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With cases of COVID-19 growing rapidly in the US and mounting evidence that the virus responsible, SARS-CoV-2, can be spread by infected people before they develop symptoms, the US Centers for Disease Control and Prevention recommended on April 3 that people wear cloth face coverings in public places. This guidance is a shift from the center’s previous position that healthy people only needed to wear masks when caring for someone who is sick. The recommendation also follows recent calls by experts on social media and other platforms for the general public to don nonmedical, cloth masks to help reduce the transmission of the novel coronavirus.

“Members of the general public should wear nonmedical fabric face masks when going out in public in one additional societal effort to slow the spread of the virus down,” Tom Inglesby, director of the Johns Hopkins Center for Health Security, tweeted on March 29.

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These experts hope the measure will reduce the rate of disease transmission by adding an additional layer of protection in places where social distancing is difficult, such as grocery stores, while reserving limited supplies of medical-grade protective equipment for health-care workers.

The internet is exploding with mask-sewing patterns and advice on which materials are best to use, but many unanswered questions remain about how exactly SARS-CoV-2 spreads and what benefit widespread wearing of nonmedical masks may offer individuals and the public. Because of the inherent variability in household materials, mask design, and mask-wearing behavior, experts caution that the practice is no replacement for social distancing.

“It is critical to emphasize that maintaining 6-feet social distancing remains important to slowing the spread of the virus,” according to the CDC’s web page on the use of cloth face coverings.

Understanding what a mask needs to do to protect the wearer and those around them begins with understanding how SARS-CoV-2 spreads. Experts think people pass the virus to others primarily through respiratory droplets. These infectious globs of saliva and mucus, expelled by talking and coughing, are relatively large and travel limited distances—they tend to settle on the ground and other surfaces within 1–2 m, although at least one study has suggested sneezing and coughing can propel them farther (Indoor Air 2007, DOI: 10.1111/j.1600-0668.2007.00469.x). Scientists have not yet reached a consensus on whether SARS-CoV-2 virus can also spread through smaller aerosols, which have the potential to spread farther and linger in the air. In one experiment, researchers found that the virus can remain infectious in aerosols for 3 h in controlled lab conditions (N. Engl. J. Med. 2020, DOI: 10.1056/NEJMc2004973). But this study has limitations. As the World Health Organization noted, the researchers used specialized equipment to generate the aerosols, which “does not reflect normal human cough conditions.”

Homemade and other nonmedical cloth masks would function like surgical masks, which are designed to minimize the spread of the wearer’s germs to surrounding people and surfaces by blocking respiratory emissions from the wearer. Respiratory emissions include saliva and mucus droplets, as well as aerosols. These masks, often made of paper or other nonwoven materials, fit loosely around the face and allow air to leak in around the edges when the user inhales. As a result, they’re not considered reliable protection against inhalation of the virus.

In contrast, tightly fitting N95 masks are designed to protect the wearer by trapping infectious particles in complex layers of extremely fine polypropylene fibers. These fibers are also electrostatically charged to provide extra “stickiness” while retaining breathability. N95 masks, which if used correctly can filter at least 95% of small airborne particles, are critical for the safety of health-care workers who are regularly encountering infected people.

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The ability to block respiratory emissions—as cloth masks and surgical masks can—is important because of growing evidence that people who are infected with SARS-CoV-2 but who have mild symptoms or are asymptomatic can unwittingly spread the virus.

“One of the challenges with the virus that causes COVID-19 is that sometimes people can have very mild symptoms that they may not even notice, but they’re actually highly infectious,” says Laura Zimmermann, the director of clinical preventive medicine for the Rush University Medical Group in Chicago. “And so they’re actively shedding the virus and can potentially infect others.”

Zimmermann says members of the Chicago health-care community have discussed the potential to distribute fabric masks to sick patients rather than surgical masks, to conserve personal protective equipment (PPE) supplies. “The cloth mask can really help out if somebody has some kind of infection, and you’re trying to basically contain the droplets,” she says.

In a recent communication, an international team of researchers reports that surgical masks can significantly reduce the amount of virus released into the air by people with respiratory illnesses, including infections by other coronaviruses (Nat. Med. 2020, DOI: 10.1038/s41591-020-0843-2).

Some experts encouraging widespread wearing of nonmedical masks point out that some countries that have successfully controlled their outbreaks also deployed this practice. “Face masks are used widely by members of the public in some countries that have successfully managed their outbreaks, including South Korea and Hong Kong,” according to a March 29 report on the US coronavirus response from the American Enterprise Institute.

Linsey Marr, an expert in airborne disease transmission at Virginia Polytechnic Institute and State University, says her thinking has evolved in recent weeks, and she no longer thinks only sick people should wear masks. Although some face masks may help reduce the wearer’s exposure to viruses, she says, the primary goal would be to reduce the spread of SARS-CoV-2 from infected individuals.

“If everyone wears masks, then less virus will be spread through the air and on surfaces, and the risk of transmission should be lower,” she wrote in an email to C&EN prior to the CDC’s new recommendation.

But people considering making their own mask are faced with many options in design and fabric choice, and it may not be easy to determine which options would be most effective. Neal Langerman, a chemical safety expert who is currently advising companies on coronavirus protective measures, notes that the permeability of household materials can vary widely and in unpredictable ways, making it hard to determine definitively which material is best for a homemade face mask. How tightly a material is woven can be a factor, as well as the type of fibers used. For example, natural fibers can swell when exposed to moisture from a person’s breath, changing the fabric’s performance in unpredictable ways. There’s also an inherent trade-off between the size of pores in the fabric and breathability—the least porous materials will also be harder to breathe through. The manufacturer of Gore-Tex, a lightweight, microporous material commonly used for outdoor clothing, received a flurry of inquiries about whether the material would effectively filter SARS-CoV-2. The company released a statement warning against using the material for homemade face masks because of insufficient airflow.

“The difficulty is that different fabrics have different specifications, and there seem to be so many options on the market,” Yang Wang, an aerosols researcher at the Missouri University of Science and Technology, tweeted. Wang is among the researchers collecting preliminary data on the filtration of nonmedical materials in light of the current outbreak.

Scientists have previously raised the idea of using improvised masks to counter a quickly spreading viral disease, and several existing studies have evaluated the filtration efficiencies of various household materials. One study of commonly available fabrics, including multiple types of T-shirts, sweatshirts, towels, and even a pocket square, found the materials blocked between 10% and 60% of aerosol particles similar in size to respiratory emissions, which is in line with the filtration efficiency of some surgical masks and dust masks (Ann. Occup. Hyg. 2010, DOI: 10.1093/annhyg/meq044). Which improvised material filtered particles the best varied depending on the size and velocity of test particles. Studies also note that a mask’s fit and how it’s worn can drastically impact its effectiveness, something that is difficult to replicate in lab conditions.

The CDC recommends using multiple layers of fabric to make a face covering. In a video, US Surgeon General Jerome Adams demonstrates how to make such a mask from items found around the home, such as an old T-shirt.

In spite of the variability in homemade mask effectiveness, there is some evidence that even a partial reduction in particle spread can help reduce the rate of disease transmission across a population. In a 2008 study, researchers in the Netherlands found that although improvised masks were not as effective as personal respirators, “any type of general mask use is likely to decrease viral exposure and infection risk on a population level, in spite of imperfect fit and imperfect adherence” (PLOS One 2008, DOI: 10.1371/journal.pone.0002618).

Langerman says his primary concern related to the general public wearing masks is that, as with any PPE, using a face mask can give the wearer a false sense of security, and they may be less rigorous with other precautions. Experts have reiterated the importance of maintaining a physical distance of 6 ft (1.83 m) or farther from other people, whether they are exhibiting symptoms or not. Langerman cautions against placing too much trust in homemade fabric masks to protect oneself or others.