. It is designed to provide the wearer protection against large droplets, splashes, or sprays of bodily or other hazardous fluids, in addition to source control. It is not intended for use as a PPE.
- Filtration: Similar to an N95, a medical mask often utilizes a meltblown nonwoven polypropylene layer for high filtration. A medical mask filters over 95% of 3 μm particles (Bacteria Filtration Efficiency). The ASTM F2100 standard also requires over 95% Particulate Filtration Efficiency (PFE) of 0.1 μm particles. However, the actual filtration efficiency during use is typically significantly lower due to leakage around the edges.
- Breathability: Medical masks are breathable, with a differential pressure below 6 mmH2O.
- Fit: Medical masks are loose fitting, thereby do not provide the wearer with a reliable level of protection when worn unmodified. However, with fit improvements like double masking with a well-fitting mask on top, adding a mask fitter, or using the knot and tuck method (see this post), a medical mask can provide a good level of protection to the wearer.
- Reuse: Medical masks are designed for single use only.
To ensure that the disposable masks you purchase comply with standards for filtration and breathability, look for “ASTM F2100” or “EN14683” on the box or package. Note that the FDA does not approve medical masks, so any claim of “FDA-approved” should be a warning sign. See this blog post for details.
Generic Disposable Masks
Generic disposable masks often look identical to medical masks, except they are not tested to any standards.
- Filtration: Unknown and can range from <10% to 99% (sources: Oberg et al., 2008, Armbrust Mask Testing Database).
- Breathability: Unknown but usually breathable, similar to medical masks.
- Fit: Loose fitting with leakage.
- Reuse: Designed for single use only.
Independent test sources like the Armbrust Mask Testing Database y MaskNerd Aaron Collin’s Database provide useful information on how a generic disposable mask performs, but they do not address quality control concerns. Use generic disposable masks with caution.
Generic Fabric Masks
Generic fabric masks are not engineered for personal protection. They are the basic and ubiquitous masks sold in stores or some DIY masks, often consisting of one or two layers of cotton, cotton blend, polyester, or polyurethane. While they provide some level of source control, they do not offer much personal protection to the wearer. Some of these masks have pockets for additional filters. In this case filtration will be improved provided the filter covers the entire breathing area.
- Filtration: As shown in this graph from Colorado State University, most 2-layer woven cotton masks can capture large particles (over 10 μm) fairly well, but not smaller particles (below 10 μm). For example, most of the 2-layer cotton masks in the same study filter less than 20% of 1 μm particles.
- Breathability: Breathability of woven fabric (e.g. woven cotton, woven polyester) is usually worse than nonwoven fabric (e.g. spunbond nonwoven polypropylene, nonwoven polyester).
- Fit: Most generic fabric masks are not engineered for gapless fit and many don’t have adjustable nose wires.
- Reuse: Many generic fabric masks are reusable. Follow the manufacturer’s washing instructions.
What Mask Should I Pick?
Selecting a mask involves evaluation of many factors, for example:
- Risk Assessment. This article (link to the corresponding study) provides insight on risk assessment.
- Viral Factors: Are there many cases in your community? How transmissible is the present variant?
- People Factors: Are people around you masked/unmasked? What activity are you involved in–are you breathing heavily or regularly? Are you fully vaccinated and boostered? Are you immunocompromised? Do you interact with high risk individuals?
- Air Quality Factors: Are you indoors or outdoors? Is it a big or small room? Is it crowded? Is it well ventilated?
- Accessibility and supply: What type of masks are available?
- Personal preference: Is reusability an important concern? How does that compare to the need for protection?
In higher risk situations, wear the most protective mask that you will wear consistently, paying attention to fit, breathability, and filtration. Choose a properly sized and fitted respirator (N95, FFP2, KN95, KF94, P2) if possible. If you lack access to a properly sized respirator, consider high filtering alternatives such as a medical mask worn with fit improvements, or a hybrid fabric mask engineered and tested to a high level of filtration.
In lower risk situations, enjoy the plethora of options from the full spectrum of masks!
The Spectrum of Masks
Options, options options.
A plethora of mask options.
Which one is good for adoption?
Breathability and filtration,
Plus fit evaluation,
Are all important considerations.
Choose with caution,
According to your situation,
A mask fit for the application.