Do face masks really reduce coronavirus spread?

Face masks are effective in helping to stop the spread of the coronavirus.
(Image credit: Shutterstock)

Editor's Note (June 2 at 11:30 a.m.): A paper discussed below has been retracted by the journal the Annals of Internal Medicine. The authors wrote that their statistical methods could not determine whether the findings were reliable, making the results of the small study "uninterpretable." The study had found that surgical and cotton masks do not effectively contain viral droplets containing SARS-CoV-2. Subsequent research has suggested that face masks are an effective method for containing the spread of the virus, used along with staying six feet apart from other people.

For the first time, the Center for Disease Control and Prevention (CDC) has recommended that even seemingly healthy people wear masks over their mouths and noses when venturing out of their homes into places where it is difficult to maintain distance from other people. But there is still major debate over how much masks — particularly the homemade fabric masks that the CDC recommends for the public — can slow the spread of SARS-CoV-2, the virus that causes COVID-19.

Researchers, writing in two new papers, attempt to tackle the efficacy of masks, one more rigorously than the other, and come to differing conclusions. One study examined the effect of masks on seasonal coronaviruses (which cause many cases of the common cold) and found that surgical masks are helpful at reducing how much virus a sick person spreads. The other looked particularly at SARS-CoV-2 and found no effect of either surgical or fabric masks on reducing virus spread, but only had four participants and used a crude measure of viral spread.

The bottom line, experts say, is that masks might help keep people with COVID-19 from unknowingly passing along the virus. But the evidence for the efficacy of surgical or homemade masks is limited, and masks aren't the most important protection against the coronavirus.

Related: 13 Coronavirus myths busted by science

"Putting a face mask on does not mean that you stop the other practices," said May Chu, a clinical professor in epidemiology at the Colorado School of Public Health on the Anschutz Medical Campus who was not involved in either new study. "It does not mean you get closer to people, it does not mean you don't have to wash your hands as often and you can touch your face. All of that still is in place, this is just an add-on."

Face mask basics

Recommendations about masks can easily get confusing, because all masks are not made equal. The N95 mask effectively prevents viral spread. These masks, when properly fitted, seal closely to the face and filter out 95% of particles 0.3 microns or larger. But N95 masks are in serious shortage even for medical professionals, who are exposed to the highest levels of SARS-CoV-2 and are most in need of the strongest protection against the virus. They're also difficult to fit correctly. For those reasons, the CDC does not recommend them for general use.

Related: How are people being infected with COVID-19?

Due to shortages, the CDC also does not recommend surgical masks for the general public. These masks don't seal against the face but do include non-woven polypropylene layers that are moisture resistant. In a surgical mask, about 70% of the outside air moves through the mask and about 30% travels around the sides, Chu told Live Science. For that reason, they don't offer as much protection as N95s.

That leaves fabric masks, which currently are recommended for general use by the CDC. Fabric masks also allow air in around the sides, but lack non-woven, moisture-repelling layers. They impede only about 2% of airflow in, Chu said.

All of this leakage in surgical and fabric masks are why public health officials generally don't believe that wearing a mask prevents anyone from catching a virus that is already floating around in the environment. Airflow follows the path of least resistance, said Rachael Jones, an associate professor of family and preventive medicine at the University of Utah who was not involved in the new research. If viral particles are nearby, they have an easy path around a surgical or fabric mask. And in the case of a fabric mask, wearers may well be wafting in particles small enough to flow right through the fabric.

But what about the other way around? When the wearer of a mask coughs or sneezes, the barrier might be enough to contain a lot of that initial jet of grossness — even if there are gaps in the fabric or around the sides. That's what the new mask studies aimed to address: Whether surgical or fabric masks did a good job of containing viruses.

Efficacy of face masks

One study, published April 6 in the journal Annals of Internal Medicine, found that they did not. That study, led by South Korean researchers, involved asking four patients with COVID-19 to cough into a petri dish 7.8 inches (20 centimeters) away. The patients coughed without masks, while wearing a disposable surgical mask and again wearing a 100% cotton mask.

Neither mask meaningfully decreased the viral load coughed onto the petri dishes. But experts not involved in the study who were contacted by Live Science were hesitant to put much stock into the findings. The researchers didn't look at distances beyond 7.8 inches to see if droplets didn't travel as far while people were wearing masks, Chu said.

"They didn't measure 2 feet or 3 feet or 4 feet," she said.

Related: Is 6 feet enough space for social distancing?

The study also returned the odd result that most swabs from the outside of patient masks were positive for coronavirus and most from the inside were negative. The authors speculate that perhaps turbulent jets of air from coughing carried the virus toward the outside of the mask, but the explanation wasn't very satisfying, according to Jones.

The other study, published April 3 in the journal Nature Medicine, used a more sophisticated method of collecting the virus particles that sick people emit. The researchers asked 426 volunteers to breathe for 30 minutes into a cone-like device that captures everything exhaled. Of these, 43 patients had influenza, 54 patients had rhinoviruses and 17 patients had seasonal coronaviruses (the kinds that cause colds, not the kind that causes COVID-19). This method allowed the researchers to quantify how much virus was found in droplet particles, which are greater than 0.0002 inches (5 microns) in diameter, versus aerosol particles, which are 5 microns or smaller. The participants were randomized to either wear a surgical mask or not wear a mask during the study.

The first key finding was that the researchers detected virus in tiny aerosol particles in all cases: influenza, rhinoviruses and coronaviruses. In the case of influenza, they cultured the captured particles and discovered that they were infectious. That's important, said study author Ben Cowling, head of the Division of Epidemiology and Biostatistics at The Hong Kong University, because there is a long-running debate among health professionals about whether influenza can spread via aerosols. The study suggests that it likely can, and that colds probably can too.

"For seasonal coronavirus and rhinovirus, we didn't attempt to culture the virus in the aerosols, but there is no reason to believe that the virus would not be infectious," Cowling told Live Science.

Related: COVID-19 may spread through breathing and talking

And as for masks? Surgical masks reduced the amount of virus released from a sick person in the form of droplets but not aerosols for influenza; the masks reduced coronavirus in both droplets and aerosols; and they didn't reduce either in rhinovirus. For the seasonal coronavirus, researchers found the virus in droplets in 3 out of 10 samples from participants not wearing masks and in aerosols in 4 out of 10 samples taken without masks. In samples taken with masks, no virus was detected in either droplets or aerosols.

The difference between viruses could have something to do with where in the respiratory tract these infectious invaders make their homes, said Cowling, who is also co-director of the WHO Collaborating Center for Infectious Disease Epidemiology and Control. For example, a virus that reproduces deep in the lungs might need to travel in smaller particles to make it all the way out into the world, while one that reproduces mostly in the nose and throat may be more easily mobilized in bigger droplets.

The results from influenza and seasonal coronaviruses suggest that surgical masks can help keep people with COVID-19 from spreading the virus, Cowling said. SARS-CoV-2 probably behaves similarly to the viruses he and his team studied, he said, and the fact that people can spread the virus before they experience symptoms is an argument for recommending masks for everyone.

But experts are still mixed on the potential usefulness of non-N95 masks.

"To me, it's not harmful to wear these masks, but it doesn't look from this study like there is a whole lot of benefit," Jones said. The sample size for seasonal coronavirus was small, she said, and there was a large amount of non-mask-related variation in how much virus people emitted, particularly given that the majority of samples without masks didn't have detectable coronavirus.

One thing everyone does agree on is that, whatever containment provided by non-fitted masks do provide, homemade fabric masks are the least effective. The recommendations that everyone wear masks are because "any kind of impediment is better than nothing," Chu said. But fabric masks are not expected to be as protective as surgical masks, she said. That's why public health officials are warning people to remain at least 6 feet apart from one another, even if they are wearing masks. In other words, homemade masks are likely to be just a small piece of the puzzle for controlling the COVID-19 pandemic.

"There's been enough research done to be able to confidently say that masks wouldn't be able to stop the spread of infection, that they would only have a small effect on transmission," Cowling said. "We shouldn't be relying on masks to help us go back to normal."

Originally published on Live Science.

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Stephanie Pappas
Live Science Contributor

Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz. 

  • R.G.
    Here is the big problem with every article on this subject that involves opinions from "experts" on the use of masks is that the people being asked know absolutely nothing about respiratory protection. Yes, there, I said it. These articles ALWAYS involve quotes from epidemiologists. Seriously, does anyone know what epidemiologist do? The study the trends of various disease processes. They don't really know anything about respiratory protection. Frankly, I'm tired of hearing from a person who can't explain the difference between a P-100 and an N-95 and the classification process. And even the researchers who just did a study on masks and SARS CoV-2 FAILED to examine N-95 masks. These people have no clue what respiratory protection involves and how to achieve it. Now, if you get an engineer who works for NIOSH, I would be interested in hearing from them.
  • x_artifex
    Why do all these articles on masks think the medical N95 is the only proper respirator on Earth? There are many other respirators that can protect you from COVID-19. Standard non-medical respirators filter out just as well - N95, P95, P100, N100's and so on. Disposable versions or filters for reusable respirators like the 3M 7500 or GVS Elipse. I've been wearing my GVS in public with P100 filters and this provides better protection than an N95.

    How is it I'm just an air brush artist but I know more about respirators than these epidemiologists and journalists?
  • x_artifex
    How incredibly unhelpful these epidemiologists and journalists are. There are people on the internet that have figured out how to make homemade masks using HEPA filters, that properly fit and work just as good as an N95. But does anyone report on that? Nope. Do these epidemiologists study it? Nope.
  • ferrencebeeb
    This whole debate is so frustrating.

    The closing point, "We shouldn't be relying on masks to help us go back to normal."
    No-one is suggesting that we should!!! Why is this point used when journalists summarise reasons not to wear masks?
    It's the same when people say that wearing a mask can actually increase your risk because people touch the front of the mask when removing it, or people stop doing other measures such as social distancing. Of course we have to educate everyone on how to wear a mask in a way that won't reduce the benefits or even increase risk, but we don't just assume that the general public are too irresponsible to be trusted to wear masks properly. At least, if we are going to assume that, then shouldn't that assumption be based on rigorous scientific tests also?

    In the 1st piece of research, "The researchers didn't look at distances beyond 7.8 inches to see if droplets didn't travel as far while people were wearing masks, Chu said."
    That is ridiculous! Hasn't it been proven that when you cough, droplets or aerosols can travel up to 6 feet? So why would you only test to 7.8 inches?

    Then the 2nd piece of research. I don't see how anyone could conclude from this that "it doesn't look from this study like there is a whole lot of benefit".
    The research showed that wearing mask reduced coronavirus droplets & aerosols from 3 or 4 out of 10 to zero respectively! How is that not beneficial?!True the sample size is small (17) but that doesn't mean you can conclude that masks are not of much benefit. It just means they could be.
    And the point saying, "there was a large amount of non-mask-related variation in how much virus people emitted". That is completely irrelevant to the argument!

    Let's just say there is insufficient evidence on mask wearing for science to say with certainty that wearing masks is beneficial.
    Isn't there also insufficient evidence to say that not wearing a mask is beneficial?
    You can choose to wear a mask, or you can choose to not wear a mask.
    The lack of evidence on this does not mean we should default to not wearing masks. It just means we have a to make a choice without using science.
    And when science can't provide an answer to which option to choose, surely we have to then rely on COMMON SENSE!
  • Eric Post
    You don't have to be a genius to realize, all cloth masks do is give politician something for the general public to pin hopes on, while their is a shortage of masks that can actually help. It's a "feel good" measure only.
  • 4TimesAYear
    Eric Post said:
    You don't have to be a genius to realize, all cloth masks do is give politician something for the general public to pin hopes on, while their is a shortage of masks that can actually help. It's a "feel good" measure only.
    Exactly. And the homemade masks just muffle people's voices and you can't hear them. Guess what people do when that happens? They lean closer. Was at a checkout last week and the cashier said something and I couldn't understand her. What happened was that both of us leaned closer so I could hear, she around the plexiglass barrier. These steps are all pointless and give a false sense of security. And who, I wonder, is going to pay for all those filters to fit into washable masks? No, we need to get back to normal - and normal does not include any masks.
  • Jane Pristly
    This is a bafflingly silly piece and it's a bit surprising that this is on "Live Science". None of these woefully incomplete studies prove anything conclusive.


    Highly densely populated cities like Hong kong and Taipei have single digit fatalities despite no real lockdown. Everyone wears masks.
    New York has similar population as HK, and lower than Taipei. It has an incredibly large outbreak. Sure, the governments started their precautions late, but people still don't wear masks. CDC and FDA are still dithering on the matter, although CDC's website now, finally, in May, has a clear dictat: "wear masks".
    Where's the study that speaks of viral load? If two people are wearing masks, even if the droplets do transmit, will the viral load not be substantially lower? Low viral load means milder Covid, which is highly desirable.

    This entire analysis is missing, and even the likes of "ReutersFact check" and snopes are relying on this "Expert" advice to convey wrong info to people. Some of these people are commenting here and there's no doubt they're from countries that have high fatality tolls.
  • Jane Pristly
    4TimesAYear said:
    No, we need to get back to normal - and normal does not include any masks.

    This is the most misguided comment you can make. I'm all for sensible reopening of the economy (with the vulnerable still quarantined to the extent they can be), but countries that have in fact worn masks have registered staggeringly positive outcomes.

    Hong Kong or Taipei are densely populated and they have *single digit* fatalities. Why? Everyone wears masks, and wears them seriously and sensibly. Lockdowns won't do diddly if people get back to "normal" as you suggest. It'll just delay the inevitable.

    Masks are absolutely vital to keeping transmissions low. Even if they don't completely prevent droplet transfer, they unequivocally lower the viral load. Low viral load means a milder infection, which is highly desirable.
  • 4TimesAYear
    The corona virus goes straight through homemade fabric masks. If the mask cannot prevent me from getting sick, I fail to see how it's going to prevent others from getting sick. Btw, before this, the reason people wore them in China, etc. was due to pollution, not for disease prevention.
    I'm a realist. Like Bill Maher says, you can't sanitize the universe. Pathogens are everywhere. Masks can't be the new normal. Neither can sanitizing everything. One young man, wise beyond his years, asked a colleague in Walmart the other day, "What are all these chemicals doing to us?" the answer to defeating COVID, again, like Bill Maher said, is a strong immune system. Lockdowns don't "do diddly", period. They kept track of the numbers in New York. Guess where the majority of hospitalizations (66%) were coming from? Those who were sheltering in place. Those that remained the healthiest were those that were allowed to keep working. All conventional "wisdom" was wrong.
  • Okegnis
    Though mask has it own benefit when compared with the population that does not wear it .
    My question now is "why can't our expect look at the likely hood of getting the virus through the other portal of entry like the ear, anus and ultimately the eyes?"Because I read a report claiming that it's possible to get it through the eyes .
    But does mask cover the eyes?