What the Data says About Wearing Face Masks

The science supports  that face coverings save lives, and yet they’re still  endlessly debated. How  much evidence is enough?

When her Danish colleagues first suggested distributing protective cloth face masks to people in Guinea-Bissau to stem the spread of the coronavirus, Christine Benn wasn’t so sure. “I said, ‘Yeah, that might be good, but there’s limited data on whether face masks are actually effective,’”  says Benn, a global-health researcher at the  University of Southern Denmark in Copenhagen, who for decades has co-led public-health  campaigns in the West African country, one of  the world’s poorest.

That was in March. But by July, Benn and her  team had worked out how to possibly provide  some needed data on masks, and hopefully  help people in Guinea-Bissau. They distributed thousands of locally produced cloth face coverings to people as part of a randomized controlled trial that might be the world’s  largest test of masks’ effectiveness against  the spread of COVID-19. 

Face masks are the ubiquitous symbol of a  pandemic that has sickened 35 million people and killed more than 1 million. In hospitals and other health-care facilities, the use of  medical-grade masks clearly cuts down trans mission of the SARS-CoV-2 virus. But for the  variety of masks in use by the public, the data are messy, disparate and often hastily assembled. Add to that a divisive political discourse  that included a US president disparaging their use, just days before being diagnosed with COVID-19 himself. “People looking at the evidence are understanding it differently,” says Baruch Fischhoff, a psychologist  at Carnegie Mellon University in Pittsburgh,  Pennsylvania, who specializes in public policy.  “It’s legitimately confusing.” 

To be clear, the science supports using  masks, with recent studies suggesting that  they could save lives in different ways: research  shows that they cut down the chances of both  transmitting and catching the coronavirus,  and some studies hint that masks might reduce  the severity of infection if people do contract  the disease.  

But being more definitive about how well  they work or when to use them gets complicated. There are many types of mask, worn in  a variety of environments. There are questions  about people’s willingness to wear them, or  wear them properly. Even the question of what  kinds of study would provide definitive proof  that they work is hard to answer. 

“How good does the evidence need to be?”  asks Fischhoff. “It’s a vital question.”  

Beyond gold standards 

At the beginning of the pandemic, medical  experts lacked good evidence on how SARS CoV-2 spreads, and they didn’t know enough to  make strong public-health recommendations  about masks.  

The standard mask for use in health-care set tings is the N95 respirator, which is designed to  protect the wearer by filtering out 95% of air borne particles that measure 0.3 micro meters and larger. As the pandemic ramped up,  these respirators quickly fell into short supply. That raised the now contentious question:  should members of the public bother wearing basic surgical masks or cloth masks? If so,  under what conditions? “Those are the things  we normally [sort out] in clinical trials,” says  Kate Grabowski, an infectious-disease epidemiologist at Johns Hopkins School of Medicine  in Baltimore, Maryland. “But we just didn’t  have time for that.” 

So, scientists have relied on observational  and laboratory studies. There is also indirect evidence from other infectious diseases. “If  you look at any one paper — it’s not a slam  dunk. But, taken all together, I’m convinced  that they are working,” says Grabowski.  

Confidence in masks grew in June with news  about two hair stylists in Missouri who tested  positive for COVID-19. Both wore a double-layered cotton face covering or surgical  mask while working. And although they passed  on the infection to members of their house holds, their clients seem to have been spared  (more than half reportedly declined free tests).  Other hints of effectiveness emerged from  mass gatherings. At Black Lives Matter protests in US cities, most attendees wore masks.  The events did not seem to trigger spikes in  infections, yet the virus ran rampant in late  June at a Georgia summer camp, where children who attended were not required to wear  face coverings. Caveats abound: the protests  were outdoors, which poses a lower risk of  COVID-19 spread, whereas the campers shared  cabins at night, for example. And because many non-protesters stayed in their homes during  the gatherings, that might have reduced virus  transmission in the community. Nevertheless,  the anecdotal evidence “builds up the picture”,  says Theo Vos, a health-policy researcher at the  University of Washington in Seattle.  

More rigorous analyses added direct  evidence. A preprint study posted in early  August (and not yet peer reviewed), found  that weekly increases in per-capita mortality  were four times lower in places where masks  were the norm or recommended by the  government, compared with other regions.  Researchers looked at 200 countries, including Mongolia, which adopted mask use in January and, as of May, had recorded no deaths  related to COVID-19. Another study looked at  the effects of US state-government mandates  for mask use in April and May. Researchers  estimated that those reduced the growth of  COVID-19 cases by up to 2 percentage points  per day. They cautiously suggest that man dates might have averted as many as 450,000  cases, after controlling for other mitigation  measures, such as physical distancing. 

“You don’t have to do much math to say this  is obviously a good idea,” says Jeremy Howard, a research scientist at the University of  San Francisco in California, who is part of a  team that reviewed the evidence for wearing  face masks in a preprint article that has been  widely circulated

But such studies do rely on assumptions  that mask mandates are being enforced and  that people are wearing them correctly. Furthermore, mask use often coincides with other changes, such as limits on gatherings. As  restrictions lift, further observational studies  might begin to separate the impact of masks  from those of other interventions, suggests  Grabowski. “It will become easier to see what  is doing what,” she says. 

Although scientists can’t control many con founding variables in human populations, they  can in animal studies. Researchers led by micro biologist Kwok-Yung Yuen at the University of  Hong Kong housed infected and healthy hamsters in adjoining cages, with surgical-mask  partitions separating some of the animals.  Without a barrier, about two-thirds of the uninfected animals caught SARS-CoV-2, according  to the paper published in May. But only about  25% of the animals protected by mask material  got infected, and those that did were less sick  than their mask-free neighbors (as measured  by clinical scores and tissue changes).  

The findings provide justification for the  emerging consensus that mask use protects  the wearer as well as other people. The work  also points to another potentially game-changing idea: “Masking may not only protect you  from infection but also from severe illness,” says  Monica Gandhi, an infectious-disease physician  at the University of California, San Francisco.  

Gandhi co-authored a paper published in late July suggesting that masking reduces the  dose of virus a wearer might receive, resulting  in infections that are milder or even asymptomatic. A larger viral dose results in a more  aggressive inflammatory response, she suggests.  

She and her colleagues are currently analyzing hospitalization rates for COVID-19 before  and after mask mandates in 1,000 US counties,  to determine whether the severity of disease decreased after public masking guidelines  were brought in. The idea that exposure to more virus results  in a worse infection makes “absolute sense”, says Paul Digard, a virologist at the University  of Edinburgh, UK, who was not involved in the  research. “It’s another argument for masks.”  

Gandhi suggests another possible benefit:  if more people get mild cases, that might help  to enhance immunity at the population level  without increasing the burden of severe illness  and death. “As we’re awaiting a vaccine, could driving up rates of asymptomatic infection do good for population-level immunity?” she  asks.  

You don’t have to do  much math to say this is  obviously a good idea.” 

Back to ballistics  

The masks debate is closely linked to another  divisive question: how does the virus travel  through the air and spread infection? The moment a person breathes or talks,  sneezes or coughs, a fine spray of liquid particles takes flight. Some are large — visible, even  — and referred to as droplets; others are microscopic, and categorized as aerosols. Viruses including SARS-CoV-2 hitch rides on these  particles; their size dictates their behavior. Droplets can shoot through the air and land  on a nearby person’s eyes, nose or mouth to  cause infection. But gravity quickly pulls them down. Aerosols, by contrast, can float in the  air for minutes to hours, spreading through  an unventilated room like cigarette smoke.  

What does this imply for the ability of masks  to impede COVID-19 transmission? The virus  itself is only about 0.1 μm in diameter. But  because viruses don’t leave the body on their  own, a mask doesn’t need to block particles that small to be effective. More relevant are the pathogen-transporting droplets and aerosols,  which range from about 0.2 μm to hundreds of  micro meters across. (An average human hair  has a diameter of about 80 μm.) The majority are 1–10 μm in diameter and can linger in  the air a long time, says Jose-Luis Jimenez, an environmental chemist at the University of  Colorado Boulder.

“That is where the action is.”  Scientists are still unsure which size of particle is most important in COVID-19 transmission. Some can’t even agree on the cut-off that  should define aerosols. For the same reasons,  scientists still don’t know the major form of  transmission for influenza, which has been  studied for much longer. Many believe that asymptomatic transmission is driving much of the COVID-19 pandemic, which would suggest that viruses aren’t  typically riding out on coughs or sneezes. By  this reasoning, aerosols could prove to be  the most important transmission vehicle. So, it is worth looking at which masks can stop  aerosols. 

All in the fabric 

Even well-fitting N95 respirators fall slightly  short of their 95% rating in real-world use,  actually filtering out around 90% of incoming aerosols down to 0.3 μm. And, according  to unpublished research, N95 masks that  don’t have exhalation valves — which expel  unfiltered exhaled air — block a similar proportion of outgoing aerosols. Much less is  known about surgical and cloth masks, says  Kevin Fennelly, a pulmonologist at the US  National Heart, Lung, and Blood Institute in  Bethesda, Maryland. In a review of observational studies, an  international research team estimates that  surgical and comparable cloth masks are 67%  effective in protecting the wearer.  

In unpublished work, Linsey Marr, an environmental engineer at Virginia Tech in  Blacksburg, and her colleagues found that  even a cotton T-shirt can block half of inhaled aerosols and almost 80% of exhaled aerosols  measuring 2 μm across. Once you get to aerosols of 4–5 μm, almost any fabric can block more than 80% in both directions, she says.  Multiple layers of fabric, she adds, are more  effective, and the tighter the weave, the better. Another study10 found that masks with layers  of different materials — such as cotton and silk  — could catch aerosols more efficiently than  those made from a single material. Benn worked with Danish engineers at her  university to test their two-layered cloth mask  design using the same criteria as for medical-grade ventilators. They found that their  mask blocked only 11–19% of aerosols down  to the 0.3 μm mark, according to Benn.

But  because most transmission is probably occurring through particles of at least 1 μm, according to Marr and Jimenez, the actual difference  in effectiveness between N95 and other masks  might not be huge. Eric Westman, a clinical researcher at Duke  University School of Medicine in Durham,  North Carolina, co-authored an August study11 that demonstrated a method for testing mask  effectiveness. His team used lasers and smart phone cameras to compare how well 14 different cloth and surgical face coverings stopped  droplets while a person spoke. “I was reassured  that a lot of the masks we use did work,” he says,  referring to the performance of cloth and surgical masks.

But thin polyester-and-spandex  neck gaiters — stretchable scarves that can be  pulled up over the mouth and nose — seemed  to actually reduce the size of droplets being  released. “That could be worse than wearing  nothing at all,” Westman says. Some scientists advise not making too  much of the finding, which was based on just one person talking. Marr and her team were  among the scientists who responded with  experiments of their own, finding that neck  gaiters blocked most large droplets. Marr says  she is writing up her results for publication. “There’s a lot of information out there, but  it’s confusing to put all the lines of evidence together,” says Angela Rasmussen, a virologist  at Columbia University’s Mailman School of Public Health in New York City. “When it comes  down to it, we still don’t know a lot.” 

You can’t do randomized trials for everything — and  you shouldn’t.” 

Minding human minds  

Questions about masks go beyond biology,  epidemiology and physics. Human behavior is core to how well masks work in the real  world. “I don’t want someone who is infected  in a crowded area being confident while wearing one of these cloth coverings,” says Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota in Minneapolis.  Perhaps fortunately, some evidence suggests that donning a face mask might drive  the wearer and those around them to adhere  better to other measures, such as social distancing. The masks remind them of shared  responsibility, perhaps. But that requires that  people wear them.  

Across the United States, mask use has  held steady around 50% since late July. This  is a substantial increase from the 20% usage  seen in March and April, according to data  from the Institute for Health Metrics and  Evaluation at the University of Washington  in Seattle. The  institute’s models also predicted that, as of 23 September, increasing US mask use to 95% —  a level observed in Singapore and some other countries — could save nearly 100,000 lives in  the period up to 1 January 2021. “There’s a lot more we would like to know,”  says Vos, who contributed to the analysis. “But  given that it is such a simple, low-cost intervention with potentially such a large impact, who  would not want to use it?” 

Further confusing the public are controversial studies and mixed messages. One study in April found masks to be ineffective, but was  retracted in July. Another, published in Junesupported the use of masks before dozens of  scientists wrote a letter attacking its methods. The authors are  pushing back against calls for a retraction.  Meanwhile, the World Health Organization  (WHO) and the US Centers for Disease Control  and Prevention (CDC) initially refrained from  recommending widespread mask usage, in  part because of some hesitancy about depleting supplies for health-care workers. In April, the CDC recommended that masks be worn when physical distancing isn’t an option; the WHO followed suit in June.  

There’s been a lack of consistency among  political leaders, too. US President Donald  Trump voiced support for masks, but rarely  wore one. He even ridiculed political rival Joe  Biden for consistently using a mask — just  days before Trump himself tested positive  for the coronavirus, on 2 October. Other  world leaders, including the president and  prime minister of Slovakia, Zuzana Čaputová  and Igor Matovič, sported masks early in the  pandemic, reportedly to set an example for  their country.  

Denmark was one of the last nations to man date face masks — requiring their use on public  transport from 22 August. It has maintained asking half to use surgical mask Canada when  going to a workplace. Although the study  is completed, Thomas Benfield, a clinical  researcher at the University of Copenhagen  and one of the principal investigators on the  trial, says that his team is not ready to share  any results. Benn’s team, working independently of  Benfield’s group, is in the process of enrolling around 40,000 people in Guinea-Bissau,  randomly selecting half of the households to  receive bilayer cloth masks — two for each family member aged ten or over. The team will then follow everyone over several months to compare rates of mask use with rates of COVID-like  illness. She notes that each household will  receive advice on how to protect themselves  from COVID-19 — except that those in the  control group will not get information on the  use of masks. The team expects to complete  enrolment in November.  

Several scientists say that they are excited  to see the results. But others worry that such experiments are wasteful and potentially exploit a vulnerable population. “If this was  a gentler pathogen, it would be great,” says  Eric Topol, director of the Scripps Research  Translational Institute in La Jolla, California.  “You can’t do randomized trials for everything  — and you shouldn’t.” As clinical researchers  are sometimes fond of saying, parachutes have never been tested in a randomized controlled trial, either. 

But Benn defends her work, explaining that  people in the control group will still benefit  from information about COVID-19, and they  will get masks at the end of the study. Given the challenge of manufacturing and distributing the masks, “under no circumstances”,  she says, could her team have handed out  enough for everyone at the study’s outset. In  fact, they had to scale back their original plans  to enroll 70,000 people. She is hopeful that  the trial will provide some benefits for every one involved. “But no one in the community should be worse off than if we hadn’t done this  trial,” she says. The resulting data, she adds,  should inform the global scientific debate. 

US baseball players wore masks while   playing during the 1918 influenza epidemic.

For now, Osterholm, in Minnesota, wears  a mask. Yet he laments the “lack of scientific rigour” that has so far been brought to the  topic. “We criticize people all the time in the  science world for making statements without any data,” he says. “We’re doing a lot of the  same thing here.” Nevertheless, most scientists are confident  that they can say something prescriptive about  wearing masks. It’s not the only solution, says  Gandhi, “but I think it is a profoundly important pillar of pandemic control”. As Digard puts  it: “Masks work, but they are not infallible. And, therefore, keep your distance.” 

By Lynne Peeples, a science journalist in  Seattle, Washington for Springer Nature Limited