[This is a guest post by my partner Alexandra van Geel.]
Disclaimer: I am not an expert, just a private individual summarizing available information. Please correct me if I’ve gotten something wrong.
I suggest: (a) the Vogmask valveless mask or (b) the O2 Canada Curve Respirator. If (b), make sure you cut some an extra filter into two small circles and put the material in the respirator to cover the exhale valves.
If you use a homemade cloth mask, pick one that lets you get a good seal all around. Use one with a pocket for a filter – preferably, one made out of polypropylene.
If you use a disposable surgical-style mask, alter it to get at tighter fit, like this.
I’ve been spending an absurd amount of time researching masks. Motivated by love for my family and – let’s face it – enjoyment of small research projects, I decided to develop an informed set of recommendations to share. I wanted to make sure that my recommendations were evidence-based and reflected practicality, including wearability and availability, at least as of the time of writing. This document is the result. Let’s get started.
The Basics: Masks vs. Respirators
To begin, I wanted to understand some of the terms that have been thrown around from “face coverings” to “surgical” or “medical” masks and of course respirators including the gold standard (in the United States), the N95 (so called because it filters out at least 95% of airborn particles). Most of these labels are common English terms yet have specific technical meanings, which I’ve summarized below.
The bottom line is that for the wearer, NIOSH-approved respirators provide the highest level of protection against viruses. Both surgical/medical masks and other face coverings provide less protection due to a combination of loose fit and different filtration ability of the material.
As we all know by now, N95 masks are in short supply and should be reserved for front-line medical personnel. So what’s the best alternative? Keep reading!
Question: What makes a good mask?
Answer: primarily, protectiveness and breathability. The difficulty is that each one often comes at the expense of the other.1 N95 masks are made out of a nonwoven synthetic material that must meet regulatory criteria relating to protectiveness and breathability, but how do other materials compare? More specifically, I sought quantitative information on:
Differential pressure / airflow resistance, which are measures of breathability. Lower values are better.
Filtration efficiency (e.g., for viruses, for bacteria, and/or for particles of a particular size range), which are measures of protectiveness. Expressed as a percentage, higher values are better.
Before describing test results for other materials, I offer two cautions.
First, test conditions matter. Filtration efficiency is a function of both particle size and airflow rates. A filtration test using larger particle sizes will produce a higher filtration rate than will an otherwise identical test using smaller particles. Also, a filtration test using a lower airflow rate will produce a numerically higher filtration efficiency value, all else equal. To make sure that regulated masks and respirators meet safety standards, NIOSH and FDA require the use of specific standard operating protocols in their respirator/mask testing. In contrast, testing efforts undertaken by third-party laboratories or researchers may use different protocols, and it is therefore important to carefully review the testing methods to appropriately interpret the results.
My second caution is that in practice, mask protectiveness depends not only on the filtration ability of the material itself but also on fit. Some research focuses only on the properties of the material and not on how well any particular mask made from the materials, might or might not fit. A mask made of the best material in the world will provide limited protection if it does not fit securely.
With that out of the way, let’s look at the “gold standard” N95, as other materials are typically compared to its performance. The “N95 FFR designation is determined by the CDC’s National Institute for Occupational Safety and Health (NIOSH) and indicates a minimum filtration of 95% for particle sizes 0.022−0.259 μm (count median diameter of 0.075 ± 0.02 μm), according to 42 Code of Federal Regulations (CFR) Part 84.10. As the [COVID-19] viral aerosols are larger than the ~0.120 μm virus itself, N95 respirators are expected to provide suitable protection” (Zhao et al. 2020).2
In short, to be certified by NIOSH, an N95 candidate must pass a specific filtration efficiency test, which is performed using a charge-neutralized NaCl aerosol (42 CFR §84.174).
Until very recently, there was almost no filtration efficiency or differential pressure information for more common materials. I speculate that prior to the current pandemic, few studies had examined the protectiveness of cloth or surgical-style medical masks because there seemed little point: in countries with more money to devote to research, N95 masks were sufficiently plentiful that there was little incentive to consider what the runner-up might be. Of course, that situation has now changed, and the table below,3 I’ve identified three studies – all from 2020 – that attempt to answer these questions. Each study used a different study design and examined different household materials. All the studies included N95 masks for comparative purposes. This is important particularly as the filtration efficiency testing protocols employed are not identical to those required under 42 CFR §84.10.
|Study Element||Konda et al. 2020||Zangmeister et al. 2020||Zhao et al. 2020|
|Fabrics||Cotton, silk, chiffon, flannel, various synthetics, and their combinations plus N95 control||32 materials: cottons, wool, nylon, polyesters, rayon, synthetic blends, poly/cotton blends, coffee filter, paper filter, HEPA filter, plus N95 control||Interfacing, cotton, polyester, nylon, and silk, paper towel, tissue paper, copy paper, plus medical masks, and N95 control|
|Air flow rates||1.2 and 3.2 CFM (~35 and ~90 L/m)||32 L/min|
|Fit-testing||Simulated by drilling holes in up to 2% of the area||Not tested||Not tested|
|Targeted particle size||∼ 0.01 to 6 μm (10 to 6000 nm)||50 to 825 nm (“a size regime relevant for the removal of SARS-CoV-2 particles”)||75 ± 20 nm|
|Filter efficiency achieved||Up to 97% (particles <0.3 μm and up to 99% (>0.3 μm)||All <50%; most <25% except for certain items made of polypropylene. Surgical masks also ~25-30%.||Excluding copy paper, range of ~5% to 26% except for the best medical mask at 33%|
|Pressure drop||2.2 Pa for N95; 2.2 to 3.0 for test fabrics||62 to 178 Pa for the top fabrics||9 Pa for N95; excluding copy paper, 1.6 (interfacing) to 244 Pa for others|
|Identified similar official test method||Mimicked Parts 3 and 5 of the EN 1822 mask filtration protocol (aerosol remains unneutralized after size selection) or the ISO 29463 testing standard (aerosol is reneutralized after size selection)||Modified version of the NIOSH standard test procedure … (fabric samples were not preconditioned in any way and the flow rate was substantially reduced)|
It is notable that the results from these three studies do not all align: Konda et al. (2020) found common materials to have a notably greater filter efficiency (up to 97%!) than did the other two studies. Zangmeister et al. (2020) points this out directly: “Combinations of cotton and synthetic fabrics identical, or nearly identical, to those outlined in Konda et al. were tested here, and the present results were not consistent with the prior findings.” Particularly given N95 masks are made from specialized (as opposed to common) materials, I have a low level of confidence in the high filtration efficiency of common materials reported in Konda et al. (2020).
In contrast to the results of Konda et al. (2020), the other two publications suggest that the tested fabrics alone or in combination generally have a filtration efficiency of below 50%, possibly substantially below. The filtration efficiency of surgical mask material was similar to the upper end of the range of that measured in more common materials. Certain polypropylene materials (a multi-layer HEPA filter and medical-grade wrap) fared better, with filtration efficiencies of approximately 75% and higher (Zangmeister et al. 2020).
The breathability of the common materials varied. Overall, multiple layers provided more filtration efficiency but also reduced breathability. For instance, Zangmeister et al. (2020) found that increasing the number of layers of a cotton poplin weave monotonically increased the filtration efficiency; however, more than four layers “exceeded the NIOSH recommended ΔP [pressure differential] during exhalation through a fitted mask (245.2 Pa, 25 mm H2O)”.
In short, as others have observed, a mask made from common fabrics is better than no mask at all, but in protectiveness are still substantially inferior to the N95. Breathability was highly variable depending on the material and number of layers.
Commercially available options
My hope, when starting this review, was to either find reassurance about the protectiveness of a homemade mask or to find an available product that I could be convinced was reasonably protective (based on evidence rather than marketing!) and tolerable. More specifically, I sought products with documented results of laboratory testing of filtration and breathability. The two best candidates I found are described below. (If anyone finds other excellent options, please let me know!)
The O2 Canada Curve Respirator
Technostat® 150 by Superior Felt & Filtration
Bacterial partical efficiency
Filter material >99.9992% (Nelson Labs Spec MIL-M-36954C)
Viral particle efficiency
Filter material >99.997% (Nelson Labs Spec MIL-M-36954C)
Filter material >99.997% (NaCl neutralized aerosol, at 236 LPM) (LMS Technologies Inc. IEST-RP-CC001.6)
Pressure drop (airflow)
<1.1 mm (<11 Pa, at 32 LPM) (Nelson Labs Exhalation Resistance Test TEB-APR-STP-0003)
0.217 mm (2.1 Pa, at 236 LPM) (LMS Technologies Inc. IEST-RP-CC001.6)
Passed (ACUTE Environmental &Safety Services CSA Z94.4-2011)
Hard shell. One size but two models for high/low nose bridge fits.
Includes exhalation valve (i.e., does not protect others unless you cover the valves with extra filter material).
“N95 is based on passing three tests looking at valve leakage, breathability, and a load test. O2 Curve has passed two of these tests at Nelson Labs, but due to the compact nature of our filter it is not compatible with the load testing procedure. The filter simply doesn’t have enough surface area to pass the test” (CEO letter)
(The above information is from the O2 Curve Performance Research Summary)
The O2 Canada Curve Respirator consists of a two-layer hard shell with a replaceable filter inserted between the inner and outer layers, and with silicone around the edges to achieve a better seal to the face. The advantage of a hard shell is that the respirator will not collapse against your face when breathing hard. A potential disadvantage is that, given its generally rigid structure, the shape may not match your facial structure well enough to seal properly. Unfortunately, the only way to know if it fits and is comfortable is to buy one and try it out. My parents did this, and my mother found it to fit well, but my father did not. Because this mask is both expensive and not returnable, acquiring one represents something of a financial risk. As a final note about fit, I will mention that this respirator is available in only one size but in two shapes, intended for different nose bridges.
The key advantage of this product – originally developed for the Asian market as a respirator for use in polluted areas – is its filter. This filter material is a product called Technostat® 150. O2 Canada’s website includes the results of independent third-party testing of their masks, and the material itself is intended for medical-grade purposes, as indicated on the manufacturer’s website. The filter is also exceptionally breathable – test results aside, my mother reported that it was much better than our homemade cloth masks in that regard. The table below summarizes the presented test results for this filter both with respect to filtration efficacy and pressure drop (breathability).
A key disadvantage of this product is that as provided, it will not similarly protect those around you because it has built-in exhalation valves not covered by the filter. These values make exhaling easier (and are consistent with the mask’s original purpose, which is use in areas with polluted air); however, for COVID purposes, it’s best to protect not only yourself but also others. Fortunately, due to the mask’s design, it is possible to cut out pieces of the filter material and put them over the valves, which should help prevent unfiltered breath from leaking out; however, this is my own speculation and has not been evaluated by any lab test. In the future, the company may make filter inserts that cover the valves, but such pre-cut filters are not presently available.
The Vogmask valveless mask
No filter brand specified.
Bacterial partical efficiency
>99.9% (Nelson Labs)
mean particle size (MPS) of 3.0 um ± 0.3 um
Viral particle efficiency
>99.9% (Nelson Labs, mean particle size (MPS) of 3.0 um ± 0.3 um)
Pressure drop (airflow)
Filter sewn into mask; not replaceable
Vogmask also originally developed masks for people wishing to reduce exposure to pollutants. Consistent with that purpose, some Vogmasks do have valves; however, the company has now introduced a line of masks that lacks valves. The Vogmask valveless mask is fabric-based and, while stiffer than the typical homemade mask, still has the potential to collapse inward against one’s mouth/nose under conditions of heavy breathing. Fit is, however, not likely to be a problem: not only does the mask have a nosepiece to allow for a customized fit, it also comes in multiple sizes intended for children through large adults.
A potential disadvantage is that this mask’s filter is sewn-in and non-replaceable. As such, the manufacturer states that the entire mask must be replaced from time to time. It can be hand-washed, but the instructions note that this will clean only the outside layers, and the inner filter will not be effectively cleaned. The company provides some estimates of how long the filter may last, but these will undoubtedly vary depending on frequency and duration of use, as well as the conditions of use.
Both my kids have preferred the Vogmask over the O2 Canada Curve respirator (I cannot swear it wasn’t appearance rather than fit or breathability that drove their preference…). Vogmask availability has unfortunately been limited. This would tend to eliminate it as a practical option(!) except that they seem to be able to regularly replenish their stock: within a few days, the website announces a new day and time for availability of additional masks, usually about a week later. Whether this pattern will continue is anyone’s guess, but if you’re interested, I’d advise keeping an eye on their website then marking that day/time on your calendar and be ready to place your order immediately.
Some tips about comfort
If you’re in a situation where you have to wear a mask for a long period of time, comfort is key and all the more so for kids. (If they’re taking it off because they can’t stand it, it won’t be protective, however great its specs might be!) So here are some possible solutions to two common problems.
Fabric collapse. I’ve found one tool that may help some people with fabric-based masks by keeping the fabric away from directly touching the nose and mouth: the 3D mask bracket. This forms a kind of frame on your face that your mask sits on top of and holds in place. I can’t provide specific recommendations about particular products, but there are a lot of options (for example). With a little luck, you’ll find something that fits your face reasonably well.
Ear fatigue. Both of the masks I’ve suggested use ear loops. Ear loops may or may not be comfortable for you, and if you use hearing aids, they are a non-starter. Both O2 Canada and Vogmask offer solutions: the O2 Canada mask has a “sport strap” (out of stock at the time of writing but anticipated back in stock shortly), which goes around the head and neck. Vogmask sells a head strap accessory (also sold out at the time of writing, but see prior text about their pattern of restocking). In addition, there are many other third-party products available to solve this problem – try searching for phrases such as mask strap extender, mask ear saver, and similar terms.
That’s all, folks, and I hope it’s been useful! Stay safe.
- Konda, A. Prakash, A. Moss, G. A., Schmoldt, M. Grant, G. D., and Guha, S. 2020. Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks. ACS Nano 14: 6339−6347.
- Zangmeister, C. D., Radney, J. G., Vicenzi, E. P., and Weaver, J. L. 2020. Filtration Efficiencies of Nanoscale Aerosol by Cloth Mask Materials Used to Slow the Spread of SARS-CoV‑2.
- Zhao, M. Liao, L., Xiao, W., Yu, X., Wang, H. Wang, Q. Lin, Y. L., Kilinc-Balci, F. S., Price, A., Chu, L., Chu, M. C., Chu, S., and Cui Y. 2020. Household Materials Selection for Homemade Cloth Face Coverings and Their Filtration Efficiency Enhancement.
It also follows that the best mask for one purpose may not be the best for all purposes: for instance, if you’re outside jogging, you may be willing to sacrifice some degree of protectiveness for better access to fresh air! ↩︎
In some cases, I was unable to discern some of the study’s methods, which is why there are some blank entries in this table. ↩︎