Summary of Coronavirus masks: What’s the latest evidence?

We have four main COVID-19 pages: on core evidence, on dangerous myths, on useful outside resources, and on mask-related evidence (the page you’re reading).

Face masks (all types confounded, and worn by both the healthy and the sick) may reduce the daily growth rate of COVID-19 infections by around 40%.[1]

What are the different types of masks?

There are three major types of masks: non-medical, medical/surgical, and N95. In this article, we’ll be covering a variety of studies on all three, from randomized trials to case reports.

When it comes to COVID-19, masks can reduce pathogen circulation from/to the mouth and nose.[2][3] They can also serve to remind you not to touch your face, thus preventing you from transmitting pathogens from your hands to your mouth, nose, or eyes.

There are three major types of masks: non-medical, medical/surgical, and N95. Research comparing them is imperfect, because you can’t ethically conduct a perfect randomized trial of masks during a pandemic. Also, most of the evidence is from hospitals, where the risk of patient/worker transmission is much higher; this evidence doesn’t always apply well to other settings.[4]

Here’s a simplistic summary of the evidence. Read on for more details.

* Enforcement policy for face masks and respirators during the COVID-19 public health emergency, revised

** NIOSH-approved particulate-filtering facepiece respirators

Non-medical masks

These include masks that are made at home (typically cloth masks, made out of T-shirts and the like) as well as other non-medical and non-N95 masks. Their potential benefit varies widely depending on the material used, the number of layers, the fit, and how they’re cleaned.[5]

Medical masks, aka surgical masks

Whereas N95 respirators are tested and standardized, surgical masks vary widely in efficacy, being able to filter between <10% and 90% of droplets. Their fit is usually much looser than the fit of N95 respirators, so side leakage is a risk. They appear to offer only minimal to moderate protection for the wearer but good protection for others if the wearer is sick[6] (and remember that it is common to have COVID-19 and not know it).

N95 respirators

In times of shortages, available N95 “masks” (properly called “respirators”) should go to healthcare workers. Don’t hoard these!

N95 respirators are the most common variety of respirators. They provide a one-two punch of filtering out small particles and sealing the face with a tight fit. They are tested to make sure they block at least 95% of particles as small as 0.3 microns.[7] Other varieties of respirators include N99 respirators, which block at least 99% of particles as small as 0.3 microns.[8]

Note that SARS-CoV-2 particle size ranges from from 0.07 to 0.09 microns[9] (SARS-CoV, the genetically similar coronavirus that causes SARS, is slightly bigger at 0.08–0.14 microns[10]), so that even N95 and N99 respirators cannot afford absolute protection. However, those respirators still show significant efficacy against particles as small as 0.01 microns,[8] whereas surgical masks show poor efficacy against particles between 0.04 and 0.2 microns.[11][12]

🔍 Digging Deeper: Personal protective equipment Due to the controversy they generate, it’s all too easy to focus on masks and forget that they’re only once piece of the standard personal protective equipment (PPE) of healthcare workers facing high-risk, aerosol-generating situations. Barring lack of supplies, their PPE includes not only an N95 respirator but also other protective clothing, such as gloves, boots, a gown, a head cover, and either goggles or a visor (remember that the novel coronavirus can enter through your mouth, nose, and eyes[13]).

COVID-19: Can masks protect you?

Direct evidence

There is preliminary evidence that both N95 respirators and surgical masks help protect the wearer from COVID-19, but no evidence (yet) for non-medical masks.

There isn’t a lot of peer-reviewed data available yet, but preliminary studies suggest that both N95 respirators and surgical masks help protect wearers, although the extent of the protection is uncertain. This evidence was gathered in healthcare settings, though, and applicability to other settings is also uncertain.

An observational study of COVID-19 in China found that 10 out of 213 medical staff without masks were infected, versus 0 out of 278 medical staff with N95 respirators.[14]

A meta-analysis of four randomized trials found low-certainty evidence that, in the context of non-aerosol-generating care, N95 respirators and surgical masks protect healthcare workers similarly against viral respiratory infections (including by coronaviruses). Consequently, the authors recommend reserving N95 respirators, when in short supply, to high-risk, aerosol-generating procedures.[7]

Let’s note that, when researchers looked at 41 healthcare workers exposed to aerosol from patients with COVID-19, they found that all 41 tested negative, even though only 15% of them wore N95 respirators (85% wore surgical masks).[7][15]

Indirect evidence

There is much more evidence on colds and the flu than on COVID-19. Non-medical cloth masks don’t seem to perform well, whereas surgical masks and especially N95 respirators do. Leave the N95 respirators to healthcare workers, though, since they are exposed to a greater variety of pathogens and aerosols. Moreover, N95 respirators are hard to fit properly.

Non-medical masks, aka cloth masks

A randomized trial found that healthcare workers wearing cloth masks were 13 times more likely to contract flu-like illnesses than those wearing surgical masks.[16] Another study found that cloth masks (made out of cotton polyester or a blend of fabrics) were significantly worse than N95 respirators at filtering out aerosol particles, but the authors suggest that well-fit cloth masks could be comparable to surgical masks.[5]

Note that the significance of aerosols versus droplets is not fully known yet for transmission of the novel coronavirus. One study has suggested that aerosols may spread well past the typically cited six feet.[17]

Medical masks, aka surgical masks

There’s lots of evidence on medical masks hindering cold and flu transmission when worn by sick people,[18] but not as much on their protecting healthy people who wear them.

From Vietnam, a randomized trial found that particle penetration was much worse for cloth masks (97%) than for surgical masks (44%),[16] and a case control study (a less rigorous type of study) reported that hospital workers not wearing masks were more likely to contract SARS (an illness caused by another type of coronavirus).[19]

On a flight with some passengers afflicted with the swine flu (which is caused by a type of influenza virus), none of the passengers wearing masks got sick, whereas around a third of those who didn’t wear masks got sick.[20] Surgical masks have also shown some limited efficacy at preventing flu transmission within households[21][22][18][6][23] and when worn by patients with immune-related conditions.[24][25][26]

In general, while surgical masks don’t have the filtration efficacy of N95 respirators, laboratory studies show a reduction of 10–50% in particles crossing the mask,[3] with much better performance for larger particles.[27][28][29]

N95 respirators

Studies that compare N95 respirators to surgical masks have very mixed results[30] and aren’t of the highest methodological quality.[31]

Some show no significant difference for influenza spread,[32][14][33][34] but better protection against bacterial infections and other respiratory illnesses.[35][36] One study of healthcare workers found that those wearing N95 respirators were 54% less likely to have respiratory viral infections, whereas those wearing surgical masks were only 12% less likely.[37]

Basically, study results are all over the place, which could be due to different clinical settings, different pathogens, different study designs, small sample sizes, or a host of other possible factors.[3] One takeaway is that N95 respirators aren’t always better by a substantial margin, so it’s probably better to leave them to healthcare workers, who know how to wear them properly and need to protect themselves from a variety of different pathogen and aerosol exposures.

COVID-19: Can masks protect other people from you?

The current evidence, direct and indirect, says yes.[38][39]

Direct evidence

There’s preliminary evidence that, if you have COVID-19, you can protect other people by wearing a mask.

One early report looked at a COVID-19 patient during the initial outbreak in China. While not wearing a facemask in a bus, he transmitted the coronavirus to five people. In another bus, he wore a face mask, and no one was infected. (Note that, at the time, he wasn’t aware of having COVID-19.)[40] While that’s weak evidence, there isn’t much direct evidence for COVID-19 yet.

A more rigorous study looked at previously known coronaviruses (not the novel coronavirus), and found that surgical masks substantially reduced virus particles in emitted droplets, with 65% of participants not shedding any detectable virus at all.[41] The overall low transmission suggests that prolonged close contact may be the most common transmission route, such as for the cold-causing rhinovirus.[42] Importantly, individual results differed widely, possibly due to some people being more contagious (which could be explained by their having a higher viral load than other people).

Indirect evidence

There’s a ton of evidence showing that sick people can protect healthy people by wearing an N95 respirator or a surgical mask. Even homemade masks have a good deal of evidence for this purpose.

Non-medical masks

Homemade masks have stronger evidence for protecting others from you than for protecting you from others.

Studies have shown that although surgical masks are around three times better at blocking microorganisms than homemade masks (made from cotton T-shirts, for instance), the latter do have some efficacy.[43][44] Of course, this efficacy varies quite a bit depending on what the homemade mask is made of and how it’s worn.

A mathematical model found that to reduce the spread of influenza by 95% in a population wearing no mask, you need 80% of the population to wear homemade masks or 50% of the population to wear surgical masks.[45] In many countries, however, surgical masks are not easily available even to healthcare workers.

Medical masks, aka surgical masks

In patients with the flu, surgical masks reduce the emission of small particles (≤5 microns) 3-fold and of large particles (>5 microns) 25-fold.[46]

When worn by the infected, surgical masks help prevent the spread of a variety of infectious droplets, including droplets carrying influenza viruses and previously known coronaviruses. The evidence is from a variety of settings, including households, college residence halls, and mass gatherings.[47][48][49][50][51][52][53][54]

Efficacy is highly dependent on adherence, though. For example, a randomized trial of Hajj pilgrims in Mecca didn’t find fewer respiratory infections in the group assigned masks, but most of the group ended up using the masks only sometimes or not at all.[55][56]

N95 respirators

N95 respirators are the best at reducing the risk of transmission of respiratory viruses, unsurprisingly, but they’re not that much better than surgical masks, and they’re substantially more uncomfortable and irritating to the skin, which could lower compliance.[57]

Reusing and disinfecting masks

Which masks can you reuse?

Extended use is better than reuse, because reuse involves touching the mask and potentially your face. You can wash and reuse cloth masks. You shouldn’t reuse disposable masks, but if you must, you should at least store them (in an area with sufficient airflow) for three days between uses.

The CDC suggests that if healthcare workers, lacking supplies, must reuse N95 respirators, then those should be stored (in an area with sufficient airflow) for 5 days (120 hours) between uses, in order to exceed the expected survival time of the novel coronavirus. The FDA recommends disposing of any N95 respirators that are at all dirty or damaged.

The WHO recommends discarding disposable masks as soon as they’re damp. If you don’t have a reusable cloth mask, layering a washable fabric on top of your disposable mask is an option, albeit an unstudied one that may impair breathing. Be careful!

For any mask, the longer you use it and the more often you reuse it, the less effective it becomes. But perhaps a bigger problem is improper mask usage. For example, one study found that nurses touch their respirators an average of 25 times per shift.[58] And of course, the average person is less adept at using protective equipment than are nurses.

If you don’t have a washable mask, you’re probably considering decontamination methods for disposable masks. The CDC provides a summary of decontamination methods in times of limited hospital supply, but the current evidence on efficacy and potential damage to the mask is almost only for N95 respirators.[59][60]

Because N95 respirators are made for one-time use, reusing them after decontaminating them is risky. The fit could worsen, and to begin with, many methods of disinfection are crapshoots (as we’ll see), which means that after the first use what you may be wearing in an infected mask.

What’s the best way to store used disposable masks?

Store used disposable masks (for at least three days between uses) where they can breathe.

When you take off your mask, remember that you’ve been breathing into it for minutes to hours. So store it somewhere where it can dry out and dissipate the moisture it has accumulated. For instance, you can hang the mask or store it in a paper bag (but not in an airtight tupperware). If you use a container, it should not deform the mask, and you should clean or change it regularly.

What’s the best way to wash masks?

You can wash cloth masks (but not surgical masks or N95 respirators) with soap and water and dry them on high.

According to the WHO, heat at 56°C (133°F) kills the SARS coronavirus (a close cousin of the novel coronavirus) at around 10,000 units per 15 minutes, so if you’re using a cloth mask, consider washing it with soap and water and drying it on high.

One study found that the efficacy of cloth masks dropped after several washing and drying cycles — by 20% after the fourth washing and drying cycle, for instance. The pores within the cloth can change in size and shape, and ear loops can get stretched out, worsening the fit.[61]

Can you disinfect your masks with alcohol or bleach?

The short answer is no.

Bleaches and brand-name products such as Clorox, Lysol, or Purell, often include a type of chloride compound that kills viruses by disrupting intermolecular interactions within the virus. So far so good. Unfortunately, Stanford University physicians found that chlorine-based solutions and 75% alcohol solutions interrupted the static charge that supports filtering.

Moreover, bleach gases remained even after multiple strategies were tried to remove them, making skin and respiratory irritation likely. This finding echoes that of a previous study in which bleach decontamination led to low levels of chlorine off-gassing when the masks encountered water.[62]

Can you disinfect your mask with UV/sunlight?

There isn't enough research to determine what effect, if any, sunlight has, but medical UV sterilizers can be effective. UVC machines may be effective as well but are more dangerous.

First, note that sunlight is not equivalent to high-powered UV sterilizing cabinets or towers, and that the devices used to disinfect masks from germs are not available to the general public.

High levels of ultraviolet germicidal irradiation (UVGI) can inactivate influenza viruses and single-stranded RNA viruses (such as the novel coronavirus).[63][64] Nebraska Medicine (a partner of the University of Nebraska Medical Center) has developed a procedure that uses UVGI to decontaminate N95 respirators. UVGI exposure increases particle penetration by only 1.25%, so it doesn’t make the N95 respirators ineffective, but it substantially weakens the layers of material and increases the chances of the straps breaking over time.[65]

UVC is a type of radiation that is blocked by the ozone layer (so it is absent from the sunlight that reaches us). Studies show that UVC can inactivate a variety of viruses, including the SARS coronavirus (a close cousin of the novel coronavirus).[66][67] Unfortunately, UVC devices are much more dangerous than other UV sterilizers (especially for non-professionals). They can cause severe sunburn and retinal damage.[68]

So what about sunlight? Alas, it isn’t a reliable solution. First, insufficiently strong UV rays won’t penetrate deeply enough to disinfect the entire mask — the sun is strong, but also far away, and Earth is protected by its ozone layer. Second, sunlight exposure often means outdoor exposure, in which case your mask may accumulate moisture or even get infected. Third and most important, sunlight has not been studied for this purpose as UV machines have. So if you choose to go the sunlight route, be aware that the effects are not predictable.

Can you disinfect your masks with steam?

Steam can be used to decontaminate masks, at the cost of reducing their filtering efficiency.

Evidence from the past few years suggests that steam can reduce viral contamination.[69] 4C Air has confirmed that subjecting N95 respirators to ten minutes of steam from boiling water has killed previously known coronaviruses, but Stanford University physicians warn that this treatment reduces the filtering efficacy of N95 respirators (from 97% to 85% after five cycles, in their test).

Those physicians steamed masks in a laboratory, however; they didn’t just hang their masks above pots of boiling water in their kitchens. There is no telling how efficacious simply doing that can be. If you choose to try anyway, make sure to perfectly dry the masks afterward. On the whole, though, washing and drying cloth masks seems safer than steaming surgical masks or N95 respirators.

Can you disinfect your mask in an oven?

Studies have used hot air to disinfect N95 respirators, but you shouldn’t try reproducing them at home. Easy mistakes can result in imperfect decontamination, a reduction of the mask’s filtration efficacy, or accidents — such as a fire.

Stanford University physicians found that an N95 respirator subjected to 75°C (167°F) for 30 minutes retained its shape, strap elasticity, and filtration efficacy even after 20 cycles. They did not test for disinfection, but they mentioned that, based on a study on the SARS coronavirus[70] (a close cousin of the novel coronavirus), 75°C for 30 minutes was sufficient for disinfection.

Likewise, other researchers have suggested that 30 minutes of heating at a similar temperature would be effective when hanging the N95 respirator by a clip and not letting it touch the hot metal of the oven (which would degrade the mask).[71]

Can you disinfect your mask in a microwave?

Definitely no if it’s an N95 respirator, a surgical mask, or a mask with any metal or plastic in it. It’s probably not a good idea for any other mask either.

One study found that a kitchen microwave killed avian pneumovirus.[72] Another found that microwave radiation was able to reduce the presence of HIV and the hepatitis C virus in fluids after two minutes of heating at 800 watts.[73] So what’s the holdup — why not just microwave your mask?

Because of three risks: melting, fire, and inadequate decontamination.

When researchers microwaved 9 models of respirators, 2 melted. [62]

If there’s any metal in the mask (metal is often used to help you fit the mask around your nose), then microwaving can be especially dangerous, possibly resulting in sparks or fire.

Microwaves work by heating water molecules. Dry mask material doesn’t fit the bill, and if the mask is damp, then anyway it may be damaged or too contaminated for reuse.

Can you disinfect your mask in a freezer?

Probably not. Previous coronavirus strains appear to be stable at low temperatures. Direct evidence on the novel coronavirus is lacking.

We don’t yet have solid evidence on SARS-CoV-2, the novel coronavirus that causes COVID-19, but other coronavirus strains appear to be stable at low and freezing temperatures for an indefinite period.[74] One study found coronavirus 229E to be stable after 25 cycles of freezing at −70°C (−94°F) for 2 hours followed by thawing at 37°C (98.6°F) in a water bath.[75] This suggests a lack of vulnerability to freezing itself. However, −70°C avoids the formation of ice crystals, which damage viruses, so freezing at temperatures that lead to the formation of ice crystals might work — but there is no direct evidence that it does.

Proper fit and potential downsides

How do I get my mask to fit right?

Because homemade masks differ widely, there is no standard way to wear them correctly. Surgical masks are somewhat loose by design, but may still be beneficial even with a somewhat poor fit. N95 respirators are the hardest to fit well, and poor fit can substantially reduce efficacy.

A study had two males and two females try five different surgical masks. All four subjects, who were not healthcare workers, failed each of the five qualitative fit tests on the first exercise (normal breathing) when they donned the masks without assistance. In other words: twenty attempts, twenty failures (and the subjects knew they were being tested; they were especially careful).[11] However, even imperfectly fitting masks may reduce the transmission rates of viruses.[76]

N95 respirators are notoriously difficult to fit correctly — to the point that, in medical settings, fit is instrumentally tested.

Can masks cause headaches?

Yes. Headaches are more likely in people who are predisposed to headaches, and they become more likely the longer you wear a tight-fitting mask, such as an N95 respirator.

The longer you wear a tight-fitting mask (such as an N95 respirator), the more your pericranial soft tissue can be compressed by tight straps,[77] causing headaches.[78]

In Singapore, during the 2002–2004 SARS outbreak, mask-associated headaches affected 37% of mask wearers in general and nearly 82% of frontline healthcare workers in particular.[79]

Among nurses working in the medical intensive care unit (MICU) of the University of Louisville Hospital, headaches were found to be one of the main reasons for suboptimal compliance with wearing N95 respirators.[58]

Headaches seem more likely in people working in a highly controlled hospital setting (such as the aforementioned MICU) because protective eyewear is sometimes used as well. This is sometimes referred to as “goggle headache” or “goggle migraine”.[80][81]

Can masks cause breathing issues?

N95 respirators and multi-layered cloth masks are more likely to contribute to breathing issues than are surgical masks and loose-fitting single-layer cloth masks.

Breathing difficulties have been associated with N95 respirators.[82]

Masks have to balance protection with breathability, to avoid causing more harm than good: the harder you breathe, the greater the risk of side leakage; and if you have pre-existing breathing difficulties, you may be unable to wear certain masks for more than a few minutes.

Can wearing a mask impair public health?

The biggest detriment was panic buying of N95 respirators, which diverted a portion from hospital settings. There are other, lesser detriments, such as the masks possibly giving their wearers a false sense of security.

Already, people hoarding N95 respirators and surgical masks have prevented their use in healthcare settings where they were critically needed.

Aside from that, masks can sometimes give a false sense of security, causing the wearers to neglect physical distancing. This is especially true for people for whom masks are less effective, such as children and people with facial hair,[83][84][76] but no mask is 100% effective for anyone, and remember that the novel coronavirus can also enter through your eyes.[13]

There is also a risk that wearers will touch their faces more because they feel the need to readjust their masks, and if those have been contaminated, the wearers can transfer the virus to their mouths, noses, or eyes. (Conversely, however, a mask could also serve as a reminder not to touch the face.)

Conflicting messages

Why have mask recommendations been so conflicting?

Imperfect as the evidence may be, it does show benefit from wearing masks. The flip-flops in messaging as the pandemic spread have had mostly logistics causes — where are masks best directed, given a limited supply?

Mask recommendations have consistently been handed out to the public without much explanation, let alone references to the state of the evidence. Granted, many people wouldn’t understand the details of the studies forming the evidence, but basic explanations could have been provided. This communication challenge, or rather failure, has eroded the public’s trust.

For instance, when people were told they shouldn’t buy masks because (1) masks do not help and (2) masks should be left to healthcare workers, they quickly realized that both reasons were mutually exclusive. Or at least, they appeared to be. It could be said that N95 respirators are useful to healthcare workers, who have been trained to fit them properly, but not nearly as useful to the general public.

Another example: When the public was told that only people with symptoms needed masks, that wasn’t entirely true. The whole truth is that, given a shortage of masks, it was to the benefit of all that symptomatic people (and healthcare workers) should be prioritized. And as the evidence and logistics evolved, so did public recommendations.

None of this was explained, though, which is a shame since lack of transparency easily erodes public trust during health emergencies,[85] while conversely the clear communication of accurate information leads to greater trust as well as better decision-making and prioritization.[86][87]

Do masks simply not work for the general public?

The message that widespread masking is more or less useless was either misguided or misinterpreted. Masks are at worst slightly useful on a population-wide level, during pandemics like this one.

If you’ve read the first half of this article, you know that, while the evidence is mixed, it clearly doesn’t say that “masks simply don’t work for the general public”. Yet this myth persists, because that’s what previous recommendations hinted at or even stated outright.

The exact wording is critical here. For example, the WHO stated that “there is currently no evidence that wearing a mask (whether medical or other types) by healthy persons in the wider community setting, including universal community masking, can prevent them from infection with respiratory viruses, including COVID-19”.

This statement is tough to parse, and when you get right down to it, at least partially incorrect. The evidence that masks can help protect healthy wearers isn’t overwhelming, but it most certainly exists. Universal community masking also has some evidence, including a preprint study suggesting efficacy against the novel coronavirus specifically.[88] (Due to obvious ethical considerations, there haven’t been large-scale randomized trials on this topic, though.)

Are masks not helpful if you’re healthy?

Masks are helpful, to protect yourself and even more to protect others.

Masks can offer some protection to healthy wearers, and more importantly, they can protect healthy people from sick wearers who look healthy.

A number of people still believe that you’re contagious only if you have symptoms. This is a myth. When infected with SARS-CoV-2, the novel coronavirus that causes COVID-19, most people don’t show any symptoms for several days or even well beyond a week.[89] Some people never show any symptoms at all.[89]

The bottom line:

Wearing a mask can protect you from other people, and other people from you. If we all wear a mask, we all protect each other. Don’t hesitate to share this article with your friends — or, if you think they’d rather watch a video, show them this one; it’s short, clear, and accurate.

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