Are you a medical professional Use N95 or surgical masks YES NO Sew a cloth mask Can you sew? YES Cut a cloth mask NO Are you a medical professional Can you sew? NO YES YES NO Use N95 or surgical masks Sew a mask Cut a mask Are you a medical professional? Can you sew? NO NO YES YES Use N95 or surgical masks Sew a cloth mask Cut a cloth mask

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On April 3, U.S. health officials recommended an arts-and-crafts project to U.S. residents: Make a cloth mask, then wear it when you go out in public.

Covering your mouth and nose is one more thing people can do in addition to social distancing and hand-washing to slow the spread of the novel coronavirus, according to the Centers for Disease Control and Prevention. Medical-grade masks are scarce and should be reserved for front-line health-care workers who are repeatedly exposed to huge amounts of the virus.

But a DIY face cover offers a bit of protection to the wearer while helping prevent them from unknowingly spreading the virus to others, a useful thing when up to a quarter of people who have covid-19 may show no symptoms, according to the CDC.

I’m in. What material should I use?

The coronavirus is extremely tiny — too tiny to be trapped by most fabrics that still allow air to flow through them.

Coronavirus Human red blood cell 0.1 micrometer average diameter 7.5 micrometer diameter E. coli bacteria 2 micrometers long Coronavirus Human red blood cell 0.1 micrometer average diameter 7.5 micrometer diameter E. coli bacteria 2 micrometers long Coronavirus E. coli bacteria Human red blood cell 0.1 micrometer average diameter 2 micrometers long 7.5 micrometer diameter Human red blood cell 7.5 micrometer diameter Coronavirus E. coli bacteria 0.1 micrometer average diameter 2 micrometers long

However, the virus seems to be most transmissible when it is stuck to much larger water or mucus droplets that come out of our mouths and noses when we cough, sneeze or talk, and a homemade mask can block those droplets.

As the mask needs to be made of something you already have around the house, cotton seems to be a good choice. The CDC recommends two layers of tightly woven 100 percent cotton fabric, such as quilter’s material or bedsheets with a high thread count.

A group of scientists and doctors at Wake Forest University recommends holding fabric options up to a bright light and choosing something that doesn’t let a lot of light show through.

They tested different masks made by community volunteers and found two of the better options to be a double layer of cotton with a thread count of at least 180, and a two-layer mask with an outer layer of regular cotton and an inner layer of flannel.

Speaking of layers, don’t just pile them on. You need to strike a balance between breathability and filtration.

Make sure you can breathe comfortably through your nose while wearing the mask — sticky nose hairs can catch some particles as well before they travel deeper into your respiratory tract. If the mask isn’t porous enough, you will be breathing around the sides of it rather than through it.

Vacuum bags, for instance, can be tough to breathe through unless you are an actual vacuum. (And be careful — some can contain fiberglass and other fibers that you do not want to inhale.)

Other materials may be too porous. The Wake Forest team suggests holding fabric up to a bright light. If a lot of light comes through the cloth, it’s probably not the best choice.

A group of Cambridge University researchers compared filtration and breathability of different types of homemade mask materials, and here’s what they found. They concluded that masks made from pillowcases or a cotton T-shirts struck the best balance.

MORE BREATHABLE LESS BREATHABLE Surgical mask A good balance between filtration and breathability 100% 90% LESS FILTRATION Vacuum cleaner bag 80% Tea towel Cotton-polyester blend 70% Antimicrobial pillowcase Linen 60% Pillowcase Silk 100% cotton T-shirt 50% Scarf 40% 30% 20% 10% No mask 0 2 4 6 8 10 12 MORE BREATHABLE LESS BREATHABLE 100% Surgical mask A good balance between filtration and breathability 90% LESS FILTRATION Vacuum cleaner bag 80% Tea towel 70% Cotton-polyester blend Antimicrobial pillowcase Linen 60% Pillowcase Silk 100% cotton T-shirt 50% Scarf 40% 30% 20% 10% No mask 0 2 4 6 8 10 12 MORE BREATHABLE LESS BREATHABLE 100% Surgical mask A good balance between filtration and breathability 90% LESS FILTRATION Vacuum cleaner bag 80% Tea towel Cotton-polyester blend 70% Antimicrobial pillowcase Linen 60% Pillowcase Silk 100% cotton T-shirt 50% Scarf 40% 30% 20% 10% No mask 0 2 4 6 8 10 12 MORE BREATHABLE LESS BREATHABLE 100% Surgical mask A good balance between filtration and breathability 90% LESS FILTRATION Vacuum cleaner bag 80% Tea towel Cotton-polyester blend 70% Antimicrobial pillowcase Linen 60% Pillowcase Silk 100% cotton T-shirt 50% Scarf 40% 30% 20% 10% No mask 0 2 4 6 8 10 12

What pattern should I use, and what if I can’t sew?

There is no one design that is the consensus gold standard for a DIY mask. There are patterns floating around everywhere, and studies have shown that any face covering is better than nothing.

Here’s one created for The Post by Grace Jun, a professor of fashion at Parsons School of Design in consultation with health experts in New York.

The CDC website has a few patterns, including one that involves a coffee filter and another that requires no sewing — just scissors and the willingness to sacrifice a T-shirt. Surgeon General Jerome Adams released a video of himself making a mask out of cloth and rubber bands.

Whatever style you choose, make sure the mask fits well, covering your nose and chin.

Sewn cloth mask Elastic ear loops Fit snugly and cover from nose bridge to below the chin. Quick-cut face mask Secure ties Do not restrict the wearer’s ability to breathe. Include multiple layers of fabric. Sewn cloth mask Quick-cut face mask Elastic ear loops Fit snugly and cover from nose bridge to below the chin. Do not restrict the wearer’s ability to breathe. Secure ties Include multiple layers of fabric. Sewn cloth mask Quick-cut face mask Elastic ear loops Do not restrict the wearer’s ability to breathe. Include multiple layers of fabric. Fit snugly and cover from nose bridge to below the chin. Secure ties Sewn cloth mask Quick-cut face mask Elastic ear loops Do not restrict the wearer’s ability to breathe. Fit snugly and cover from nose bridge to below the chin. Include multiple layers of fabric. Secure ties Sewn cloth mask Quick-cut face mask Elastic ear loops Secure ties Fit snugly and cover from nose bridge to below the chin. Do not restrict the wearer’s ability to breathe. Include multiple layers of fabric.

Once you’ve got your new mask secured to your face, leave it on while you’re outside and don’t touch the front of it — just as you wouldn’t touch your face.

I wore my creation to the store. Can I take it off now?

Yes, and you should. Once you’re back home after wearing your spiffy new mask in public, remove it without touching the front and without touching your eyes, nose or mouth.

You’ll want to clean it before wearing it again. You don’t need to wash your mask differently than other laundry because the soap in detergent should destroy any viruses.

[Which DIY mask pattern should you use? Even experts can’t pick one to recommend.]

Why can’t I just buy a surgical mask or respirator?

The CDC asks that those masks be saved for health-care workers, who are exposed to a far higher dose of the virus than a person would encounter on a quick trip to the grocery store.

These two masks worn by health-care workers offer very different types of protection.

N95 respirator Designed to reduce the wearer’s exposure to very small particles and droplets and to protect patients from the wearer’s respiratory emissions. Nose clamp and mask must be fit tested to ensure there are no gaps. Surgical mask Designed to reduce the wearer’s exposure to droplets and to protect patients from the wearer’s respiratory emissions. Offers only limited protection from inhaling small particles. N95 respirator Surgical mask Designed to reduce the wearer’s exposure to very small particles and droplets and to protect patients from the wearer’s respiratory emissions. Designed to reduce the wearer’s exposure to droplets and to protect patients from the wearer’s respiratory emissions. Nose clamp and mask must be fit tested to ensure there are no gaps. Offers only limited protection from inhaling small particles. N95 respirator Surgical mask Designed to reduce the wearer’s exposure to very small particles and droplets and to protect patients from the wearer’s respiratory emissions. Designed to reduce the wearer’s exposure to droplets and to protect patients from the wearer’s respiratory emissions. Offers only limited protection from inhaling small particles. Nose clamp and mask must be fit tested to ensure there are no gaps. N95 respirator Surgical mask Designed to reduce the wearer’s exposure to very small particles and droplets and to protect patients from the wearer’s respiratory emissions. Designed to reduce the wearer’s exposure to droplets and to protect patients from the wearer’s respiratory emissions. Offers only limited protection from inhaling small particles. Nose clamp and mask must be fit tested to ensure there are no gaps. N95 respirator Surgical mask Designed to reduce the wearer’s exposure to very small particles and droplets and to protect patients from the wearer’s respiratory emissions. Designed to reduce the wearer’s exposure to droplets and to protect patients from the wearer’s respiratory emissions. Offers only limited protection from inhaling small particles. Nose clamp and mask must be fit tested to ensure there are no gaps.

The more exposure, the more likely a person is to get sick — and the level of exposure probably also plays a big role in the severity of the disease.

“I think dose is really important,” said Vineet Menachery, a virologist at the University of Texas Medical Branch in Galveston who specializes in studying coronaviruses. “The higher the dose you get, the more sick you are likely to get.”

A previous version of this story had a chart that incorrectly compared the size of the coronavirus to other particles. It also included an extra layer of cotton in the description of a mask tested at Wake Forest University.