David Kyle Johnson
King’s College (PA)
Why do you have to wear a mask in class? The short answer is because wearing a mask makes it less likely that you will unknowingly pass COVID on to others. The school is requiring everyone to do it because that makes a campus outbreak much less likely. But since the issue of mask wearing is so hotly debated in public discourse, before the semester starts, it will be worth our time to gain a basic understanding of (a) how masks work, (b) how we know they work, (c) how we know they are not dangerous—and (d) why the pseudoscientific arguments and misinformation that you have heard is wrong.
How Masks Work (To Help Prevent the Spread of COVID)
There is no longer any debate in the scientific community about whether masks can help prevent the spread of COVID-19. There was some debate back in March and April 2020, but once the evidence was clearly laid out, and the function of masks was made clear, the issue was settled and the experts agreed: yes, widespread mask use can significantly curtail the spread of COVID. Still, misinformation is prevalent and has confused the issue.
For example, it’s commonly believed that, if masks are going to help mitigate the spread, they will do so by filtering the air that a person inhales, thus protecting the person who wears the mask. This is why many think mask wearing should be a choice. “If I want to risk getting COVID, that’s my right.” This is also why many initially thought that mask use would be ineffective. While there is some evidence that cloth masks may be able to offer some protection to their wearer by filtering inhaled air, that protection is limited. Indeed, boxes of surgical masks come with a warning that they are not a reliable means of protecting the wearer from infection.
In reality, however, this is irrelevant because the primary way masks help mitigate the spread is by protecting others from the person wearing them, if that person happens to be infected. This is called mitigating source spread. And they do so filtering the air a person exhales—when they sneeze, cough, speak, or breathe. This helps because a virus doesn’t travel by itself; it travels in droplets of moisture propelled from an infected person’s mouth.
The largest droplets, which carry the majority of the viral load, settle on people and objects in the nearby vicinity of the person expelling them. (This probably expands beyond 6 feet.) If they end up in someone’s nose, mouth, or eye, that person could get infected. The smallest, called aerosols, carry the least viral load,  but are so light they can float in the air for hours and travel much further. If they are inhaled into a person’s lungs, that person could get infected. Those in the middle carry a midsize viral load, but can actually evaporate and aerosolize, after they leave an infected person’s mouth. Consequently, they might be the worst, because they can deliver pretty large viral loads over significant distances for hours.
Now, there is still scientific debate about whether droplets or aerosols are the primary mode of infection for COVID. But since masks can capture any of these kinds of droplets, that doesn’t matter. When they do, they lower the number of infected droplets in the air and environment, and thus reduce the risk of exposure for other people.  Once the experts started thinking in terms of a mask’s ability to protect others from their wearer, and realized how common it was for people to have COVID at not know it, experts realized the usefulness of widespread mask use and began recommending it.
Why are masks effective in capturing such droplets? The biggest droplets, which carry the majority of the viruses expelled from your mouth, are too big to even attempt to get through the mask; they are just intercepted by its surface. The smaller ones are stopped by something called impaction as they try to make it through the mask. The tiny pathways in the mask are too convoluted (twisty/turny); the particles can’t make their way through and end up adhering to the mask material. The smallest droplets are diffused by the mask because of something called Brownian motion–the erratic way they move.Indeed, because they don’t travel in straight lines, they are pretty easy to catch.
Now it’s important to note that, while all masks filter exhalations, certain kinds of masks do that better than others. The early woven cloth masks were effective, but not as effective as later non-woven ones—what we today call “medical” or “surgical masks.” (According to Anna Davies, while both types are effective, surgical masks are about three times more effective.) But different kinds of cloth, layered or combined, can offer better filtration, by making the pathway particles would have to travel to get through the mask more convoluted. Onur Aydin found that layering cotton fabrics can make homemade masks almost as effective as surgical masks, and Abhiteja Konda found that layering different kinds of fabrics (like cotton with flannel) could also increase their efficacy. Such masks filtered 80% of particles smaller than 300 nanometers (0.3 microns) and 90% of those larger. Filters can also improve mask efficacy. But if a mask has an unfiltered exhaust valve, they are almost completely useless; they might even project the droplets of an infected person farther. (So don’t use them!)
Now, if an infected droplet is already in the air that a masked person is about to inhale, the mask will be less likely to catch that particle. This is why they protect their wearer less efficiently. But masks are very efficient at filtering the air a person exhales. This is partly due to air flow; unlike when you are breathing in, when you are breathing out, you are forcing most of the air directly through the mask. But, more importantly, the longer a droplet is in the air, the more opportunity it has to evaporate and become smaller (and even aerosolize)—and (generally) the smaller it is, the less likely a droplet is to be caught by a mask. So infected droplets in the air that an unmasked person has exhaled are going to be smaller and thus have a better chance at making it through your mask. But most of the infected droplets exiting your mouth won’t; they will be larger since they have not yet had a chance to evaporate. So your mask is much more likely to catch them.
So, all in all, masks unquestionably work to help prevent the spread of COVID because they filter droplets (those on the right of the picture), including most of them that would become aerosols (those in the middle), and can even capture those that start out small (on the far left)—although, admittedly, a bit less efficiently.
So, if you are in a classroom with a non-masked infected person, you are pretty likely to be infected even if you are wearing a cloth mask and are more than six feet away. Just by breathing, they are spewing loads of infected droplets and aerosols into the air and surrounding environment. However, if that infected person is wearing a good cloth or surgical mask, you are much less likely to be infected—even if you are not wearing a mask and only 6 feet away. This is why it is said “My mask protects you; your mask protects me.” Your choice to not wear a mask does not put you at risk; it puts others at risk. And it is for this reason that the “it’s my right to not wear a mask” argument has no legs to stand on. While you might have a right to endanger yourself, you unquestionably do not have a right to endanger others.
How We Know Masks Help Prevent the Spread Of COVID
Given what we know masks do, the fact that mandating them helps curb the spread of COVID is just common sense. But the evidence also bears this out.
For example, transmission rates slowed in hospitals, German cities, North Texas, and in US states after mandates went into effect. (In Germany, they reduced growth rates by 40%.) What’s more, U.S. States with mandates have seen much less spread than those without. The same is true in countries around the world where mask use is popular. Now some will argue that such correlational studies can’t prove anything because “correlation doesn’t entail causation.” But this is a misuse of that logical rule. A single correlation does not guarantee causation, but enough of them can imply it strongly enough to produce knowledge. For example, deaths rates dropped 27% after seat belt mandates were enacted in New York State, and similar numbers were seen in all states that enacted such laws. Something similar happened for deaths in motorcycle accidents after helmet mandates were enacted. When the physical connection is obvious, and the correlation is repeated, it most definitely entails causation.
Modeling has also confirmed the effectiveness of mask mandates. According to Richard Stutt and Steffen Eikenberry, if masks are just 50% effective, they could help bring down infection rates to non-epidemic levels and reduce the death rate by as much as 45%. Large reviews of observational and comparative studies have also concluded that mask mandates are highly effective, as have collections of collaborating experts. And, of course, there were those infected hair stylists in Missouri who didn’t pass it on to 140 of their clients because they were wearing a mask. The IHME found that mask mandates could save 33,000 by October 1st, and (according to Brooks) if everyone wore a mask, we could get the pandemic under control in four to eight weeks. 
If one is being stubborn, however, one might demand that this is not good enough. We need randomized control trials (RCTs) for mask safety and efficacy—the supposed “golden standard” in science. Well, we actually do have some. Nancy Leung, for example, performed a randomized controlled trial in which she masked some people (but not others) with respiratory illness and tested for infected droplets and aerosols; she found that “Surgical face masks reduced detection of coronavirus RNA in both respiratory droplets and aerosols.” But anti-mask activists are likely to insist that such studies are not good enough. We need a RCT that measures how many people got sick with infected people wearing, or not wearing, a mask.
What such arguments fail to recognize, however, is not only the limits that ethical considerations put on such studies. They also fail to recognize that, while RCTs are great (indeed necessary) for testing drugs and treatments, RCTs are not necessary or even appropriate for other scientific fields or questions. Indeed, RCTs are often easily misused and can lead to confusion. To understand why, consider an example.
Suppose you wanted to know about the safety and efficacy of Kevlar vests regarding their ability to protect people from bullets. So someone explains the science—the physics of how Kevlar resists bullets—and show you how it works in a lab. Maybe they shoot bullets at plastic dummies, some wearing and some not wearing Kevlar vests, and see how they fare. They also show you correlational studies of how, say, death and injuries drop in army platoons after Kevlar vests are issued. That would be good enough, right? Of course. At that point, you would now know that they work.
And if anyone went further and performed a RCT on Kevlar vests, it would be pretty much useless. Why? Because the only way to do a completely controlled and bias free fully analogous RCT on Kevlar vests would be to put 1000 people in a room, give half of them Kevlar vests at random, and then start blindly shooting at them. Obviously, that kind of study cannot ethically be done. So, at best, an RCT could only compare, say, different groups of soldiers, already out in the field, who are either wearing or not wearing vests.
But since the real world is filled with all kinds of randomness and chaos, if such a study failed to find a difference in death and injury rates between the two groups, it wouldn’t be because vests offer no protection. It would be because real-world randomness threw off the results—because, say, the non-vested soldiers fought in less dangerous battles, or because the vested soldiers didn’t wear them right. To be clear, enough very carefully monitored studies might be able to show what certain kinds of vests are needed in certain circumstances; “Soldiers in X conditions need Y type vests.” But given what we already know about how Kevlar works, and what we have proved Kevlar can do in a lab, it would be absurd to think that a single RCT (or even a bunch of RCTs) could ever prove that they offer no protective effect to their wearer.
That’s how it is with masks. We understand the science of how they work. We know they block droplets and aerosols; we know that is how COVID is spread; we know mask mandates make more people wear masks, and we know COVID is spread pre- and asymptomatically. We even have trials where their use reduces the number of infected particles in the air. Combine that with the above examples from around the world of rates dropping with mask use and mandates, and that’s all you need. We know they work. No ethically dubious RCTs, where we see who gets sick after we throw healthy people into rooms with COVID patients who may or may not be wearing masks, are necessary.
And any other kind of limited RCT that we did in the field on masks would be pretty useless. Even if it found no significant results, that wouldn’t tell us masks don’t work. Random variables (like proper mask use) skewing the results would always be the more likely explanation. If they are carefully designed enough, such studies might be able to point towards one kind of mask being more effective than the other, or more needed in this or that situation. But proving masks don’t work at all is going to be next to impossible. Such studies can’t be controlled well enough to overturn something that is already well established. (And people who cite such studies, but leave out the part where the author acknowledges this, are trying to mislead you.) And since the evidence we have (which I reviewed above) has already well established that masks can block droplets and aerosols, and thus reduce the number of infected particles in the air, we know people wearing them will help reduce the spread of COVID-19.
So we know mask use helps mitigate the spread of COVID. And that, dear student, is why you have to wear a mask in class. But you probably still have a few questions, like…
Are Masks Dangerous?
You are bound to find stories and examples on the internet about how masks are dangerous. Some people claim they restrict your oxygen, for example, or can cause carbon dioxide poisoning. You may have even heard a story about a guy who passed out while driving his car and wearing a mask, or saw a video of a carbon dioxide detector going off the scale while under some kid’s mask. But in reality, such concerns are nonsense.
First of all, anecdotes are not evidence. People fall asleep or pass out while driving all the time; with mask use so prevalent, it’s not surprising it would eventually happen while someone was wearing a mask. This is most certainly a time where correlation does not equal causation. Such stories are only evidence that people don’t understand what masks are for. Unless you are driving with people outside your household, you don’t need to wear a mask while driving.
Second, carbon dioxide detectors are designed to detect slight increases in indoor/household air, not the amount of carbon dioxide in the air coming directly out of your mouth. If you breathed directly on one without a mask, it would start going off too. Such “experiments” prove nothing. What’s more, I can find you a lot more videos of people wearing two or three masks while the oxygen level in their blood is shown to be completely normal. Masks do not restrict oxygen or hold in carbon dioxide.
And third, the claim is self-contradictory. The same people who say masks are dangerous are those that say they don’t work. But which is it? Either masks are so porous that they can’t block droplets and aerosols, or they are so non-porous that they trap carbon dioxide and restrict oxygen. You can’t have it both ways. Carbon dioxide and oxygen are gases; droplets and aerosols are liquid. Masks can’t restrict gas flow and choke you to death, while at the same time not restrict infectious liquids. (They can, however, be designed to block liquids but not gases—which they are.)
What’s more, masks have been worn by medical and other professionals for over 100 years. If we were going to see some negative side effects, we would have seen them by now. The notion that they are dangerous is absurd.
Debunking the Pseudoscience
Stuff you have seen online might also raise some questions, like…
How can masks work if they can’t block everything?
The answer here is simple: Masks don’t have to block everything to lower risk. By merely blocking some infected particles, masks reduce the risk of infection to others, by lowering the number of infected particles in the air. As I discussed above, if they were just 50% effective, they could keep COVID below epidemic levels—and tests show they are 80% to 90% effective.
Or think of it this way. Bullet proof vests can’t block everything either. Armor piercing bullets can still get through, and your head, legs, and arms are still exposed. That doesn’t mean they “don’t work”—that Kevlar vests offer no protection. This argument commits what’s called the “all or nothing” fallacy.
Didn’t the CDC and the WHO change their tune on masks?
Yes, but that doesn’t mean masks don’t work. It means that the CDC and WHO updated their recommendations based on the evidence, as any good scientific organization should. In fact, they didn’t do so quickly enough. The old recommendations were based on thinking in terms of a mask’s ability to protect its wearer; that is why they did recommend mask use for higher risk patients—they do offer a little protection in this regard. But once they shifted to thinking in terms of source spread (the mask’s ability to protect others from the wearer), and realized how long those with COVID can be asymptomatically infectious, they updated their recommendation.
What about those studies that say masks don’t work?
Online you will find people citing studies they say prove “masks don’t work.” In reality, they are usually studies about whether cloth or surgical masks offer enough protection to healthcare workers in high-risk environments. As we have already discussed, they usually don’t. (Healthcare workers should usually be wearing N95 respirators.) But whether or not masks protect their wearer is completely irrelevant to whether they can protect others from their wearer. As we discussed above, the air that someone exhales is filled with large droplets; the air that someone inhales is not.
Worse still, even if the studies are about source spread, anti-maskers will selectively quote them to make it seem like the studies entail something that they don’t—by, say, leaving out the part where the authors admit that the study was limited and inconclusive (which they all must be, given what we discussed above about RCTs). This is called quote mining.
To use a real world example: a study that says “Parachute use did not reduce death or major traumatic injury when jumping from aircraft in the first randomized evaluation of this intervention” sounds like convincing evidence that parachutes don’t work…until you read the next sentence: “However, the trial was only able to enroll participants on small stationary aircraft on the ground, suggesting cautious extrapolation to high altitude jumps.”
Likewise, “Although mechanistic studies support the potential effect of hand hygiene or face masks, evidence from 14 randomized controlled trials of these measures did not support a substantial effect on transmission of laboratory-confirmed influenza” sounds convincing … until you read “Most studies were underpowered because of limited sample size, and some studies also reported suboptimal adherence in the face mask group.”
And then sometimes the very study anti-maskers quote to show masks don’t work actually proves that they do. Take Faisal bin-Reza’s study for example. I’ve seen anti-maskers point out that he says
“None of the studies established a conclusive relationship between mask/respirator use and protection against influenza infection.”
That sounds convincing! But if you bother to actually look at that sentence in its full context, you will realize that the authors were actually comparing masks and respirators and saying that they were equally effective (not grouping them together and saying they were both ineffective). What’s more, that quote comes from the part of the study about influenza, and the authors specifically state that their findings about influenza cannot be extrapolated to things like SARS because “SARS is an unusual acute viral respiratory infection with a very different epidemiology to almost all other respiratory viral infections. It is fundamentally different from human inﬂuenza.” (This is significant because the virus that causes COVID (SARS-CoV-2) is much more like the virus that causes SARS (SARS-CoV-1) than it is like the viruses that cause the flu.) Furthermore, the authors of the study specifically state their study did find that “mask and⁄or respirator use was independently associated with a reduced risk of severe acute respiratory syndrome [SARS].” Funny how they left that part out, right?
Now, of course, the anti-masker might go on to quote more studies—but once a person establishes that they have a habit of misrepresenting the evidence, you can no longer trust that they are telling you the whole truth about what studies say and show. But if you bothered to look closer anyway, you would see that the studies are either irrelevant, being quote mined, or actually contradict their conclusion.
Isn’t Humidity to Blame?
If you really dig deep, you might find people who try to blame the humidity for COVID transmission, and thus argue that masks are useless. But while low humidity might be the explanation for why certain long-standing respiratory disease rates spike in the winter, that has nothing to do with whether or not masks can filter particles and thus reduce infection rates for COVID. Regardless of the humidity, they can reduce the number of particles in the air. What’s more, according to Rachel Baker
“in a pandemic like the one we’re in now, what decides how quickly the new virus spreads is how many people are susceptible, or not immune, to it. Climate would play a bigger role only as more people become immune.”
And given how COVID spread like wildfire in Texas and Florida in July, she seems to be right.
So, while we all know that wearing a mask is a pain, we also know it works—and it’s a relatively small inconvenience compared to its benefits. Please know, your professor hates trying to teach in it more than you hate sitting and listening in it. So please do your part, and wear your mask—over both your mouth and your nose. I can’t emphasize this enough: it does very little good if it’s not over your nose. Remember–we’re all in this together! My mask protects you, and your mask protects me. So, I thank you in advance for protecting me!
 This paper was inspired by my course “Scientific, Pseudoscientific, and Medical Reasoning” which was moved online in the spring of 2020 due to the COVID pandemic.
 See Rick Kushman, “Your Mask Cuts Own Risk by 65 Percent,” UC Davis, July 6, 2020, https://www.ucdavis.edu/coronavirus/news/your-mask-cuts-own-risk-65-percent/. Caitlin McCabe, “Face Masks Really Do Matter. The Scientific Evidence Is Growing,” The Wall Street Journal, July 18, 2020, https://www.wsj.com/articles/face-masks-really-do-matter-the-scientific-evidence-is-growing-11595083298 See WH Seto,et al., “Effectiveness of Precautions Against Droplets and Contact in Prevention of Nosocomial Transmission of Severe Acute Respiratory Syndrome (SARS),” The Lancet 361, no 9368 (May 3, 2003): 1519-20, https://doi.org/10.1016/S0140-6736(03)13168-6. and Hiroshi Nishiura, et al., “Rapid Awareness and Transmission of Severe Acute Respiratory Syndrome in Hanoi French Hospital, Vietnam,” Am J Trop Med Hyg 73, no. 1 (July 2005): 17-25, https://pubmed.ncbi.nlm.nih.gov/16014825/. Lijie Zhang, et al., “Protection by Face Masks against Influenza A(H1N1)pdm09 Virus on Trans-Pacific Passenger Aircraft, 2009,” Emerging Infectious Diseases 19, no. 9 (September 2013): 1403-10, https://doi.org/10.3201/eid1909.121765. See Christian J. Kähler and Rainer Hain, “Fundamental Protective Mechanisms of Face Masks Against Droplet Infections,” Journal of Aerosol Science 148 (2020) https://doi.org/10.1016/j.jaerosci.2020.105617. C. Raina MacIntyre, et al. “A Cluster Randomised Trial of Cloth Masks Compared with Medical Masks in Healthcare Workers,” BMJ Open 5 no. 4, (2015) https://doi.org/10.1136/bmjopen-2014-006577. This study suggested that 95% of viruses in aerosols could be blocked by homemade masks, and 97% could be blocked by surgical masks: Qing-Xia Ma, et al, “Potential Utilities of Mask-Wearing and Instant Hand Hygiene for Fighting SARS-CoV-2” Journal of Medical Virology (2020) https://doi.org/10.1002/jmv.25805. This is a study out of Hong Kong which suggested that people wearing a mask was very effective at reducing transmission of alpha coronaviruses”; Nancy H. L. Leung, et al., “Respiratory Virus Shedding in Exhaled Breath and Efficacy of Face Masks,” Nature Medicine 26 (2020): 676-80, https://doi.org/10.1038/s41591-020-0843-2.
 See also WH Seto, et al., “Effectiveness of Precautions Against Droplets and Contact in Prevention of Nosocomial Transmission of Severe Acute Respiratory Syndrome (SARS),” The Lancet 361, no 9368 (May 3, 2003): 1519-20, https://doi.org/10.1016/S0140-6736(03)13168-6; see also Hiroshi Nishiura, et al., “Rapid Awareness and Transmission of Severe Acute Respiratory Syndrome in Hanoi French Hospital, Vietnam,” Am J Trop Med Hyg 73, no. 1 (July 2005): 17-25; Harvey Fineberg, Rapid Expert Consultation on the Possibility of Bioaerosol Spread of SARS-CoV-2 for the COVID-19 Pandemic (April 1, 2020), (The National Academies Press, 2020), chapter 1 and 2, https://www.nap.edu/read/25769/chapter/1#2.
 Here I am thinking of droplets approx. 120 mircons and up.
 The idea that they travel only around 6 feet is based on outdated evidence; they likely travel farther. See See Zeshan Qureshi, et al., “What is the Evidence to Support the 2-metre Social Distancing Rule to Reduce COVID-19 Transmission?” CEBM, June 22, 2020, https://www.cebm.net/COVID-19/what-is-the-evidence-to-support-the-2-metre-social-distancing-rule-to-reduce-COVID-19-transmission/.
 Only about 1 in every 700 of the smallest droplets (smaller than 10 microns), expelled from an infected persons mouth, contain even a single virus. Adrien Burch, “A Microscopic Perspective on Airborne COVID-19,” Medium, March 31, 2020, https://medium.com/better-humans/should-you-be-worried-about-catching-COVID-19-from-aerosols-6c97d023bb6d.
 Here I am thinking of droplets smaller than 10 microns.
 Depending on the humidity, droplets between 10 and 120 microns can evaporate and aerosolize at different rates.
 Neeltje van Doremalen, N., Bushmaker T., Morris D.H., Holbrook M.G., Gamble A., Williamson B.N., Tamin A., Harcourt J.L., Thornburg N.J., Gerber S.I., Lloyd-Smith J.O. “Aerosol and surface stability of SARS-CoV-2 as Compared with SARS-CoV.: N. Engl. J. Med 382 (2020):1564–1567. https://www.nejm.org/doi/10.1056/NEJMc2004973
 See Mahesh Jayaweera (et. al.), “Transmission of COVID-19 virus by droplets and aerosols: A critical review on the unresolved dichotomy” Environ Res. 188 (Sep 2020): 109819. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7293495/
 Talib Dbouk and Dimitris Drikakis, “On Respiratory Droplets and Face Masks,” Physics of Fluids 32, no. 063303, published electronically June 16, 2020, https://doi.org/10.1063/5.0015044. Bhanu Bhakta Neupane, Sangita Mainali, Amita Sharma, and Basant Giri, “Optical Microscopic Study of Surface Morphology and Filtering Efficiency of Face Masks,” PeerJ 7,no. e7142 (2019), https://doi.org/10.7717/peerj.7142.
 Wycliffe E Wei, Zongbin Li, Calvin J Chiew, Sarah E Yong, Matthias P Toh, and Vernon J Lee, ”Presymptomatic Transmission of SARS-CoV-2-Singapore, January 23-March 16, 2020,” MMWR Morb Mortal Wkly Rep. 69, no. 14 (April 2020): 411-5, https://doi.org/10.15585/mmwr.mm6914e1.
 Depending on your definition, aerosols range from around 100 microns to 0.1 micron. “Various sources will put the cutoff at 2 µm, 5 µm, 10 µm, 20 µm, or even 100 µm.” Justin Morgenstern, “Aerosols, Droplets, and Airborne Spread: Everything You Could Possibly Want to Know,” First10EM, April 6, 2020, https://first10em.com/aerosols-droplets-and-airborne-spread/. For simplicity, I’ll define aerosol as a droplet that is 10 µm in size. Brownian motion dominates in particles less than 0.3 µm in size.
 Cloth masks of only one material seem to have very little effectiveness: Samy Rengasamy, Benjamin Eimer, and Ronald E. Shaffer, “Simple Respiratory Protection – Evaluation of the Filtration Performance of Cloth Masks and Common Fabric Materials Against 20-1000 nm Size Particles,” Annals of Occupational Hygiene 54, no. 7 (October 2010): 789-98, https://doi.org/10.1093/annhyg/meq044. This is why those who are just wearing bandanas or pulling their t-shirt over their mouth, are not doing anyone much good.
 Both types “significantly reduced the number of microorganisms expelled by volunteers,” “the surgical mask was 3 times more effective.”) Anna Davies, et al., “Testing the Efficacy of Homemade Masks: Would They Protect in an Influenza Pandemic?” Disaster Medicine and Public Health Preparedness 7, no. 4 (August 2013): 413-8, https://doi.org/10.1017/dmp.2013.43; Milton (2013) found that surgical masks decreased emission of large particles by 25 fold, and aerosols by 3 fold in flu patients. See Donald K Milton, et al., “Influenza Virus Aerosols in Human Exhaled Breath: Particle Size, Culturability, and Effect of Surgical Masks,” PLoS Pathogens 9, no. 3 (March 2013): 1-7, https://doi.org/10.1371/journal.ppat.1003205.
 Aydin et al. (2020), suggests that layering greatly increases the filtering efficiency of cloth masks while also maintaining some breathability, Onur Aydin, et al., “Performance of Fabrics for Home-Made Masks Against the Spread of Respiratory Infections through Droplets: A Quantitative Mechanistic Study,” medRxiv,preprint, submitted July 8, 2020 https://doi.org/10.1101/2020.04.19.20071779.
 Abhiteja Konda, Abhinav Prakash, Gregory A. Moss, Michael Schmoldt, Gregory D. Grant, and Supratik Guha, “Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks,” ACS Nano 14, no. 5 (2020): 6339-47, https://doi.org/10.1021/acsnano.0c03252.
 “Overall, we find that combinations of various commonly available fabrics used in cloth masks can potentially provide significant protection against the transmission of aerosol particles.” Konda, “Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks.”
 Dr. Marty, a professor of infectious diseases at Florida International University told Good Morning America “But if you add that filter, then you’re also adding a really good protection for yourself.” See Becky Worley, Anthon Kane, Robyn Weil, and Angeline Jane Bernabe, “Face Masks With Filter add Another Layer of Protection, Experts Say,” GMA, July 16, 2020, https://abcnews.go.com/GMA/Wellness/face-masks-filters-add-layer-protection-experts/story?id=71811792.
 Unless it is smaller than 0.3 microns.
 Some clarification here is useful. Technically, depending on how you classify “aerosols” (definitions range from 5 microns to 100 microns), most of the particles you breathe out could be classified as aerosols–and depending on their size, the mask will filter them with different efficiencies. Even cloth masks are very good at filtering down to 10 microns, pretty good down to 5 microns, but not great below 5. See B.B. Neupane, “Optical Microscopic Study of Surface Morphology and Filtering Efficiency of Face Masks.” PeerJ. 2019; 7: e7142. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6599448/
In one way, this is concerning because according to Adrien Burch (2020), the average size for aerosols leaving your mouth is 3 microns. The good news is, despite the fact that they make up the largest number, they only represent 0.00024% of the liquid leaving your mouth during a cough. Consequently, very few of them are infected (at worst 1 out of every 700). A full 99.99976% of the viruses sprayed during a cough are carried in droplets — not aerosols.’) So the majority of transmission happens from droplets. What’s more, the deadliest aerosols are those that started out as droplets, but then evaporated down; they have higher concentrations of the virus. Masks catch those. So the inability of masks to filter out 3 micron (or 0.3 micron) particles does not greatly hinder their ability to keep infected particles out of the air, and thus does not prevent them from efficiently preventing the spread of COVID.
Adrien Burch, “A Microscopic Perspective on Airborne COVID-19,” The Medium, March 31, 2020, https://medium.com/better-humans/should-you-be-worried-about-catching-COVID-19-from-aerosols-6c97d023bb6d.
 From the abstract of: Richard O. J. H. Stutt, Renata Retkute, Michael Bradley, Christopher A. Gilligan, and John Colvin, “A Modelling Framework to Assess the Likely Effectiveness of Facemasks in Combination with ‘Lock-down’ in Managing the COVID-19 Pandemic,” Proceedings of the Royal Society A (2020) https://doi.org/10.1098/rspa.2020.0376.
 Anthony D Sung, et al. “Universal Mask Usage for Reduction of Respiratory Viral Infections after Stem Cell Transplant: a Prospective Trial,” Clin Infect Dis 63, no. 8 (October 2016): 999-1006, https://doi.org/10.1093/cid/ciw451. Xiaowen Wang, Enrico G. Ferro, Guohai Zhou, et al., “Association Between universal Masking in a Health Care System and SARS-CoV-2 Positivity Among Health Care Workers,” JAMA,published electronically July 14, 2020, https://doi.org/10.1001/jama.2020.12897. In this study, cases of COVID-19 declined after mask mandates were put into effect in hospitals (that required all health care workers and patients to mask up). The study concluded that such mandates reduce the transmission of SARS-CoV-2.
 Timo Mitze, Reinhold Kosfeld, Johannes Rode, and Klaus Wälde, “Face Masks Considerably Reduce COVID-19 Cases in Germany: A Synthetic Control Method Approach,” IZA (June 2020) http://ftp.iza.org/dp13319.pdf. This study shows the impact of mask mandates in Germany. In Jena, for example, the first German city to enact such a mandate, COVID-19 cases fell by almost 25% in 20 days. The study concluded that similar mandates could reducethe daily growth rate by 40% in the long term, although it did acknowledge that, outside Germany, different norms and climatic conditions in other countries might result in different protective outcomes.
 “HSC COVID-19 Report #5 – July 20, 2020,” University of North Texas Health Science Center at Fort Worth, https://www.scribd.com/presentation/469858261/COVID-19-Report-July-20-Updated?fbclid=IwAR1ta8C-x5yYfpqQ5eghmiPFr42ndbA6rYCmTv3WbcGU9tDt3a_RU1BOIL0.
 In those 15 US States, they likely prevented up to 450,000 cases in under two months. Wei Lyu and George L. Wehby, “Community Use of Face Masks and COVID-19: Evidence from a Natural Experiment of State Mandates in the US,” Health Affair 39, no. 8 (2020): 1-7, https://doi.org/ 10.1377/hlthaff.2020.00818.
This was a retrospective analysis which examined the effects that different governmental orders to wear face masks had on COVID-19 growth rates, from April 9-May 15, 2020. It estimated that they prevented between 230,000 and 450,000 cases by May 22 (a reduction of 14-27%).
 Kasra Zarei and John Duchneskie, “Coronavirus Cases Rise in States with Relaxed Face Mask Policies,” The Philadelphia Inquirer, June 24, 2020, https://www.inquirer.com/health/coronavirus/COVID-19-coronavirus-face-masks-infection-rates-20200624.html.
 American Thoracic Society, “Countries with Early Adoption of Face Masks Showed Modest COVID-19 Infection Rates,” Medical Xpress, June 24, 2020, https://medicalxpress.com/news/2020-06-countries-early-masks-modest-COVID-.html.
 Joseph Berger, “Death Drops 27% With State’s Seat-belt Law, The New York Times, May 1, 1985, https://www.nytimes.com/1985/05/01/nyregion/death-drops-27-with-state-s-seat-belt-law.html.
 Samantha M. Tracht, Sara Y. Del Valle, and James M. Hyman, “Mathematical Modeling of the Effectiveness of Facemasks in Reducing the Spread of Novel Influenza A (H1N1),” Plos One 5, no. 2 (February 2010): 1-12, doi.org/10.1371/journal.pone.0009018.
 Stutt, et al., “A Modelling Framework to Assess the Likely Effectiveness of Facemasks in Combination with ‘Lock-down’ in Managing the COVID-19 Pandemic.” To keep the infection rate (R0) below 1.0, the authors argue for widespread use of face masks. “[F]acemask adoption by entire populations would have a significant impact on reducing COVID-19 spread.” “[I]n summary, our modelling analyses provide support for the immediate, universal adoption of facemasks by the public.”
See also, Eikenberry, et. al. “To mask or not to mask: Modeling the potential for face mask use by the general public to curtail the COVID-19 pandemic.” Infectious Disease Modelling doi: 10.1016/j.idm.2020.04.001 https://www.medrxiv.org/content/10.1101/2020.04.06.20055624v1
 Derek K Chu, et al., “Physical Distancing, Face Masks, and Eye Protection to Prevent Person-to-person Transmission of SARS-CoV-2 and COVID-19: A Systematic Review and Meta-analysis,” The Lancet 395, no. 10242 (2020): 1973-87, https://doi.org/10.1016/S0140-6736(20)31142-9. This was a review of 172 observational studies and 44 relevant comparative studies. The authors concluded “Face mask use could result in a large reduction in risk of infection.”
 Kimberly A. Prather, Chia C. Wang, and Robert T. Schooley, “Reducing Transmission of SARS-CoV-2,” Science 368, no. 6498 (June 2020): 1422-24, https://doi.org/10.1126/science.abc6197. In this paper, aerosol chemists and an infectious disease specialist argue that, because “airborne spread from undiagnosed infections will continuously undermine the effectiveness of even the most vigorous testing, tracing and social distancing programs,” the widespread use of masks are necessary to help prevent the spread of COVID. Both analytical information about the virus and information about countries where masks are commonplace was used.
Catherine M. Clase, et al., “Cloth Masks May Prevent Transmission of COVID-19: An Evidence-Based, Risk-Based Approach,” Annals of Internal Medicine, published electronically May 22, 2020, https://doi.org/10.7326/M20-2567. This study, done by an international research team of medical doctors and other medical specialists not only concluded that cloth masks worn by the public will reduce COVID-19 transmission rates but that the benefits of widespread mask use outweigh any risks that may be brought about by wearing masks (such as improper use).
 M. Joshua Hendrix, Charles Walde, Kendra Findley, and Robin Trotman, “Absence of Apparent Transmission of SARS-CoV-2 from Two Stylists After Exposure at a Hair Salon with a Universal Face Covering Policy – Springfield, Missouri, May 2020,” Weekly 69, no. 28 (July 1, 2020): 930-32, http://dx.doi.org/10.15585/mmwr.mm6928e2.
 “New IHME COVID-19 Model Projects Nearly 180,000 US Deaths,” IHME,June 24, 2020, http://www.healthdata.org/news-release/new-ihme-COVID-19-model-projects-nearly-180000-us-deaths.
 For the quote, see McCabe, “Face Masks Really Do Matter.” For the evidence behind it, see John T. Brooks, Jay C. Butler, Robert R. Redfield, “Universal Masking to Prevent SARS-CoV-2 Transmission – The Time is Now,” Jama, published online July 14, 2020, https://doi.org/10.1001/jama.2020.13107.
 For more such evidence, see “Face Masks – A Summary of Relevant Research Papers for COVID-19,” Sound Reason & More,June 11, 2020, https://soundreasonandmore.wordpress.com/2020/06/11/face-masks-a-summary-of-relevant-research-papers-for-COVID-19/.
 Leung et al. 2020. Respiratory virus shedding in exhaled breath and efficacy of
face masks. Under review. https://www.researchsquare.com/article/rs-16836/v1
 Rebecca A. Clay, “More than One Way to Measure,” American Psychological Association 41, no. 8 (September 2010): 52, https://www.apa.org/monitor/2010/09/trials; Roger Mulder, et al., “The Limitations of Using Randomised Controlled Trials as a Basis for Developing Treatment Guidelines,” Evidence-Based Mental Health 21, no. 1 (2018): 4-6, http://dx.doi.org/10.1136/eb-2017-102701.
 Jingyi Xiao, et al., “Nonpharmaceutical Measures for Pandemic Influenza in Nonhealthcare Settings—Personal Protective and Environmental Measures,” EID26, no. 5 (2020): 967-75, https://dx.doi.org/10.3201/eid2605.190994.
 Faisal bin-Reza, Vicente Lopez Chavarrias, Angus Nicoll, and Mary E. Chamberland, “The use of masks and respirators to prevent transmission of influenza: a systematic review of the scientific evidence,” Influenza and Other Respiratory Viruses 6, no. 4 (December 2011): 257, https://do8i.org/ 0.1111/j.1750-2659.2011.00307.x.