Two face masks may be better than one if you're doubling up on single-layer cloth masks or combining a double-layered cotton mask with another material like silk, chiffon, or flannel.
Multiple masks may do more harm than good in some cases, though, like if they make it so difficult to breathe you adjust them to let in more air or if they offer a false sense of security.
When it comes to preventing the spread of COVID-19, we know that more distance between people is better than less, smaller crowds are safer than larger, and outdoor activities are less risky than indoor ones.
We also know that a wide variety of face masks work to protect both the wearer and those around them. So are two better than one?
It depends on what types of masks you're referring to and their fit to your face, Stephen Morse, a professor of epidemiology at the Columbia University Medical Center, told Insider.
And whether you're wearing one or two, it's important to remember that while masks can greatly reduce your risk of contracting COVID-19, they're not bulletproof. Other mitigation measures are critical, too.
Two masks are better than one if each are a single cloth layer
In general, face masks work by blocking the large respiratory droplets that spread the virus when people sneeze,or cough. They can also serve as a barrier against smaller aerosols, a less common but still possible way the virus transmits.
Wearing them works so well that, if almost everyone did so, they could prevent tens of thousands of deaths in a few months, researchers have predicted.
But different masks serve different populations and situations better than others. For instance, N-95 masks are the most protective and therefore best for healthcare workers, while healthy people in the general population can stay pretty safe outside with a mask made from a cotton t-shirt, though a two-layer cotton mask is better.
Doubling up may offer extra protection if, for example, you're using two single-layer cotton masks, ideally with a high thread count (around 600). That way you can mimic a mask that has two layers, which research has shown is superior to a single-layered cloth mask, though both aren't as protective as a surgical mask.
Mixing masks could help them last longer, or simply provide comfort
In some cases, mixing mask types may be appropriate too, Morse said. For example, healthcare workers who are in short supply of N95s can benefit from wearing a surgical mask on top in order to keep them cleaner and help them last longer. Dr. Anthony Fauci did this at some early press briefings, Morse noted.
You can also mimic a so-called "hybrid" mask by combining two layers of 600-thread-count cotton paired with another material like silk, chiffon, or flannel — an April study found such masks filtered at least 94% of small particles and at least 96% of larger particles, Business Insider's Aria Bendix and Yuqing Liu previously reported.
That aligns with the World Health Organization's recommendation that fabric masks have three layers: an inner layer that absorbs, a middle layer that filters, and an outer layer made from a nonabsorbent material like polyester.
Dr. Deborah Birx, the White House coronavirus response coordinator, has also recommended wearing a shield over a cloth mask.
Morse said on occasion he's also worn a surgical mask underneath a cloth face covering, mostly for comfort in order to prevent the inner lining from sticking to his face when he breathes.
"Does this offer extra protection? It probably depends on how well they fit to begin with," Morse said. "The real limit is likely to be comfort. Can you breathe through the combo? Otherwise, you might not fit it closely enough to your face, and then it could be less protection than one good, well-fitting mask."
A well-fitting mask should seal around the nose and mouth, preventing droplets from escaping through any gaps.
Wearing multiple masks is not so protective that you can let your guard down
Ultimately, doubling up on masks likely won't exponentially increase your protection. For example, a mask made from a 100% cotton t-shirt has been shown to be 97% effective at filtering respiratory droplets, while a double-layer cotton mask is 99.5% effective. (The latter is much more effective, though, at filtering aerosols: 82% to 51%, respectively.)
Doubling up may also do more harm than good if they give you a false sense of security, leading you for example to spend more time with strangers indoors or slip up on social distancing.
"You cannot become overly reliant on any one measure," WHO infectious disease specialist Maria Van Kerkhove said during a media briefing Monday. "It can't just be testing alone, it can't just be case identification alone, it can't just be masks alone, it can't just be physical distancing alone. All of these different interventions need to be used."
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