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What To Look For In A Face Mask, According To Science

In a matter of weeks, face masks went from being considered unnecessary, and possibly harmful, to mandatory in many places across the U.S. You’re forgiven if you’ve got a bit of whiplash.

Currently, the scientific consensus is that face masks are at least somewhat effective at preventing the spread of COVID-19, and both the World Health Organization and the Centers for Disease Control and Prevention recommend wearing a cloth face covering. But which mask should you choose? Is the hand-stitched one your father-in-law made just as good as the one produced by your favorite fashion retailer? Are disposable surgical masks just as good? How many layers of fabric should it have?

To help you wade through the mountain of options, we’ve taken a look at the (admittedly limited) science available and spoken to experts to create a science-backed guide to mask shopping.

Do I need to wear a mask?

Unless you’re under the age of 2, have a specific medical condition that makes it hard for you to wear a mask, or are able to keep six feet away from other people at all times (hello, hermits!), you should wear a mask whenever you leave your house, according to current recommendations from the CDC, WHO and public health experts. And it may be a requirement in your city or state.

What does a mask do?

We now know that the main way the novel coronavirus is spread is through respiratory droplets: tiny bits of spit and other materials that are expelled from your mouth and nose when you sneeze, cough, talk or breathe. If you’re carrying the virus, it can hitch a ride on these droplets and spread to surfaces or to other people — entering their nose or mouth and causing infection. A mask can physically block these droplets from travelling further than your face. While they can also provide some protection against COVID-19, there are lots of ways to come into contact with infectious droplets (touching a surface where droplets have landed, or having droplets land on your eyes or skin), but fewer ways of spreading droplets (likely only through your mouth and nose). Because of this, masks do a much better job at preventing an infectious person from spreading the disease than they do of protecting a healthy person from catching it. Wearing a mask is about not spreading the virus yourself rather than providing protection for you.

“You wear one to protect me and I wear one to protect you,” said Dr. Scott Segal, an anesthesiologist at Wake Forest Baptist Health in North Carolina who has run lab tests of face mask materials.

Should I go disposable or reusable?

In lab tests, disposable surgical masks perform well for both breathability and blocking respiratory droplets. Depending on the material, both kinds of masks can be equally effective and safe. But disposable masks are really only designed to be worn once then thrown away (they start to break down and become less effective after wearing), which isn’t great from an environmental perspective. Already, piles of single use personal protective equipment have been washing up on shorelines. If a disposable mask is the only option, wear it, but a reusable, washable mask is a better long-term solution.

What material is best for a reusable face mask?

A lot of scientists have been trying to figure this out, and the answer isn’t exactly straightforward (is anything straightforward when it comes to COVID?). Different researchers have set up devices that spray tiny droplets at fabric and then measured how much of it comes through the other side, while also measuring air flow to determine breathability. What they’ve found is that it’s less about the type of fabric — cotton, linen, silk — and more about the quality of fabric, according to Segal. Higher quality fabrics have a tighter weave and thicker thread that do a better job of blocking droplets from passing through.

But you also want the fabric to be breathable, according to Taher Saif, a mechanical engineer at the University of Illinois who has been researching face mask material. Saif said if breath can’t get through the mask, it will find another way out, allowing respiratory droplets to spread.

“Imagine you take aluminum foil and make a mask out of it,” Saif said. “All the air will be forced out the sides. If you sneeze or cough, the majority of the droplets will escape out the side and you lose the point of the mask.”

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So you should be able to comfortably breathe through the mask. But measuring exactly how effective a specific fabric is at blocking droplets requires a lab, and just because one lab finds a mask made from a 100 percent cotton T-shirt to be effective doesn’t mean the cotton T-shirt you have in your drawer is the same material. In lieu of more precise measurements, Segal offered a rule of thumb: hold the material up to a bright light.

“Look at the light coming through the fabric,” Segal said. “If it outlines individual fibers and you can see the light through fabric, it’s probably not as effective. The less of that you can see, the better the filter.”

How many layers is best?

According to research that has been published or prepublished so far, two layers do a better job than one, and three layers do a better job than two. Take a look at these two videos from Saif’s study, comparing the same T-shirt material in one layer versus two:

In its guidelines, the WHO recommends a minimum of three layers, and many major manufacturers seem to be using this as a rule of thumb. So why not go four layers? You could, but remember you don’t want to sacrifice breathability (see above). Still, choosing a mask with more layers will generally add more protection.

Does thread count matter?

Thread count refers to the number of threads per square inch of fabric, but more isn’t necessarily better, according to Segal.

“Satin sheets have super high thread count but you can hardly breathe through them because it’s so tight,” Segal said. “That may have to do not just with thread count but also tightness of weave.”

What style should I buy?

The most effective masks are ones that cover both your nose and mouth and form a somewhat snug seal at the edges of the mask. Without a professional grade mask like an N95 respirator, you’re not going to get an airtight seal, but that’s okay — we’re aiming for good enough, not perfect. For everyday use, the experts I spoke to said a comfortable but snug mask does the trick.

Should I buy handmade or mass produced?

The efficacy of the mask has more to do with the material than whether it was made in a factory or at your kitchen table. Even homemade, no-sew, T-shirt masks have performed well in lab tests, especially when made with multiple layers. Check out the material and choose (or make) the mask you find most comfortable and will be likely to wear.

What about those exhalation vents?

Exhalation vents were designed for masks that serve a very different purpose than the ones we should be wearing now. Those masks are meant to prevent the wearer from inhaling particles, not to block them from exhaling particles, so the vents make it easier to breathe while still stopping the bad stuff from getting in. With COVID-19, masks are meant to block your respiratory particles from getting out, not to block other things from getting in, so an exhalation vent defeats the purpose of the mask.

What if I pick the wrong one?

At the end of the day, many experts also say that any face covering is better than no face covering. Do your best to find a mask that is breathable and that you don’t mind wearing, and you should be in good shape.

To drive the point home, Saif held up a Kleenex during our video interview and said: “Even this would be better than nothing.”

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Kaleigh Rogers is FiveThirtyEight’s technology and politics reporter.