Wear a mask.
Those three words, the simplest of commands, mask a staggering degree of scientific complexity.
Make no mistake: amid this pandemic, you should wear a mask when in close contact with others outside your immediate circle, when in enclosed spaces with others outside the family bubble, and when in crowds outside or inside. But the science behind mask-wearing – like so much of the science related to the COVID-19 pandemic – remains a work in progress.
The State of Colorado mandates – and many Colorado counties require – masks. The Centers for Disease Control and Prevention recommends wearing masks; the World Health Organization does too; UCHealth requires masks in hospitals, clinics and administrative areas. One model created by Colorado scientists assumes that masks reduce a wearer’s contagiousness by as much as 50%.
“If you’re not social distancing, I think you should be wearing a mask,” said Dr. Michelle Barron, medical director for infection control and prevention at UCHealth University of Colorado Hospital on the Anschutz Medical Campus and a professor at the University of Colorado School of Medicine. “It’s really to prevent others from being exposed to your droplets and spittle without even knowing it.”
The scientific questions surrounding masks – homemade or store-bought cloth masks and surgical-style masks, not the still-scarce N95s that health care workers need more than we do – are more nuanced. The mask-skeptical arguments rest on a couple of pillars. One is that there’s little in the way of convincing, peer-reviewed proof that regular folks wearing cloth masks are protecting themselves or anyone else from COVID-19. Another is that many such masks aren’t great in the first place, and even good ones often aren’t being worn properly (that open-air nose defeats the purpose). Yet another is that masks may lull the wearer into a false sense of security in the crowded places and tight spaces where transmission risk is highest.
No less an authority than University of Minnesota epidemiologist Michael Osterholm said in June: “The messaging that dominates our COVID-19 discussions right now makes it seem that, if we are wearing cloth masks, you’re not going to infect me and I’m not going to infect you. I worry that many people highly vulnerable to life-threatening COVID-19 will hear this message and make decisions that they otherwise wouldn’t have made about distancing because of an unproven sense of cloth-mask security. Distancing remains the most important risk reduction action we can take.”
Then why wear a mask?
The science that supports wearing masks
One reason is that the science of masks is evolving, and all indications are that it’s evolving toward the theory that masks are an effective way to slow the pandemic’s spread. There are indeed few peer-reviewed studies on the effectiveness of masks on slowing the spread of the coronavirus. That’s because the human disease the SARS-CoV-2 virus causes is still just months old. COVID-19 studies take time; so does peer review. More research, rest assured, is coming.
Until then, we must largely rely on work that predates COVID-19 and mathematical models of the disease based on best estimates of how well masks and other coronavirus countermeasures work.
A 2015 study done in Vietnam compared cloth masks to surgical masks among 1,607 hospital health care workers and found that those wearing cloth masks ended up catching influenza more often (there was no mask-free control group). The same research group recently revisited the topic amid the COVID-19 epidemic. They concluded that, while health care workers need N95-class protection, “The general public can use cloth masks to protect against infection spread in the community,” particularly in light of the many mild and asymptomatic coronaviruses cases.
A 2013 British study concluded that “a homemade mask should only be considered as a last resort to prevent droplet transmission from infected individuals, but it would be better than no protection.” A 2008 Dutch study considered N95-style respirators, surgical masks, and cloth masks and found that they all would reduce exposure to airborne influenza virus in that order. An April 2020 reviewconsidering N95-class respirators, surgical masks, and homemade cloth masks came to much the same conclusion, as did a June 2020 review in The Lancet. That study reviewed 172 observational studies and concluded that wearing masks reduce the risk of coronavirus infection – albeit with “low certainty.”
The science of masks: cloth masks helped during the 1918 Spanish Flu Pandemic
Consider, though, that a century ago, there were only cloth masks, and they indeed lowered infection rates for health care workers and others during the 1918 Spanish Flu pandemic, the Manchurian plague epidemic of 1920-1921, and, in the 1930s and 1940s, tuberculosis. A study of U.S. mask mandates found that mask wearing may have averted 230,000 to 450,000 coronavirus cases by late May. Another modeling study that estimated that, if 95% of the U.S. population wore masks, 18% fewer would die from the virus by Oct. 1 (including 125 fewer people in Colorado).
The Colorado COVID-19 Modeling Group’s interactive model assumes masks reduce the wearer’s contagiousness by 50%, though they add “there remains considerable uncertainty about this assumption.” Among other sources, the Colorado group cites a preprint review article by Jeremy Howard and colleagues that in turn discusses other modeling groups’ work predicting that cloth masks to have enormous impact when widely worn. That article concludes that near-universal mask wearing could itself turn the pandemic’s tide, with a positive economic impact of thousands of dollars per person per mask. (Goldman Sachs, the investment bank, estimated that masks could prevent the need for further lockdowns that could wipe out 5% of GDP.)
The Colorado COVID-19 model shows masks to have an enormous impact: if everyone wore masks, statewide intensive care unit bed occupancy wouldn’t exceed 71. With half of us wearing masks, a peak of 460 ICU beds would be filled with coronavirus patients by April 2020. If none wore a mask, 1,116 ICU beds would be filled by next January.
Finally, masks – or something like masks – were shown to reduce COVID-19 transmission among Syrian hamsters by about two-thirds. That study was among several that have led scientists to believe that not only droplets that quickly fall to the ground, but also tiny virus-carrying particles that can float about for hours – aerosols – are important COVID-19 transmission routes.
The physics behind mask-wearing
University of Colorado Boulder Prof. Jose-Luis Jimenez, an expert on aerosols, atmospheric chemistry and air quality, turned his attention from air pollution to viral infection with the rise of COVID-19. His team’s coronavirus model – one intended to help colleges understand the risk of in-person classes – assumes that cloth masks reduce coronavirus contagiousness by 50% if the wearer is the potential spreader and by 30% if the healthy wearer is exposed to someone with COVID-19. A mask’s effectiveness is ultimately driven by physics, Jimenez explains. It’s complicated – “one could write a PhD dissertation on the computations of the fluid dynamics here,” he said – but the basics are comprehensible to those of us less versed in Brownian motion and van der Walls force.
Cloth masks are more effective in protecting others from the wearer than the other way around because, assuming a good fit, we exhale in respiratory jets, Jimenez says: “Velocity gives inertia, and inertia makes the particles impact into the mask material.”
When inhaling, the velocity of incoming particles is somewhat lower, and that means fewer collisions of droplets and aerosols with the mask material.
He says that besides proper fit, a mask’s material (thick being typically better than thin) and the number of layers matter: two layers have a better chance of snagging particles than one, for example. N95 masks, he adds, are more effective in part because they include electrically-charged fibers that attract airborne virus-carrying particles. And beware of those masks with exhalation valves, Jimenez says, as they don’t protect others at all. Finally, he says, talking emits far more respiratory particles than breathing, and singing or shouting emits more particles than talking.
“Removing the mask to talk – as we see some public officials do – removes a lot of the benefit of wearing masks,” Jimenez said.
Though scientists continue to study the effectiveness of cloth masks in slowing the spread of COVID-19, there’s more than enough evidence to support their widespread use, Barron says.
“At the end of the day, I don’t want anybody else sick,” she said. “Even if masks were only 10% effective, that would still be better than nothing. This can spread like wildfire. Even if you’re young and healthy, others are vulnerable. It’s not just your risk.”
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