Well over a year into the COVID-19 pandemic, science has learned quite a lot about the novel coronavirus — including how to stop its spread. You know the drill: social distancing, frequent hand-washing and, perhaps most important, wearing a mask.
Respiratory droplets are the most common way the virus spreads, and research has shown that masking up helps protect yourself and others from spread. That research has evolved and changed some of the recommendations behind mask-wearing, and that has led to some confusion in the general public — including the concept of double-masking: wearing two masks instead of one to improve protection.
Clearing up some of that confusion will help you protect yourself and others from the spread of COVID-19.
Filtration and Fit
How well a mask works depends on two things: filtration and fit, according to Linsey Marr, professor of civil and environmental engineering at Virginia Tech who studies airborne disease transmission. “Good filtration removes as many particles as possible, and a good fit means that there are no leaks around the sides of your mask, where air — and viruses — can leak through. Even a small gap can degrade the performance of your mask by 50%,” she says.
The Centers for Disease Control and Prevention recommends that everyone wear a mask when they go out in public. “Masks are effective at reducing the spread of COVID-19,” says Dr. John Brooks, chief medical officer of the CDC’s COVID-19 response. While any mask is better than none, the CDC further recommends that everyone wear a mask with at least two layers, because, in general, more layers block more particles.
Each additional layer adds more filtration capability, Marr explains. “If one layer is 50% effective, then combining two of them gets to 75% efficiency. Imagine if 100 particles approach the first layer. Half of them, or 50, are trapped, while the other 50 pass through. When those 50 hit the next layer, half of them are removed, leaving only 25 that pass through.”
Indeed, a recent study published in JAMA that Brooks authored found that wearing multilayer cloth masks were more effective than single-layer masks, blocking as much as 50% to 70% of exhaled droplets and particles.
So two layers are better than one. But the term “double-masking” has “unfortunately introduced substantial consumer confusion,” Brooks says. “CDC prefers consumers think about ‘fit to improve performance.'” That’s because correct fit is just as important as multiple layering. “Improving the fit of a mask makes it even more effective,” Brooks says.
[READ: Skin Health When Wearing a Mask.]
Choosing Your Masks
There are a variety of mask types to choose from:
— Surgical or medical procedure masks.
— Disposable or single-use masks.
— Gators and bandanas. These are typically recommended as a last resort.
— Reusable, washable cloth masks.
— Multilayered single mask or respirators.
— Masks with an expiatory valve. These are not recommended for COVID protection, as they allow for droplets from inside the mask to escape.
Many multilayered single masks, including KN95, N95 and others, “are designed to be worn alone. Adding a surgical mask over these will not decrease their effectiveness and can keep the multilayered single mask clean, allowing for easy disposal of the surgical mask and the continued use of a clean N95. Placing a surgical mask under a N95 is not recommended, as it will affect the fit and overall effectiveness of the N95.
Medical procedure masks and surgical masks are fairly similar. The exception with medical procedure masks, Brooks says, is that they are less protective and made for sterile environments. These loose fitting, single-layer masks allow particles to escape. Adding a cloth mask over a surgical mask helps negate this issue of filtration and fit, and fits under double-masking criteria.
Wearing two medical procedure masks doesn’t add much in terms of better fit to prevent the air leaks around the edges, Brooks adds. And wearing two cloth masks may be more difficult to breathe through or block peripheral vision, which could lead to a trip or fall injury, he says. Thus, the CDC recommends double-masking only in the case of a cloth mask worn over a medical procedure mask.
If two layers are better than one, are even more layers better still? “Some three-layer masks may offer more protection than some two-layer masks, depending on their fabric and construction,” Brooks says. But masks with four or more layers may make it too hard to breathe.
A Better Fit
The best strategy is to find a two-layer mask or to wear a cloth mask over a medical procedure mask, and to make sure whatever you wear fits correctly. Marr recommend two ways to improve the fit and performance of your mask:
— Ensure your mask has a metal nose bridge that bends closely to fit around your nose.
— Choose a mask with straps that can be tightened around your head, not just your ears, because you can get a tighter fit.
“You should feel the mask sucking inward when you breathe in, and if you hold your hands around the sides of the mask, you should not feel any air leaking out when you breathe out,” she says.
The CDC also outlines several methods to improve mask fit on its website.
“Any mask is better than no mask, and masks that fit work best,” Brooks says. He also stresses the need to stay vigilant, even as case numbers appear to be dropping in most places across the country. “With the emergence of more transmissible SARS-CoV-2 variants, it is even more important to adopt widespread mask-wearing, as well as to redouble efforts with use of all other nonpharmaceutical prevention measures, until effective levels of vaccination are achieved nationally,” he says.
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Update 10/22/21: This story was previously published at an earlier date and has been updated with new information.