When you look at photos of Americans during the 1918 influenza pandemic, one feature stands out above all else: masks. Fabric, usually white gauze, covers nearly every face. Across the country, public health experts recommended universal mask wearing, and some cities ordered residents to wear them under penalty of fine or imprisonment. The Red Cross made thousands of cloth masks and distributed them for free. Newspapers published instructions for sewing masks at home. “Make any kind of a mask … and use it immediately and at all times,” the Boston commissioner of health pleaded. “Even a handkerchief held in place over the face is better than nothing.”
After the 1918 pandemic, the prophylactic use of masks among the general public largely fell out of favor in America and much of the West. The US Centers for Disease Control and Prevention has almost never advised healthy people to wear masks in public to prevent influenza or other respiratory diseases. In the past few months, with medical supplies dangerously diminished, the CDC, US surgeon general Jerome Adams, and the World Health Organization have urged people not to buy masks, paradoxically claiming that masks are both essential for the safety of health care workers and incapable of protecting the public from Covid-19.
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Recently, some experts have disputed this contradictory advice. They propose that widespread use of masks is one of the many reasons why China, Japan, South Korea, and Taiwan have controlled outbreaks of coronavirus much more effectively than the US and Europe. “Of course masks work,” sociologist Zeynep Tufekci wrote in a New York Times editorial. “Their use has always been advised as part of the standard response to being around infected people.” Public health expert Shan Soe-Lin and epidemiologist Robert Hecht made a similar argument in the Boston Globe: “We need to change our perception that masks are only for sick people and that it’s weird or shameful to wear one … If more people donned masks it would become a social norm as well as a public health good.” Last week, George Gao, director-general of the Chinese Center for Disease Control and Prevention, said that America and Europe are making a "big mistake" by not telling the public to wear masks during the ongoing pandemic.
It is unequivocally true that masks must be prioritized for health care workers in any country suffering from a shortage of personal protective equipment. But the conflicting claims and guidelines regarding their use raise three questions of the utmost urgency: Do masks work? Should everyone wear them? And if there aren’t enough medical-grade masks for the general public, is it possible to make a viable substitute at home? Decades of scientific research, lessons from past pandemics, and common sense suggest the answer to all of these questions is yes.
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The two most widely used types of masks are N95 respirators and surgical masks. N95s are typically round or duck-billed and, when properly fitted, form a tight seal around the nose and mouth. Stiff and snug, they can be uncomfortable to wear for long periods of time. Surgical masks, also called procedural masks when worn outside the operating room, are usually soft, pleated rectangles secured to the face with strings or ear loops and pulled under the chin. Although they are more comfortable than N95s, they are also looser, allowing more air to leak through the sides. Both surgical masks and N95s contain an inner mesh of tiny plastic fibers that functions as a filter. And both masks are disposable by design, typically discarded whenever they become too wet, dirty, or damaged.
Masks reduce the spread of infectious disease by catching microbes expelled by the wearer and protecting the wearer from microbes in their environment. When we cough, sneeze, talk, or simply breathe we emit a plume of air and droplets, which are largely composed of saliva, mucus, salts, and—if we are infected—potentially dangerous microbes. The smallest of these droplets, sometimes called aerosols, may hover or drift through the air for hours, potentially exposing anyone who enters that airspace. Larger droplets may travel only a few feet—or up to 26 feet if propelled by a sneeze—before falling to the ground or onto another surface, such as someone’s skin or clothes.
Respirators were originally designed to protect miners, firefighters, and soldiers from dust, smoke, toxins, and other harmful particles in the air. N95s are so-named because they filter out 95 percent of particles with a diameter of 0.3 microns, which are the most difficult particles to trap. Think of the filter in a mask not as a sieve but as a thicket—a dense tangle of minuscule filaments. To make it through, particles must flow seamlessly with air currents, weaving around every obstruction. Large particles are too heavy to pivot quickly, so they crash. Extremely tiny particles are buffeted by individual air molecules, bouncing about like pinballs and colliding with filaments. Particles 0.3 microns wide are just the right size to ride a stream of air through a filter’s fibrous maze, but it is still possible to thwart them with enough twists and turns.
Because N95s block the vast majority of particles that try to pass through them, they are formidable barriers against microbes. For context, the bacterium that causes anthrax is 0.8 microns wide and 1.4 microns long, whereas influenza viruses and coronaviruses are usually between 0.08 and 0.12 microns. But microbes expelled from someone’s respiratory tract are rarely naked: the droplets they travel in range in size from 0.6 to more than 1,000 microns.
Although surgical masks are not tightly sealed like N95s, the filters they contain are still a major impediment to microbes. The CDC and other health agencies often say that surgical masks catch only spurts of bodily fluids and very large respiratory droplets, and that they cannot filter tiny infectious particles. But this is simply not true.
For a 2009 study of influenza transmission, nine infected volunteers coughed five times onto a Petri dish while wearing a surgical mask, an N95 respirator, or no covering. Nearly every time someone coughed without a mask, influenza virus showed up on the dish, but no virus was found when the volunteers wore either type of mask. Similarly, in a study still under review, 246 participants with symptoms of respiratory infection breathed into a droplet-collecting device called the Gesundheit-II for 30 minutes. When volunteers were bare-mouthed, coronavirus was detected in 30 to 40 percent of their sampled droplets; when they wore a surgical mask, no coronavirus was detected. Another study using a realistic manikin that simulated human breathing concluded that, when accounting for leakage, a surgical mask can filter at least 60 percent of 0.3 micron particles. A similar manikin study demonstrated that surgical masks reduce exposure to aerosolized influenza virus by sixfold, on average.
Scientists have also tested whether masks reduce infection in randomized controlled trials. Results from these studies are inconsistent: Many fail to find definitive support for mask wearing, but a few are somewhat encouraging. Neither hand sanitizer nor face masks alone produced a statistically significant effect on rates of influenza-like illness among 1,437 college students in Michigan; together, however, they reduced the rate by 35 to 51 percent. Similarly, surgical masks appeared to reduce the spread of flu within 84 households in Berlin when they were used within 36 hours of symptoms.
Because so many trials find only a marginal benefit or none at all, some health agencies have decided against recommending masks to the general public. But the inconsistency of randomized trials does not negate the robust physical evidence that masks block respiratory droplets and microbes. Rather, these trials underscore that the efficacy of a mask depends on how it is used. In a study of 143 households in Sydney, people who diligently wore surgical masks as instructed reduced their daily risk of respiratory infection by an estimated 60 to 80 percent, but fewer than half the participants kept up the demanding routine.
In fact, this very issue has been cited (and even exaggerated) by health authorities in order to dissuade the public from using masks. "Folks who don't know how to wear them properly tend to touch their faces a lot and actually can increase the spread of coronavirus," Jerome Adams told "Fox & Friends" at the beginning of March. Yet in the same interview, Adams described how many seconds it takes to correctly wash one’s hands. The CDC and WHO have poured considerable resources into numerous websites, tweets, and videos that encourage frequent handwashing and meticulously demonstrate proper technique. If it’s possible to educate the public about better hand hygiene, why not teach them how to wear masks, too?
Meanwhile, several studies have tested the performance of masks improvised from household materials. A 2008 paper found that masks made from kitchen towels were about half as protective as surgical masks. For a study published in 2013, scientists compared the filtration efficiency of surgical masks to linen, silk, a scarf, a kitchen towel, a pillowcase, a vacuum cleaner bag, and masks that volunteers made from 100-percent-cotton T-shirts. The surgical mask performed best, followed by the vacuum cleaner bag and kitchen towel, but the latter were too thick and stiff to be worn for long periods of time. The T-shirt masks were comfortable, though, and one-third as effective as the surgical masks. “Our findings suggest that a homemade mask should only be considered as a last resort,” the authors wrote, “but it would be better than no protection.” A 2010 study reached a nearly identical conclusion.
The collective evidence makes a strong case for universal mask wearing during a pandemic. Masks are not a substitute for other interventions; they must always be used in combination with social distancing and hand hygiene. But even during a lockdown, some people need to leave their homes for essential tasks, such as buying food and medicine. With diseases like Covid-19, many individuals may be infected but asymptomatic, spreading the virus without realizing it. In parallel, some healthy people may not be able to adequately isolate themselves from infected partners, family members, and housemates. Masks could help reduce the spread of disease in all these scenarios. “Masks work in both directions,” virologist Julian Tang explained. “If everybody wears a mask, it’s double protection. Even if a mask is not 100-percent sealed, it is still a significant reduction in risk of transmission.”
“Originally, I agreed that only sick people should wear masks,” said Linsey Marr, an environmental engineer who studies disease transmission. “After observing this pandemic, I now think that if we had an infinite supply of masks, everyone should be wearing them when they go out in public.” Benjamin Cowling, an infectious disease epidemiologist, agreed: “If there were a plentiful supply of cheap face masks, I believe there would be a recommendation for mass masking. We need to consider the use of masks going forward as supplies permit and develop evidence-based guidelines for homemade versions.”
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Given the shortage of respirators and surgical masks in many parts of the world, textile companies, scientists, and citizen tailors are racing to invent and craft alternatives. Hanes, Fruit of the Loom, and Parkdale Mills are manufacturing masks from three-ply underwear fabric. Fashion designer Christian Siriano and his 10 seamstresses hope to produce several thousand masks each week. The fabric company Joann [has released]](https://www.joann.com/make-to-give-response/) video tutorials for sewing masks at home. Scientists in Hong Kong are teaching people how to make a mask from tissue paper, paper towels, tape, and a twist tie, coupled with a transparent file folder binder-clipped to glasses to serve as a face guard. Joe Fan, assistant hospital chief executive at the University of Hong Kong, says the paper mask can filter submicron particles about 80 to 90 percent as well as a surgical mask, but this data has not yet been published or replicated by other researchers. Engineers at Stanford University are investigating how to spin Styrofoam into a fine mesh that could be a proxy for the plastic filter used in N95s. Meanwhile, other researchers are exploring the possibility of sterilizing masks with heat or hydrogen peroxide vapor so that hospital workers can safely reuse them. Homemade cloth masks should be frequently boiled or washed.
Like the surgeon general, critics of universal mask wearing frequently argue that most people will not wear masks properly and may accidentally infect themselves in the process (masks should always be removed by their strings or ear loops, not by touching the contaminated front-piece). Proponents counter that any microbes caught in someone’s mask are microbes they would otherwise have inhaled and that direct evidence of harm from incorrect mask use is scant. Respiratory illnesses such as influenza and Covid-19 are primarily spread by droplets expelled from the nose and mouth. By covering these body parts, masks fight respiratory disease at its source. “To be honest, it’s common sense,” says Tang. “If you put something in front of your face, it’s going to help more than not.” If enough people wear masks at least somewhat correctly at least some of the time, the overall benefits could be dramatic. A 2011 review of high-quality studies found that among all physical interventions used against respiratory viruses—including handwashing, gloves, and social distancing—masks performed best, although a combination of strategies was still optimal.
Masks are more than physical armor against disease—they also make us more psychologically resilient. We seem to be hardwired to incessantly touch our faces. In addition to intercepting our fingers, masks can alter our habits, teaching us not to reach for our faces in the first place. Masks further function as an important social signal. In 1919, inspired by America’s use of masks to combat influenza and eager to embrace Western modernity, Japanese health authorities began recommending that people wear masks in crowds, on public transportation, and anywhere that might pose a high risk of infection. “The nation was brought together through the mask,” which provided “a sense of control over the invisible threat of a pandemic,” writes sociologist Mitsutoshi Horii. The 2004 outbreak of avian influenza strengthened the cultural importance of masks in Japan. Habitual mask wearing became a “civic duty,” both to protect others and to take responsibility for oneself.
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Considering how badly the US government has botched its response to the ongoing pandemic, and how much better most Asian countries have fared so far, it’s difficult to believe that Japan once regarded America’s management of a viral outbreak as progressive. Had the US federal government listened to expert warnings about an inevitable pandemic and taken the necessary precautions years ago—by investing in domestic mask production, for instance—we would not be faced with such a dire shortage of basic medical equipment today. Mask manufacturers around the world are working overtime and expanding their operations, but it remains uncertain whether they will meet the surging demand; some of the necessary machines cost millions of dollars and take months to construct.
Even if production increases dramatically, masks must go to health care workers and patients before anyone else. N95s are especially crucial for hospital staff who perform intubations and other procedures that generate infectious aerosols. Fortunately, the available evidence suggests that for most people in most situations, an N95 is not a necessary form of protection against Covid-19. If we eventually have a surplus of surgical masks, which are much more comfortable and affordable than respirators and still provide excellent protection, they would be an ideal choice for universal masking. In the meantime, homemade masks made from tightly woven yet breathable fabric are the best option and certainly better than nothing. A piece of cloth will never be as good as a manufactured filter, but it can still smother the brunt of a cough or sneeze and impede other people’s respiratory droplets. Neighbors should form teams of tailors. Scientists should continue studying how best to disinfect and reuse masks. Engineers should reinvent the medical mask altogether, replacing disposable varieties with something more durable, sustainable, perhaps even self-sanitizing.
To overcome the present crisis we must summon more than ingenuity and industry, however. We need solidarity. As we move closer to a phase of the pandemic in which people are allowed to mingle again but there is still no vaccine—and therefore still a chance of new outbreaks—universal masking may become even more imperative. The US desperately needs to revive the ethic embodied by the legions of gauze-wrapped faces in photos from 1918. “You must wear a mask not only to protect yourself, but your children and your neighbor,” the Red Cross implored a century ago. “The man or woman or child who will not wear a mask now is a dangerous slacker.”
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