What should you stuff into your face mask to make it work better?

Do DIY Masks Help Stop Coronavirus?

Here’s what you need to know about homemade face coverings

Artist Kate Tully sewing a face mask at her Illinois home on April 18 to help fight the coronavirus pandemic.PHOTO: NAM Y. HUH/ASSOCIATED PRESS

ByJo Craven McGinty Updated April 24, 2020 3:22 pm ET

Even before Gov. Andrew Cuomo ordered New Yorkers to cover their faces in public to prevent the spread of Covid-19, instructions for creating homemade masks were circulating.

But different sources offered conflicting advice.

Is T-shirt fabric the best material for fashioning a mask…or the worst? Is thick cloth more impervious to airborne viruses…or is a tightly woven sheet? Are vacuum bags essential…or will a simple bandanna do?

Anything is better than nothing, according to filtration experts, but there are differences in the intended function and level of protection offered by the coverings.

Percentage of particles the size of the coronavirus blocked by various household materials

Source: David Pui and Qisheng Ou, Center for Filtration Research at the University of Minnesota

Note: Researchers tested only the materials, not masks made of the materials, and didn’t account for poor fitting, breathability and other issues.

THIS TAKEN FROM A BAR CHART — NUMBER CONVERSION IS AN ESTIMATE

Furnace filters (three to five layers of electrostatically charged polypropylene and polyolefin) == near 100%

Automotive filters == about 95%

Vacuum bags = about 90%

Swiffer dry sweeping cloth (five layers) == 60%

T-shirt fabric (five layers) == 60%

Bed sheets (five layers) == 50%

Papertowels (five layers) == = 45%

Coffeefilters == 20%

Simple masks, whether medical or makeshift, prevent wearers from dispersing viruses through coughing, sneezing, talking or breathing.

Respirators, like the N95 masks meant for health-care workers, do the same but also protect wearers from inhaling viruses once they become airborne.

The differences are a function of the material and construction.

Respirators are made of nonwoven fabric that’s enhanced with an electrostatic charge to trap particles that might otherwise penetrate its surface. The masks fit tightly around the nose and mouth and are intended to reduce the wearer’s exposure to particles smaller than 100 microns; a micron is one-millionth of a meter.

Surgical masks are made of similar material but might not have an electrostatic charge. They’re primarily designed to prevent the wearer’s spit or mucus from reaching patients, but sometimes have fluid-resistant properties that also protect the wearer from patients’ spatter.

Homemade masks made from fabrics such as woven or knitted cotton are not charged; they’re more porous than masks made of nonwoven material; and they don’t fit as snugly.

But when viruses are ejected, they’re encapsulated in droplets of mucus or saliva, and almost any face covering will trap those particles.

“Normally, you think about wearing a mask to protect yourself,” said Mike Bell, deputy director of the Division of Healthcare Quality Promotion at the U.S. Centers for Disease Control and Prevention. “That’s not what this is about. It’s about not spraying respiratory secretions and spit into the air around you.”

A cough or sneeze will launch droplets of mucus or saliva measuring 80 to 300 microns at speeds of 50 miles an hour to 100 miles an hour, according to David Pui, director of the Center for Filtration Research at the University of Minnesota.

But even breathing or talking will expel particles.

One experiment by researchers at the National Institutes of Health and the University of Pennsylvania found that speaking the words “stay healthy” generated thousands of droplets that were invisible to the eye.

A 100-micron particle will sink at a rate of about one foot a second, Dr. Pui said, and in very dry conditions, it will evaporate in around seven seconds—so some droplets might settle to the ground before releasing a virus—but particles less than one micron will float indefinitely.

The new coronavirus measures 0.12 micron.

“It’s extremely small, almost 1,000 times smaller than the width of a human hair,” said Sergey Grinshpun, director of the Center for Health-Related Aerosol Studies at the University of Cincinnati.

The pores in fabrics used to hinder dust mites and other allergens are around 3.5 microns to six microns—or 30 to 50 times as large as the virus—but smaller particles can still get caught in the fibers and be collected there, Dr. Pui said.

“It’s like a highway barrier,” said Juan Hinestroza, an associate professor of fiber science at Cornell University. “You may be going 90 mph, but if you hit a barrier, your velocity will change.”

Still, with medical-grade masks and respirators in short supply, Dr. Pui and his colleague Qisheng Ou have tested how well different household materials work as alternatives.

They tested several materials, and found that five layers of T-shirt fabric or Swiffer dry sheets blocked 60% of particles the size of the coronavirus; five layers of bed sheets blocked 50%; five paper towels blocked 40% and two coffee filters blocked 10%. In addition, they found that three to five layers of a furnace filter made of electrostatically charged polypropylene and polyolefin blocked 98% of the particles.

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It’s important to note that the researchers tested only the materials—not masks made of the materials—so the findings don’t account for problems like air leaks caused by poor fit, and some of the items might not be breathable or pliable enough to use as face masks.

“That’s the trade-off,” said Yiqi Yang, a textile expert at the University of Nebraska Center for Materials and Nanoscience. “If you want better filtration, you will have less air flow. It will feel more difficult to have oxygen.”

More efficient materials, like furnace filters, could be inserted into the pocket of a cloth mask, Dr. Pui said. But if the wearer can’t inhale or exhale through the material and instead pulls air from around the mask’s edges, it could defeat the purpose.

The best plan, the experts said, is to don a mask, whatever the material, and prevent the virus from becoming airborne to begin with.

Write to Jo Craven McGinty at Jo.McGinty@wsj.com

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