What air cleaner test reports don’t tell you

99.8% pathogen reduction after 60 minutes! FDA approved! Reactive compounds destroy SARS-CoV-2!

The market for air cleaners is booming and you’ve undoubtedly seen these types of claims. I’ve spoken to countless business owners, building managers, engineers, teachers, and concerned citizens over the last several months who have had questions about the effectiveness and safety of a wide variety of air cleaning technologies that are being heavily marketed to combat COVID-19 transmission in indoor environments. The marketing claims are often bold, and sound quite promising:

  • ABC technology uses reactive molecules [or super oxides or positive and negative ions] to destroy SARS-CoV-2 on surfaces and in air!
  • XYZ technology has just been granted emergency FDA approval!
  • DEF technology reduces viable SARS-CoV-2 [or surrogate organism] by 99.99% in 60 minutes!

These claims are frequently made based on results from test reports from third-party test labs such as EMSL, MRI Global, Innovative Bioanalysis, Analytical Lab Group, and Aerosol Research and Engineering Laboratories, to name a few. As far as I can tell, these are generally manufacturer- or distributor-funded lab tests intended to demonstrate the effectiveness of an air cleaning device for removing particles, killing microorganisms, or whatever the device is purported to do.

However, there are consistent problems with many of these test reports. Others have recently pointed out of some these issues. Here I will demonstrate just a few of these common issues using specific examples, primarily from one particular company, but only because they were in the news recently. They are far from alone, and quite typical of how efficacy is reported. 

I recently noticed in test reports made available by a company named Aerus, which utilizes a patented technology they call ActivePure® in a number of products including their Medical Guardian Air device, their smaller Pure & Clean unit, and their subsidiary Vollara’s Air and Surface Pro device. You may have heard of this company, especially recently, as Deborah Birx, the former White House Coronavirus Coordinator in the Trump administration, just joined the company as chief medical and science advisor. The press release I read suggests that “Dr. Birx encourages every indoor environment to have ActivePure Technology.” Let’s look deeper into how this technology performs using test reports provided by the manufacturer.

There are surprisingly few details on the mechanisms involved in their ActivePure® technology on the Aerus website, but a few Youtube videos, as well as their published patent, provide some insights into what the technology is: a combination of UV-PCO (ultraviolet photocatalytic oxidation), often combined with ionization, and sometimes both combined with more conventional HEPA (high efficiency particulate air) filters. Key words used to describe the technology include “oxidizers”, “super oxides”, “active PCO”, “disinfecting molecules”, and more. In other words, it is a form of “additive” air cleaning technology that aims to “seek out and destroy pathogens,” among other claims. Here I focus primarily on inactivation of microorganisms on surfaces or in air (or both), as that’s clearly a high priority for consumer interests and marketing efforts right now.


Marketing materials love to show big impacts

You can read some of their marketing materials that summarize test reports conducted by third-party labs directly on their website. Here is a common approach to marketing these results from one of their air cleaners, the Medical Guardian device, in which they show a percentage reduction in the viability of MS2, a bacteriophage commonly used as a surrogate for for influenza virus and other respiratory viruses (it’s much safer and easier to use surrogates than the real deal). 

The graphic reports 99.9999% reduction in MS2 bacteriophage RNA virus after 60 minutes (also note that this approach is in no way unique to this particular company and its technology).

Sounds great, right? Well, let’s look deeper into the report to learn more. You can download a compilation of their third party lab test reports directly from Aerus (and I have also hosted them here in case the link ever disappears). 


The first example test report: pathogen surrogates, large chamber, but unconventional reporting

The first report in this packet documents a series of tests conducted by Aerosol Research and Engineering Labs. They tested the viability of several surrogate pathogens in air, with reasonably well documented and detailed methods (which isn’t always the case for some of these third party reports). They show a picture of the unit and mention that it is an ion generator and PCO in Figure 1. They mention the volume of the chamber – 562 cubic feet – and even show a diagram. This is a rather large chamber compared to a lot of other test reports, as we will see later.

They compared results with the air cleaning device operating to a control, which is again another basic competency that I’m glad to see but that is not always done in other lab reports. The unit they tested (Medical Guardian) includes the ActivePure® technology and also a HEPA particle filter. Its data sheet suggests airflow rates of 90 to 300 cfm are achievable depending on fan speed setting. It is not immediately clear what fan speed setting the unit was set to when it was tested.

They aerosolized a number of surrogate organisms to mimic various bacteria, viruses, and molds (fungi), one at a time. And they report “log-reduction” values in both figure and table form, including for both intervention tests (run in triplicate) and a control test without the air cleaning technology operating. Every “log reduction” is a factor of 10^(log reduction) lower than the original starting point (i.e., a log reduction of -3 is 10^3 or 1000 times lower than the original starting point, or 0.001 of the original concentration). In a separate test, they also injected polystyrene latex (PSL) beads as a measure of non-biological particle removal. Let’s look at some of these test results.

First, in Figure 4 of the test report, they show “Log reduction” values over a period of 20-60 minutes for 1 µm particles during the PSL bead injection and decay test, including for both air cleaner on and off periods (i.e., intervention vs. control):

These “Log reduction” values appropriately start at 0, as at time = 0 hours (i.e., the beginning of the test), the initial concentration is at its highest, and therefore this is no reduction at that moment. At a “Log reduction” value of -1, that is the time at which under a given condition the concentration of 1 µm particles have decreased by 1-log, i.e., 90%. Subsequently,  2-log means concentration reductions of another 10x, or 99% from the total. 3-log means 99.9% lower, 4-log means 99.99% lower, 5-log means 99.999% lower, and so on. It is a convenient way to show rapidly decreasing concentrations, and clearly these particles decrease much faster with the air cleaner operating than the control condition without the air cleaner operating, as one would hope to see.

However, “Log reduction” is a fairly unconventional way to show these types of data. The main issue is that it doesn’t allow for easy comparison to other standalone air cleaning devices because no conventional test methodology reports log reductions at a specific time. Instead, a more conventional and useful approach is to use loss rates between air cleaner on and off conditions to calculate the unit’s CADR (clean air delivery rate). This is a well known metric, widely used in the industry (e.g., by AHAM, the Association of Home Appliance Manufacturers), research communities, consumer groups, and recommended by the U.S. Environmental Protection Agency (EPA). 

Fortunately, we can estimate a CADR based on the loss rates shown here. We have to transform the “log reduction” values into “natural log reduction” values to be useful and to confirm to a first-order exponential decay model for a well-mixed chamber environment (read more about that approach here). So what I’ve done is transform the “log reduction” values at each time stamp into linear values (where C/C0 = 1/(10^(log reduction))) and then into the natural logarithm of the concentration at each time step compared to the initial time step (i.e, ln(C/C0)). I first translated “log reduction” values visually into a spreadsheet (so it’s an imperfect, but close, translation), then completed the other two transformations. These are simple calculations to get the results on terms that are more commonly used and from which I can estimate first-order loss constants. Here is a table of those values, followed by a figure showing the ln(C/C0) values over time from both test conditions.

1 µm PSL Control Test
Time (hr) LogReduction C/Co ln(C/C0) LogReduction C/Co ln(C/C0)
0 0 1 0 0 1 0
0.1 0.05 0.89125094 -0.1151293 1 0.1 -2.3025851
0.2 0.12 0.75857758 -0.2763102 2 0.01 -4.6051702
0.3 0.2 0.63095734 -0.460517 3 0.001 -6.9077553

We can fit a straight line through the ln(C/C0) versus time data in the figure above to estimate first order loss rate constants under each condition. My approximations of loss rate constants result in ~1.5 per hour during the control condition (i.e., no air cleaner operating) and ~23 per hour with the air cleaner operating. This “background” loss rate is just due to deposition of particles to chamber surfaces and any ventilation provided to the chamber. A difference of ~21.5 per hour is significant – a good sign that the air cleaner is doing something!

The last step is to multiply this difference in loss rates by the volume of the chamber, then divide by 60 to convert from hours to minutes, and estimate the CADR. Doing that results in an estimated CADR of about 200 CFM (cubic feet per minute) for these sized non-biological particles:

Loss rate summary
  Loss, 1/h
Control 1.46
Test 23
Delta 21.54
Delta, 1/min 0.359
Chamber Vol, ft3 562
CADR, cfm 201.8

That’s not too bad! A typical standalone or portable air cleaner with a HEPA filter would be expected to deliver a CADR of 200 CFM or more (and often quite a bit more). What’s interesting with this finding is that this particular air cleaner device is able to deliver ~200 CFM of clean air in terms of non-biological 1 µm particles, which, since this device includes a combination of ActivePure® technology PLUS a HEPA filter, gives us a sense of what the “baseline” level of performance might be for this device. It’s impossible to know whether or not the bulk of the removal is happening because of the HEPA filter or because of the UV-PCO and/or ionization components, but we do know that UV-PCO doesn’t actually target particle removal, so we can generally rule that out, and my hunch is that the HEPA filter is doing most of the heavy lifting here. Some ionization technologies/products have shown to be effective at removing particles in some studies, but not in others, including a recent study of our own. Regardless, this gives us a good anchor point that characterizes the basic performance of this device.

Next, we can move into reviewing the biological test results to see if the additional oxide generation or other active/additive technologies are doing more to reduce the concentration of viable biological particles, whether through removal by HEPA or by inactivation by active technologies or both, than the baseline rate of non-biological particle removal.

Table 3 of the report summarizes “net log reduction” results, which are the same types of values as “log reduction” presented above, albeit with the background loss rates from the control (i.e., air cleaner off) test subtracted out such that they are just the additional loss rates added by the use of the air cleaner. The units and magnitudes are otherwise similar (i.e., -5.6 log reduction means 10^-5.6 = 1/2.5^10-6 of the original value, or 99.9977% lower than the original value). The two viral surrogate tests (MS2 and Phi X174) are flagged for having concentrations after 60 minutes of testing that were low enough to be at or near the detection limits of the measurement methods they were using, so I’m not going to use those values. This means the loss rates are underestimated because the true value could have been lower and the methodological approach just couldn’t measure that low. It is worth noting that after 60 minutes for both of these tests, the net log reduction was around 4-log for one and 5.6 log for another. Let’s pick an even better performer to be generous: their Staphylococcus epidermidis results, which is used as a surrogate for methicillin resistant Staphylococcus aureus (MRSA) and achieved an average net log reduction of almost 6-log after 60 minutes of testing. The figure below shows their reported results using the “log reduction” approach (not the “net log reduction” approach), which shows results for both the control period (i.e., air cleaner off) and the air cleaner on period.

Similar to the non-biological removal tests, the table below transforms the “log reduction” values at each time stamp into linear values (where C/C0 = 1/(10^(log reduction))) and then into the natural logarithm of the concentration at each time step compared to the initial time step (i.e, ln(C/C0)). Below is a table of those values, followed by a figure showing the ln(C/C0) values over time from both test conditions.

MRSA surrogate Control Test
Time (mins) Time (hr) LogReduction C/Co ln(C/C0) LogReduction C/Co ln(C/C0)
0 0 0 1 0 0 1 0
15 0.25 0.35 0.44668359 -0.8059048 2.05333333 0.00884437 -4.7279747
30 0.5 0.44 0.36307805 -1.0131374 4.54 2.884E-05 -10.453736
45 0.75 0.5 0.31622777 -1.1512925 5.83333333 1.4678E-06 -13.431746
60 1 0.44 0.36307805 -1.0131374 6.39333333 4.0427E-07 -14.721194

Again, we fit a straight line through the ln(C/C0) versus time data in the figure above to estimate first order loss rate constants under each condition. My approximations of loss rate constants for these biological tests result in ~1.4 per hour during the control condition (i.e., no air cleaner operating) and ~16.6 per hour with the air cleaner operating. A difference of ~15.3 per hour isn’t bad, but is actually a bit lower than the ~21.5 per hour from the non-biological particle tests. Again multiplying this difference in loss rates by the volume of the chamber, then dividing by 60 to convert from hours to minutes, the effective CADR for this surrogate organism test is about 130 CFM:

Loss rate summary
Control 1.68
Test 15.27
Delta 13.59
Delta, 1/min 0.2265
Chamber Vol, ft3 562
CADR, cfm 127.3

In other words, the operation of this air cleaner with its combination of fan, HEPA filter, and patented UV-PCO and ionizer combination and whatever else is included in this particular device, yields an effective CADR for biological inactivation or removal of about 130 CFM, which is actually lower than the non-biological test results. So while 99.9999% reduction in airborne MRSA surrogate after an hour sounds super impressive in the marketing materials below, it’s actually not that great. It’s not zero, so that’s good! But 130 CFM of air free of MRSA surrogate is not that impressive and is easily achievable by conventional technologies. Moreover, since this unit actually has a HEPA filter in the device, it’s quite plausible that the HEPA filter is doing most of the heavy lifting here to inactivate or remove surrogate organisms. In addition to that, none of these tests mention anything about the chemical compounds that are emitted by the device via the UV-PCO + ionization process or their impacts on indoor chemistry and potential for chemical byproduct formation. This is something we recently studied using a popular bipolar ionization device, and we found that while the technology as tested appeared to remove some volatile organic compounds (VOCs), it increased the concentration of several others and even generated a few new previously undetected compounds. This potential for chemical byproduct formation is concerning, yet almost none of these test labs have ventured into this territory, and the peer-reviewed literature on this topic is extremely scarce. 


The second example test report: SARS-CoV-2, a cabinet, and more of the same

Now, it’s worth noting that this particular test report dates back to March of 2019 — well before COVID-19. These types of tests and the resulting reports are commonplace for technologies that purport to kill or inactivate or destroy microorganisms. Since COVID-19, numerous device manufacturers have also tested their products in commercial laboratories for their effectiveness in removing or killing SARS-CoV-2. It seems that even fewer labs are equipped to handle this dangerous virus. Aerus turned to a different lab this time, MRI Global, to conduct testing of the decontaminating ability of ActivePure® technology for SARS-CoV-2.

The test lab used what looks to be the same, or at least similar, Medical Guardian unit with 300 CFM of airflow. The lab placed the device in a biosafety cabinet that was 6 feet x 4 feet x 4 feet, or 96 cubic feet. That’s a 300 CFM air cleaner in a cabinet the size of a closet. If you divide 300 CFM by 96 cubic feet, that’s an effective recirculation rate (flow divided by volume) of about 187 per hour. There isn’t an indoor environment on planet earth for which these conditions would be relevant. 

The lab conducted tests with a control where the unit was operating without the ActivePure® technology operating, although they don’t show their control data in the test report (they only show comparisons on what appears to be a net-log reduction basis, i.e., with the control rates subtracted out). They didn’t aerosolize SARS-CoV-2 but rather inoculated test coupons and placed them on the floor of the cabinet. They sampled after 1 hour, 3 hours, 6 hours, and again at a 7th hour. Between the 6th and 7th hour, for some reason, the lab increased the relative humidity (RH) in the cabinet, which seemed to escalate the loss rate (but that seems quite unrelated to the performance of the unit itself). For that reason, I will look only at the 1 h, 3 h, and 6 h data:

There isn’t much data to work with here, but since there was no reduction in viable SARS-CoV-2 after 1 hour, I will use that as the time zero starting point, and shift the time since that time for the next two time stamps to 2 hours later and 5 hours later. I use the reported net log viable reduction, presumably with the control rates subtracted out, to again calculate a linear C/C0 value at each time step followed by a ln(C/C0) value at each time step. A brief table of these results is below, followed again by a figure showing the “net” ln(C/C0) values over time, which I can use to estimate net, i.e., additional, loss rates over the control condition. 

Time (hr) Time (hr) Net Log reduction C/Co ln(C/C0)
1 0 0 1 0
3 2 1.17 0.0676083 -2.6940246
6 5 1.69 0.02041738 -3.8913688

Once again, 97.9% reduction in viable SARS-CoV-2 on test coupons after 6 hours sounds pretty impressive! But if you calculate an effective first order loss rate constant from these data, you obtain a value of only ~0.9 per hour. The fit isn’t perfect — there are a small number of data points and some deviation from a perfect first order decay profile, but it’s not an unreasonable estimate. What does that mean practically? Well, converting this to an effective SARS-CoV-2 CADR yields a CADR of almost nothing (~2 CFM):

Loss rate summary
  Loss, 1/h
Delta (net) 0.8567
Delta, 1/min 0.01427833
Chamber Vol, ft3 96
CADR, cfm 1.4

About 2 CFM. This is with an air cleaning device with an airflow rate of 300 CFM, which is enough to cover a large room like a classroom or large bedroom, operating in a cabinet that was not much larger than the device itself. And the additional CADR for viable SARS-CoV-2 introduced by the use of the ActivePure® technology was essentially zero. Another impressive looking claim (99.7% removal after 6 hours) that doesn’t translate to reality. (I would also note that I would really like to see the background condition loss rate data because my hunch is that they left the HEPA filter installed but UV-PCO + ionization functions turned off, and that the loss rates were probably a lot higher in magnitude with the HEPA filter only compared to the additional benefit from turning these functions on). 

Now, operating a big air cleaner in a small cabinet seems a bit ridiculous, I know. But it gets worse.


The third example test report: SARS-CoV-2, an even smaller chamber, and again more of the same

Aerus also contracted with the University of Texas Medical Branch (UTMB) to test another of its devices that utilizes their ActivePure® technology for inactivating SARS-CoV-2. This is the third report in the packet linked above. In these tests, the lab placed one of two Aerus devices — their Pure & Clean unit and their subsidiary Vollara’s Air & Surface Pro unit — in a small (150 L) chamber and tested the impact on aerosolized SARS-CoV2. The Pure & Clean unit appears to operate at an airflow rate of 40 to 60 CFM, while the small 150 L chamber is only about 5 cubic feet in volume. Divide flow rate by volume in this setup and you get an effective recirculation rate of between 450 and 680 per hour! Again, a relatively large air cleaner in a very, very small volume:

Here are the test results:

Once again, great looking results like >99.8% inactivation of SARS-CoV-2 within minutes. Those kinds of results will get you FDA approval for use in hospitals. And once again, I took a similar approach here in attempting to calculate effective CADR values for SARS-CoV-2 from this somewhat bizarre test configuration. However, I could only use one data point for the air cleaner on condition because SARS-CoV-2 was reduced below detection limits within 3 minutes in this configuration. So the CADR is likely an under-estimate, but I don’t know by how much. Here are my results in table form:

    Control Test
Time (mins) Time (hr) LogReduction C/Co ln(C/C0) LogReduction C/Co ln(C/C0)*
0 0 0 1 0 0 1 0
3 0.05 0.71 0.19498446 -1.6348354 3.07666667 0.00083817 -7.0842868
10 0.16666667 1.30333333 0.04973552 -3.0010359 3.07666667 0.00083817 -7.0842868
15 0.25 1.27333333 0.05329257 -2.9319584 3.07666667 0.00083817 -7.0842868
30 0.5 2.41 0.00389045 -5.5492301 3.07666667 0.00083817 -7.0842868

Again, I plot these ln(C/C0) values over time from both test conditions to estimate loss rates:

This results in an estimated loss rate of ~12 per hour in the control condition and ~142 per hour with the technology activated. These are huge values, but largely because of the size of the air cleaner compared to the small chamber. Multiplying the difference in loss rate estimates by the chamber volume yields another small effective CADR for viable SARS-CoV-2, this time about 12 CFM:

Loss rate summary
Control 11.9
Test 141.7
Delta 129.8
Delta, 1/min 2.16333333
Chamber Vol, ft3 5.295
CADR, cfm 11.5

A CADR of 12 CFM is again lower than the airflow rate of 40 to 60 cfm of the unit, assuming they operated it at one of those flow rates (it is unclear). Once again, an additional 12 CFM of inactivation in a real room achieves almost no additional removal.


Why does this matter? 

So what have we learned here? Using just one example air cleaning technology and three different commercial test lab reports, we have gained some insights into how limited many of these tests and reports are, often lacking important details to even understand how tests were performed and/or reporting results in a way that look quite favorable to manufacturers. In turn, marketing materials make those results look even more impressive with big claims like 99.99% inactivation of pathogens and so on.

None of this is new in this industry, but it is certainly in greater focus than ever before with so much attention on reducing the transmission of COVID-19 in indoor environments. And while I use this particular company Aerus and its technology to demonstrate these issues, they are in no way unique to this company. These issues are widespread throughout the industry, and have been for years. Other technologies that I’m aware of that have similar issues in their published test reports include the Aerisa 2000 ion generator (I estimate a CADR of about 57 CFM for MS2 in the test configuration reported), GPS bipolar ionization units (I can’t estimate a CADR from many of their test reports because they don’t report a chamber volume!), and many more.

With something like $170 billion in the recently signed COVID relief bill being directed to K-12 schools and colleges for upgrading ventilation systems and portable air cleaning units (see screenshot from the actual bill below), it is imperative that we understand how to interpret manufacturer claims of the effectiveness of air cleaning technologies in addition to the potential safety concerns associated with many “additive” air cleaning technologies that rely on the addition of reactive constituents to indoor air to do their dirty work.


Learn more by analyzing air cleaner test reports on your own!

If you find yourself reviewing test reports like these, you may be interested in analyzing them on your own. Fortunately, my colleague Elliott Gall at Portland State University has developed an online air cleaning efficacy calculator tool that you can use. You can input data from test reports in a variety of ways that data are commonly shown — including % reduction over time, log reduction over time, or translating from a plot of log reduction over time — and calculate an effective CADR for the test conditions. You can then translate that to an effective air change rate equivalent or your space.


What kind of mask should I be wearing to protect against COVID-19?

November 10, 2020 Update: this post has been updated to incorporate new literature and resources and several helpful suggestions from colleagues. Updates are noted throughout.

We’ve been on an unnecessarily confusing roller coaster ride in the United States in the last few months regarding the role that masks and face coverings can play in slowing or stopping the transmission of COVID-19.

We’ve gone from the CDC telling the public not to wear masks in March 2020, to some public understanding of the idea that “I wear a mask to protect you, not me!“, to now where I think we should have been all along, which is: “my mask protects you, and might also protect me, so we should both wear masks.”

The reasoning behind this latter thinking was outlined well in a perspective piece in Science in June 2020:

“No masking maximizes exposure, whereas universal masking results in the least exposure” (Prather et al., 2020)

In fact, universal masking is now thought to be key to stopping the transmission of COVID-19, and the CDC now acknowledges that the widespread use of masks (or “face coverings” — I’ll primarily use “masks” for simplicity*) can “slow and stop the spread of the virus — particularly when used universally within a community setting.”

Now, as the country ventures into reopening K-12 schools and universities, the question I’ve been getting a lot lately is: “What kind of mask should I be wearing?”

I’ve asked the same question to myself numerous times, and I have found the landscape on masks confusing and difficult to generalize. 

Fortunately, numerous outlets have synthesized a lot of useful information already. FiveThirtyEight had an early summary of masks, including the basic concepts and the impacts of fabric materials, layers, and vents (the answer on the latter: don’t use vents because although they can protect you from others, they expel your own unfiltered respiratory emissions, which presents a potential hazard to others if you happen to find yourself among the many symptomatic, asymptomatic, or pre-symptomatic hosts of SARS-CoV-2).

More recently, Wirecutter published a helpful article on selecting cloth masks, which interviewed over 20 subject matter experts and focused especially on fit, comfort, and breathability, with some information on particle filtering effectiveness. And Emily Oster and Galit Alter at COVID Explained have a short, helpful primer on masks. Additionally, new test data continue to be published in a variety of outlets. I’m sure you’ve seen the headlines.

By now, I think there are some lessons to be learned that can help guide your mask selection based on factors that are known to influence their effectiveness and performance, which I will summarize here.  

What do masks do?

One way that masks can help prevent infected individuals from infecting others is that they reduce the velocity of air expelled during breathing, talking, sneezing, and/or coughing. Reducing the velocity of air expelled from respiratory activities means that the expelled air won’t travel as far, and that’s a good thing because it means that virus-containing respiratory droplets hitching a ride on the expelled air shouldn’t travel as far either.

This phenomenon has been demonstrated in recent years through mock-up experiments and Schlieren imaging:

A visualization from NIST illustrates airflow when coughing, but it does not show the movement of virus particles (Staymates, 2020)


Masks can also filter respiratory particles of various sizes. And particle filtration can work in two directions: filtering particles during exhalation (which protects others) and filtering particles during inhalation (which protects yourself).

Depiction of aerosol filtration efficiency mechanisms for a fabric mask material (Konda et al. 2020)


Particle filtration efficiency, or the ability of a mask to filter airborne particles, often depends on a lot on particle size and a lot on mask material.

And the overall filtration efficiency of a mask depends on both the particle filtration efficiency and how well the mask fits (and thus how much unfiltered air flows around the mask).

The combination of these factors means that when respiratory particles containing the virus are expelled from individuals wearing an effective mask, there won’t be as many particles expelled beyond the mask and they won’t travel as far, and that’s a good thing:

Masks with additional layers of fabric will reduce the number and transport distance of respiratory droplets (Bahl et al., 2020)

Similarly, if an individual is wearing a mask and breathing air laden with airborne particles containing the infectious virus, then their mask can filter out a fraction of those particles, reducing their own exposure.

But, as mentioned, this filtration ability, and therefore overall mask effectiveness, can depend a lot on particle size.

Why does particle size matter?

The importance of particle size — such a seemingly small detail (pun intended) — in the transmission COVID-19 has been a hotly debated topic in the last several months.

The key issue in this debate is whether SARS-CoV-2 (and thus COVID-19) is transmitted primarily through small aerosols, large droplets, or touching surfaces (or some combination of the three).

Organizations like the WHO and CDC maintained for months that SARS-CoV-2 was primarily transmitted through large droplets that travel only short distances (less than 1 or 2 meters) where they can deposit directly onto the eyes, mouth, or nose of others and/or deposit on nearby surfaces (“fomites”) that are subsequently touched by others who then touch their eyes, mouth, or nose. Both organizations have considered transmission by larger particles (“droplets” in their nomenclature, conventionally meaning larger than 5 or 10 µm in diameter) to be dominant and transmission by smaller particles (“aerosols” in their nomenclature, conventionally meaning smaller than 5 or 10 µm in diameter) to be negligible. 

While this way of thinking has dominated for most of this pandemic, there is also an increasing recognition of the importance that small aerosols likely play in transmission. In fact, so much recognition that the WHO updated their position in July 2020 to recognize the potential for airborne transmission via smaller aerosols. [November 2020 Update: the CDC similarly updated their position to include transmission by smaller aerosols in October 2020]

They updated their position in part because two leading researchers on respiratory emissions and modes infectious disease transmission — Lidia Morawska and Don Milton — published an open letter appealing to the medical community at large to recognize the likelihood of airborne transmission of COVID-19 through small aerosols. The letter was backed by 239 signatories, largely from the aerosol science community (including myself). (The infectious disease community has now published their own letter with over 300 signatories warning that epidemiological data and clinical experiences in healthcare settings continue to support that the main mode of transmission is “short range through droplets and close contact.” So the “debate” continues, but I think it has more to do with how the two communities define “airborne” than anything else.)

From my perspective, we know that a wide range of particle sizes are emitted during human respiratory activities, including very small particles (conventionally called “aerosols” in medical communities) that can travel long distances and stay suspended in air for long periods of time, as well as very large particles (conventionally called “droplets” in medical communities) that don’t travel very far and don’t remain suspended in air for very long. The definition of “aerosols” versus “droplets” has long centered on an arbitrary cut-off of 5 or 10 µm in size. The reality of respiratory emissions is broader and more complex.**

In one of the most comprehensive studies of respiratory emissions of which I am aware, researchers demonstrated two distinct distributions of human respiratory emissions generated during activities like speaking and coughing, organized chiefly by particle size, including one part of the distribution with an average diameter of ~0.5 to ~2 µm and another part of the distribution with an average diameter of ~50 to ~300 µm:

Particle size distributions resulting from (a) speaking (uncorrected data), (b) speaking (corrected data), (c) voluntary coughing (uncorrected data), and (d) voluntary coughing (corrected data) (Johnson et al., 2011)

Particles in the smaller distribution exist in the respirable “fine particle” size range, which can easily remain suspended in air for minutes to hours in poorly ventilated spaces. Particles in the larger distribution exist as ballistic droplets that fall to the ground rapidly.

Particles near the arbitrary cut-offs of 5 or 10 µm (and smaller) can also remain suspended in air for at least several minutes and can easily mix within a room, contrary to the conventional “droplet” definition in medical communities:

Simulations of droplet trajectories for initial diameters of (a) 0.1 µm, (b) 10 µm, (c) 100 µm, and (d) 200 µm (Chen and Zhao, 2010)

We’ve also learned from recent research on other respiratory viruses, including influenza, that many respiratory viruses are commonly found in the smaller aerosol size ranges in isolated studies of human respiratory activities as well as in samples collected from various indoor environments with infected individuals present, including in particle size fractions smaller than 5 or 10 µm (and even smaller than 1 µm). While the actual viruses in question are only perhaps ~60 nm to ~140 nm in size (i.e., ~0.06 to ~0.14 µm), they exist in the environment, either in air or on surfaces, encapsulated in respiratory fluid and thus in particles larger than their own size alone.

Similarly, and more recently, we’ve also learned that SARS-CoV-2 genetic material has been found in both aerosol (< 10 µm) and surface samples in hospitals with COVID-19 patients, including in Singapore, Wuhan (China), and Nebraska. While these previous studies reported the detection of viral RNA in aerosol and surface samples, they did not assess viability (but viability has long been assumed likely based on what we know about viability in aerosols and on surfaces via controlled experiments).

It was only in early August 2020 that we learned of the detection of viable, infectious SARS-CoV-2 in aerosol samples from a hospital at the University of Florida, including nearly 5 meters (nearly 16 feet) from COVID-19 patients. Granted, we don’t know if these viral concentrations were high enough to actually get someone sick, but finding viable SARS-CoV-2 in aerosols over 15 feet away from patients in highly engineered, well-ventilated, and well-filtered hospital settings does not exactly give me confidence that COVID-19 is transmitted only through large ballistic droplets that land on someone’s eyes, mouth, or nose.

Why does this all matter?

This definition of relevant particle sizes matters because if COVID-19 is transmitted substantively through small aerosols in addition to (or instead of) large droplets, it affects how we effectively control its spread.

Substantive aerosol transmission means we need more than physical distancing and face shields. It means we need better ventilation and air cleaning in buildings. And probably most importantly, it means we not only need universal masking, especially in indoor environments with other people present, but we also need those masks to be effective.

And if we need effective masks, it means we need to understand what particle sizes we are trying to protect ourselves and others from.

Are we worried about large respiratory droplets containing the virus that may be so large as to be visible to the naked eye, shot ballistically into our faces? Or are we worried about breathing small aerosols containing the virus that go undetected to the human eye?

To me, this distinction is less about “airborne” transmission, which seems to conventionally mean “highly infectious and able to transmit long distances” to the infectious disease and medical communities, versus “close contact” transmission. Rather, it is more about whether or not small respirable particles (or “aerosols”) contribute to COVID-19 transmission over any distances (including both close- and/or long-range distances). Unfortunately we still don’t even seem to know about the importance of aerosol versus droplet types of transmission in humans, let alone in animals, but lots of evidence points towards the likely importance of smaller aerosols.

Therefore, my position is that we should err on the side of caution in our use of masks for the public and we should seek to prevent the transmission of both small aerosols and large droplets. At least some of the medical community seems to agree with this idea.

With the assumption that both smaller aerosols and larger droplets may contribute substantively to transmission, my goal is to seek the most effective masks that we can reasonably acquire and comfortably use (in addition to continued physical distancing and other measures). I’m not talking about extreme measures like N95 or fan-powered respirators; rather I am seeking to better understand widely available fabric, cloth, and medical/surgical masks.

[November 2020 update: a recent summary of literature on face masks was also published in Nature in October 2020, and another in Mayo Clinic Proceedings.]

Filtration effectiveness of masks for aerosols and/or droplets

The reason that particle size is so important for masks is because the particle removal efficiency of different mask types can vary widely by (1) particle size, (2) mask material/fabric, and (3) how well a mask fits on your face.

Unfortunately, I cannot think of a single study that has thoroughly tested a wide spectrum of these factors influencing mask effectiveness across a widely representative sample of non-medical-PPE masks and materials.

Instead, most studies, including those published both pre-pandemic and during the current pandemic, have usually investigated just one or two of these factors in some depth and/or breadth.

[November 2020 update: note that the above conclusions are changing as we speak, as additional studies are being conducted to evaluate mask effectiveness using a range of methodologies. Additionally, the New York Times published a great visualization on how masks work in October 2020.]

There are also numerous epidemiological studies that have investigated the impacts of mask wearing in various populations, which can help translate from our understanding of mechanisms like filtration efficiency and mask fit to observable impacts in real human populations. However, these studies also necessarily have to gloss over some of these mechanistic details.

Here I summarize some relevant literature in these categories and then use that information to provide some fairly generalizable recommendations for masks for the general public.

I sort the literature broadly into the following categories: (1) measurements of filtration efficiency of masks (or mask materials alone) for aerosols and/or droplets, (2) measurements of mask fit effects, and (3) epidemiological investigations of mask wearers.

The filtration efficiency of masks and mask materials for both medical-PPE and non-medical-PPE style masks has been measured for a wide variety of materials subject to a wide variety of challenge aerosol sizes and types, including those ranging from ultrafine particles (i.e., < 0.1 µm) to respirable particles (i.e., 0.5-10 µm) to large visible droplets (i.e., > 1 mm). Test rigs have been setup to test materials by themselves, full masks by themselves, and full masks worn by mannikins or actual people. Some of these tests have also been setup to measure the filtration efficiency of masks used for inhalation or exhalation, or both.

There are even standardized respirator and mask testing equipment for making these types of measurements, although others have gotten creative in how they measure filtration efficiency of masks and mask materials (for better or worse), including reducing the costs of testing by implementing low cost air quality sensors and improving testing speeds by cell phone imaging.

N95 respirators are commonly used as the standard reference for comparison in these measurements, as they meet NIOSH N95 classification requirements, meaning they have a particle removal efficiency of at least 95% when challenged with ~0.18 µm size particles using a standard respirator fit test instrument. This size range is barely larger than common respiratory viruses on their own, and it corresponds to what is typically near the “most penetrating particle size” for many types of filtration media. Somewhat counterintuitively, the filtration efficiency for many media types is commonly highest for both the largest particles (e.g., > 5 µm) and the smallest particles (e.g., < 0.1 µm), but is lowest for those “Goldilocks” sizes in the middle (e.g., 0.1-1 µm). So, comparisons to N95 are a pretty extreme (and perhaps even unreasonable) target for evaluating the filtration efficiency of many cloth masks, but it’s a good standardized reference for comparison — a sort of “platinum level” of achievement that other masks strive to attain.

An excellent example of one of these types of studies is summarized below. Researchers aerosolized a variety of different size particles into a test chamber with a mannikin head wearing one of several types of masks, including a couple of N95 masks, a few cloth masks, and a surgical mask. They found some variability in the N95 masks, but overall their removal efficiency for all particle sizes tested was quite high (i.e., >70% typically, and increasing to >90% with larger particle sizes). N95 mask efficiencies lower than 95% were probably due to mask fit issues (i.e., air leakage around an imperfectly sealed mask). These effects are important to capture because they diminish mask efficiency compared to testing media filtration efficiency by itself.

The surgical mask they tested was just as efficient as the N95 masks for 1 µm and 2.5 µm particles, but was less efficient for particles smaller than 1 µm (~60% efficiency). Two of the three cloth masks performed worse than both the N95 and surgical masks, especially for smaller particles (i.e., only 50-60% removal of 1 µm particles but only 15-30% removal of 0.03 and 0.1 µm particles), although one of the cloth masks performed just about as well as both the N95 and surgical masks (i.e., ~80% removal for 1 µm particles). How confusing is that!

An example of a mask efficiency test using a mannikin and several sizes of particles (Shakya et al., 2017)

What is not shown above is the removal efficiency of these masks for “droplets.” However, if a fabric or fibrous media filtration device can capture 100% of 2.5 µm particles, I am generally not concerned about their ability to capture much larger “droplet-sized” particles (the typical U-shaped efficiency curve helps us here). Droplet-sized particles are so large that they are difficult to test in a setup like this — they don’t hang around long enough or in sufficient numbers to be counted.

So, my take from this study (with an admittedly very small sample size) is that if you’re concerned about ~1-2.5 µm particles — which I think is probably a reasonable proxy for virus-containing respiratory particles — then the cloth mask #1 and surgical mask performed about as well as (if not better than) the N95 respirators (taking fit into account), and I would recommend both of those masks. If you want to be on the safe(r) side and focus on the smaller size range of ~0.5 µm, then the surgical mask starts to look less convincing (but with big error bars, so it’s actually hard to say) while cloth mask #1 still looks good. I would rather not wear mask #2 and #3 given these results.

You might be wondering… what exactly are these three cloth masks and why are they different? Fortunately the study shows photos of each one:

Photos of the masks tested in Shakya et al. (2020)


Although no other specific details are reported, cloth masks #2 and #3 look like they may be single or double fabric layers — not unlike what is commonly available and fashionably worn in the U.S. today. Cloth mask #1 seems to be thicker and perhaps integrates multiple fabric types (it also has exhaust valves, which should be avoided or taped if they can’t be avoided). 

Testing of masks and mask materials has continued, with a number of recent publications that can further guide us.

In an early study during this pandemic (in April 2020), researchers measured the filtration efficiency of several common fabrics, including “cotton, silk, chiffon, flannel, various synthetics, and their combinations,” for a range of particle sizes from ~0.01 µm (10 nm) to ~5 µm. Fabrics were tested solely as fabrics — not shaped into mask form. They were also compared to N95 and surgical mask materials. The study found a wide variety of removal efficiencies by the different fabrics, ranging from 5% to 80% for < 0.3 µm particles and from 5% to 95% for > 0.3 µm particles:

Particle filtration efficiencies of mask materials (Konda et al. 2020)

For reference, the removal efficiency of the tested N95 mask was ~85% for < 0.3 µm particles and ~99.9% for > 0.3 µm particles. The tested surgical mask performed similarly, with a removal efficiency of ~76% for < 0.3 µm particles and ~99.6% for > 0.3 µm particles. Clearly, as we have also learned from other prior testing, N95 respirator masks and surgical masks are both useful for blocking the transmission of both aerosols and droplets. What this study adds is that two layers of cotton quilt performed similarly to both of these standard reference materials, as did a high thread count cotton fabric (thread count of 600, with two layers slightly better than one), as well as two layers of chiffon. Various combinations of different fabrics such as cotton and chiffon, cotton and silk, and cotton and flannel also performed well. And again, gaps, which would ostensibly be caused by inadequate mask fit (but was just simulated herein by creating an artificial gap), greatly reduced the effectiveness for all tested materials, including surgical masks and N95 respirators. 

[November 2020 update: while I have left the above table from Konda et al. 2020 in this piece because it demonstrates an early test of both fine and ultrafine particle filtration efficiency of mask materials in 2020, several researchers have also pointed out some serious methodological flaws that limit its accuracy and utility, but the broader point reminds: different materials have varying filtration efficiencies for different types and sizes of particles.]

Another early study posted in April 2020 used a respiratory fit testing device to measure the particle removal efficiency of several home-made masks as well as a type of 3M surgical mask for particles ~20 nm to ~1 µm in size (without size-resolution in this range). The tested masks included several 2-ply cotton masks, some with additional filters, and each with an additional nylon layer introduced. All of the homemade masks, which were worn on people, had lower particle removal efficiency than the surgical masks (which was ~75%), although some were within ~4% of the surgical masks while others were over up to ~60% lower. Adding a layer of nylon stocking over the masks minimized the flow of air around the edges of the masks and improved particle filtration efficiency for all masks, which brought the particle filtration efficiency for five of the ten fabric masks above the 3M surgical mask baseline. 

Another study published in May 2020 tested the filtration efficiency of common fabric combinations and masks using a “fluorescent virus-like nanoparticle” challenge. This was an unconventional test setup resulting in their N95 mask reference achieving less than 50% removal efficiency, which is inconsistent with conventional aerosol testing. But if we can use this as a reference, several materials (and material configurations) were found to have removal efficiency similar to the N95 mask, with a few performing even better than the N95. Multiple layers of cotton materials acting alone, as well as in combination with other fabrics such as flannel or terry cloth or non-woven polypropylene, all had performance similar to, or better than, the N95 references in this test setup.

Others have characterized material filtration efficiency using more conventional aerosol equipment, and some have published their data online in a spreadsheet. This includes materials that can be used as filter inserts into face masks or respirators (including custom 3D printed respirators), such as household air filters coffee filters and various fabrics. In this work, researchers reported a conservative measure of filtration efficiency in the “Goldilocks” range I mentioned earlier (0.3 µm) for something like 200 materials under a variety of conditions. Coffee filters weren’t that great, but vacuum cleaner HEPA filters were excellent. Again, increased thread count on fabrics helped increase efficiency, with 600 and 1000 thread count fabrics achieving over 50% efficiency in this size range. I think one can safely extrapolate to the efficiency of 0.5 or 1 µm particles being even higher than that, even though it wasn’t explicitly tested. 

You’ve probably read about another recent study because it showed how ineffective neck gaiters are. Unfortunately, this study got a lot of press, much of which overstated and contorted its very own claims about how their testing of the neck gaiter marketplace was in no way representative of the variety of products available. The researchers reported results of exactly one (1) neck gaiter, which they report was made of ‘fleece’ (but who wears a fleece neck gaiter in the middle of the summer?). The novelty of their work really was less about the testing of 14 mask products (which is not a large sample size, as the researchers clearly note), but more about how they use cell phone imaging to characterize mask efficiency in capturing respiratory emissions from people wearing masks and placing their head into their custom designed box.

In fact, they use a respiratory emission tracking technique using smart phone imaging that, while quite intriguing and potentially quite useful, isn’t actually compared to conventional aerosol filtration techniques, so it’s really hard to say how relevant the efficiency measurements are. There is reference in the paper to a minimum detection limit of 0.5 µm, but they also report results in “droplet” nomenclature. They also show results for droplet distributions beginning around 0.1 mm in diameter (100 µm) and extending to over 1 mm (over 1000 µm), and they mention that the pixel analysis they use can’t measure the size of droplets less than 120 µm.

So…. what does this “neck gaiter study” tell us? I think it tells us about ballistic large droplet efficiency and probably not much else. If that’s useful to us, then we might want to avoid a fleece neck gaiter, a bandana, or a knitted mask because of their low removal efficiency, but rather we should select a three-layer cotton-polypropylene-cotton mask, a two-layer polypropylene mask, or either a “two-layer cotton, pleated style mask” (“Cotton5” with >90% efficiency) but perhaps not a “two-layer cotton, pleated style mask” (“Cotton3” with only ~75% efficiency). Wait, what?

Droplet transmission through facemasks (Fischer et al., 2020)

Aren’t those last two masks the same exact type? They are, nominally. In fact, they look very similar:

‘Cotton3’ vs. ‘Cotton5’ from above — can you tell the difference?

For whatever reasons, the “droplet” removal efficiency of these two otherwise similar masks are pretty different (although they’re at least both over 50%). Perhaps fit issues (loops versus straps) or perhaps an unknown fabric issue. And this comparison also says nothing about their aerosol removal efficiency.

So while this study garnered a lot of attention, I worry that it doesn’t provide much new information on what I’m more concerned about — aerosol filtration efficiency. I’m aware of at least one other really well-done droplet efficiency testing study, but again I’m not convinced that that’s what we need to be focusing on.

[November 2020: a recent pre-print from researchers at NIOSH provides better evidence on the neck gaiter situation, and as expected, they’re much better than the above study showed.]

Another recent study tested medical-style surgical masks and respirators similar to N95 masks worn on human subjects, each generally meant for healthcare settings, using respirator fit testing equipment, and interestingly found that surgical masks with ear loops removed less than 40% of particles smaller than 1 µm while surgical masks with ties removed over 70% of the same size particles (the N95 respirator removed >98% of particles in the tested size range, as expected). I’ve compiled a graphic of their results here:

Two surgical masks with very different particle removal efficiencies (Sickbert-Bennett et al., 2020)

This large discrepancy in surgical mask performance does not give me confidence; in fact, I think it highlights how important mask fit can really be. Otherwise, how can someone see these two products on the market, which ostensibly look the same, and understand that one might give you greater than 70% protection against respiratory aerosols while the other might give you less than 40% protection?

[November 2020 update: mask data continue to roll in, with better and better quality as well. I am providing a few additional links below. Many thanks to those researchers who continue to collect and/or communicate this information.]

Additional mask/material effectiveness studies since the original posting of this article include:

Mask fit effects

No matter how good a mask or mask material is at filtering relevant particles under test conditions, if the mask doesn’t fit well and seal around the user’s face, the effectiveness — especially in filtering aerosols during inhalation — can be greatly diminished.

Fortunately, mask fit has been widely studied as well (and mostly pre-pandemic). Also, as we’ve already discussed, some of the aforementioned studies incorporate mask fit effects as well. 

What we generally see for smaller aerosol removal efficiency by masks is something like this:

The removal efficiency of a facemask on a mannikin head increases with increasing edge-sealing (Lai et al., 2012)

Normal wearing of this particular mask (which looks to be a surgical-type mask) led to a particle removal efficiency (of small, ultrafine particles in this case) of less than 50%. Increasing amounts of sealing with adhesive tape increased removal efficiency to nearly 100%. This is an excellent example of how important mask leakage is, especially for small particles.

Those that have studied mask leakage over the years have shown that leakage can lead to 5 to 20 times more particles penetrating through otherwise decent masks; that there is a wide variability in leakage among commercially available masks of all types; that surgical masks don’t work very well for aerosols unless they have a vaseline seal (no thank you!); that nylon hosiery materials wrapped around the head can help achieve a good seal for all sorts of mask materials; and that no one seems to know how to fit a surgical mask correctly to their head, which creates enormous leaks. Oh, and that all surgical masks are not equal, as those used in hospital settings tend to have higher efficiency than those used in dental settings.

A few studies have also taken the next, rare, leap to measure viral content in respiratory emissions of sick human subjects wearing masks and not wearing masks. While small in number, these studies have generally shown that wearing surgical masks can reduce the viral content of influenza virus expelled by human subjects with influenza by a large amount in larger particles (> 5 µm) (i.e., 25-fold, on average, but with variability) and by a smaller amount in smaller particles (< 5 µm), (i.e., ~3-fold, on average, again with variability).

Another recent study showed similar results for coronavirus (not SARS-CoV-2) and influenza virus. Surgical masks reduced the viral content of both pathogen types in large particle (> 5 µm) emissions from infected subjects, but with some variability. However, surgical masks reduced viral content emitted from patients with seasonal (coronavirus) colds down to nothing, whereas their use led to a non-significant decrease in influenza content emitted from patients with influenza:

Efficacy of surgical masks in reducing respiratory virus shedding in droplets (>5 µm) and aerosols (<5 µm) (Leung et al., 2020)

To me, this combination of information means that surgical masks are probably quite useful, but their high potential for leakage presents some potential problems if we’re worried about smaller aerosols (which, again, I think we should be). 

Epidemiology of masks

Finally, I am aware of several studies that have explored the utility of masks from an epidemiological perspective — that is, they evaluate infection rates among real people, ideally grouped into those that wore masks (or certain types of masks) and those who did not, or at least grouped into some similar classification. 

When I first read the only study of which I am aware that compared the effects of cloth masks versus surgical masks on respiratory illnesses in healthcare workers using a randomized trial, my heart sank. They tracked over 1600 healthcare workers in hospitals in Vietnam and grouped them into those who wore (1) medical masks (like surgical masks), (2) cloth masks, and (3) a control group (usual practice, which included mask wearing — they did this for ethical reasons; they couldn’t tell one group to not wear masks at all). They used their assigned masks for 4 straight weeks. The group that wore cloth masks experienced a 13-fold higher risk of influenza-like illnesses compared to the medical mask group — not 13%, but 13 times higher risk!

But if you read a little further, it makes plenty of sense: the surgical/medical masks had a particle filtration efficiency of ~56% while the cloth masks had a particle filtration efficiency of only ~3%!

Although it is unclear which particle sizes were tested in this evaluation, I assume it was a standard respirator fit test for small particles (i.e., < 1 µm) because they reported N95 masks filtered >99% of particles in the same test. Regardless, it makes plenty of sense to me that a randomized control trial in healthcare workers demonstrated that wearing cloth masks with essentially 0% removal efficiency for what are probably smaller aerosols was associated with an extraordinarily higher risk of acquiring influenza like illnesses compared to a surgical mask group with ~50% removal efficiency. In fact, in a backwards way it adds some confidence to the importance of aerosol transmission of those influenza-like illnesses. 

Beyond this study, other epidemiological evidence gives support to the effectiveness of mask wearing. For example, do you remember that story of a hair stylist in Missouri testing positive while seeing well over 100 clients? Well, it turns out that two hair stylists worked at the salon, both of whom were symptomatic with confirmed COVID-19, but both wore masks throughout their work days, as did the 139 clients they saw. Apparently no symptomatic secondary cases were reported among 104 clients that were later interviewed, and follow-up testing of 67 of those clients revealed no positive tests. One of the stylists reported wearing a two-layer cotton face covering while the other wore either a two-layer cotton face covering or a surgical mask. About half the clients reported wearing cloth face coverings and about half reported wearing surgical masks. 

Others have compiled and synthesized the epidemiological evidence on mask-wearing and have reported, for example, that physical distancing combined with masking has reduced other respiratory illnesses across the world (with stronger effects for N95 than surgical masks). However, others have reported that medical (surgical) masks and cloth masks have not been effective in stemming the transmission of respiratory viruses in healthcare workers compared to respirators (like N95). Granted, healthcare settings may not be widely applicable to non-healthcare settings because of the differences in exposures and contact times involved, so it is difficult to generalize much from the epidemiology data on respiratory viruses broadly, and doubly difficult with SARS-CoV-2.

So where does this leave us on masks for the general public?

Simple recommendations for masks for the general public

With some synthesis of the information above, I feel reasonably comfortable providing the following advice:

  • We should all be wearing masks of some kind in public, including in any indoor spaces and in crowded outdoor environments, especially in communities with significant prevalence of COVID-19 
  • We should strive for masks that effectively filter both larger droplets and smaller aerosols
  • Your mask should be at least two layers, and the more layers the better (although too many layers could increase airflow resistance and lead to leaks around the material)
  • Your mask should fit tight against your face, but should be reasonably comfortable to breath through
  • Mask material is important. Consider using:
    • High thread count cotton (600+ or higher) and/or multiple (~3) layers of cotton
    • Combinations of different fabrics in alternating layers (e.g., high thread count cotton + chiffon, silk, nylon, or flannel)
    • Nylon (or similar) wrappers placed over your mask to help it adhere close to your face
  • Masks that incorporate separate filters can be effective, as long as:
    • The secondary filter spans most of the width of the mask and also your mouth such that airflow and aerosols pass through the filter rather than around it
    • The secondary filter is known to have a high removal efficiency, either from the database listed above or from more formal testing (such as ASTM standards or HEPA certification for face mask materials)
  • The more information a mask manufacturer or seller provides, the more you can evaluate its claims against the recommendations and evidence above
    • Conversely, the less information a mask manufacturer or seller provides, the less inclined I am to trust its performance
  • Face shields can be use in addition to a good mask to provide additional eye protection, but they do not protect against aerosol inhalation and should not be used alone
  • Masks with valves should not be used because they can expel infectious aerosols if worn by an infected person, providing a false sense of security to others

In the end, it is also worth noting that some of these considerations might be important only on the margins, but I think those margins may become important as we begin spending more time indoors with others, reopening schools and offices and heading into the fall and winter.

For example, if you and I are both wearing a mask that is 50% effective, then we each experience a 75% reduction in transmission risk: 50% of your emissions are captured by your mask, and then my mask captures another 50% of that value, or 25%, providing a 75% net reduction for both you and me.

A 50% effectiveness mask may very well have been plenty effective for limiting transmission in, say, grocery stores and outdoor farmers markets over the spring and summer, but when we return to spending 2, 4, or 8 hours a day with others in indoor spaces, some of whom may be infected, then I’ll take any additional improvement in effectiveness that I can reasonably achieve. 



*I consider the distinction between “mask” and “face covering” not particularly useful, but if you want to draw a distinction, I would say that “masks” usually refer to hospital or industrial grade PPE such as respirators and surgical masks, while “face coverings” usually refer to garments of a more improvised nature, typically made of cloth or other fabrics. While “face coverings” can be shaped into “masks,” they’re explicitly not considered the same as hospital or industrial grade PPE like N95 masks. However, the distinction between high quality “face coverings” that are shaped and worn like “masks” and conventional “masks” such as surgical masks can be small in terms of performance. 

**Linsey Marr at Virginia Tech has a really nice slide deck on viruses in air for those interested in learning more. I also have a slide deck on this topic, as well as modes of transmission, from my 2020 summer course ENVE 576 Indoor Air Pollution.