#fabirc

Blue dyes. The coal tar colours of Farbwerke vorm. Meister, Lucius & Brüning, Hoechst on Main, Germany, applied in calico printing. 1908.

More and more people are wearing homemade masks at supermarkets, hardware stores, workplaces and more. The goal is to slow the spread of the virus that causes COVID-19. Now two new studies provide data on which fabrics to use. Also important, they show: a snug fit.

To see why, it helps to understand how the virus travels through air. People infected with COVID-19 breathe out some of the virus particles in very small droplets of spit, snot or water vapor. Without something to stop them, the larger droplets will fall within a few feet. That’s one reason why physical distancing between people matters. But the tiniest droplets, called aerosols, can remain aloft for a few hours before falling onto surfaces. That’s also a reason for frequent handwashing, cleaning of surfaces and other sanitizing steps.

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Masks can help to limit the spread of aerosols. But medical-grade masks are in short supply. Hospitals and health practices need them most. Among the public, therefore, many people have begun making masks at home. It’s been unclear, however, which materials might work best.

Aerosols can range from about 6 micrometers (or microns) down to 10 nanometers across, notes Supratik Guha. In comparison, the materials scientist explains, “a human hair is about 7,500 microns.” (A micron is one millionth of a meter; a nanometer is one billionth of a meter.) Guha works at Argonne National Laboratory in Lemont, Ill., and at the nearby University of Chicago.

To see which fabrics worked best in blocking out aerosols, Guha and his colleagues performed tests. They set up a chamber to create aerosols of salt in the size range of the droplets that can carry the COVID-19 virus. A fan blew these aerosols toward tubes. A piece of fabric — sometimes layers of one or more types of fabric — covered the near end of each tube. The team then measured what share of the aerosols made it through the fabrics.

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Overall, a combination of fabrics worked best. And not just any fabrics. The top performers paired 600-thread-count cotton (meaning 600 threads were woven to make up each square inch of fabric) with two layers of either silk or chiffon. On average, each type stopped at least 90 percent of the particles.

What does this mean if you make your own mask? “Use tighter fabrics with tighter weaves,” Guha says. A tighter weave has smaller holes for particles to sneak through. “Try to use combinations of materials,” he adds, “They filter the particles in different ways.”

For example, tightly woven cotton works as a mechanical filter. Like a sieve, it keeps airborne bits that are too big from going through the holes between its threads. Chiffon or silk can work a second way, too. The structure of the molecules that make up those fabrics lets them attract electrons or give them up, Guha explains. That lets them attract charged aerosols. This electrostatic property lets the threads and aerosols bind to each other.