Which Materials Make The Best Homemade Masks? New Study Evaluates 32 Materials

By Nicholas Gerbis
Published: Monday, July 6, 2020 - 4:15pm
Updated: Monday, July 6, 2020 - 4:16pm

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person wearing a mask
Sky Schaudt/KJZZ
A homemade mask made from cotton.

As Arizona tops 100,000 coronavirus cases, it's never been more important to practice social distancing and wear a face covering. Now, a new study in the American Chemical Society journal ACS Nano evaluates the best materials for homemade masks.

With N95 respirators reserved for medical settings, the World Health Organization and the Centers for Disease Controls and Prevention have called for better guidelines for mask material, design and construction.

Unfortunately, most of what we know about the topic dates back to the 1918 influenza pandemic, when our understanding of micrometer-sized droplets and nanometer-sized aerosols was far less advanced.

When researchers from the National Institute of Standards and Technology tested 32 materials using salt particles sized to match the coronavirus's droplets and aerosols, they found 100% cotton — in plain, block and poplin weaves — gave the best protection.

A synthetic twill weave and a polyester poplin weave also performed in the top five.

Other synthetics, like polyester knit and chiffon, performed the worst. Cotton muslin and a blend of 65% polyester/35% cotton also performed poorly.

The materials tested included 14 cotton, one wool, nine synthetic, four synthetic blends and four synthetic/cotton blends.

The results suggest a woven cotton hand towel in six layers, or a light flannel in four, would remain breathable and protect well, despite both having only moderately dense weaves.

The researchers suggest the raised nap of those fabrics might offer more surface area for capturing aerosols.

The researchers measured most samples in two layers.

No measured cloth masks performed as well as an N95 respirator.

Face masks protect the population by trapping droplets and aerosols expelled as an infected mask wearer breathes, coughs or speaks. They also somewhat filter air inhaled by mask wearers.

Opponents of mask wearing sometimes say pores in component materials are too large to stop virus-carrying particles measuring 250-500 nm.

But particles on such tiny scales are subject to far more than just collisions and tangles. They drift in fluid air, bounce around in random Brownian motion and even stick to cloth like hair to a static-charged balloon.

The study proposes a complex array of factors at work in cloth masks, and suggests appearances can be deceptive, since the cotton cloth that offered the best protection consisted of a visibly loose weave.

Still, the researchers caution their laboratory results based their breathability estimates on air pressure changes, not actual user experiences.

Moreover, their recommendations do not take into account WHO guidelines, which call for mixing layers that absorb water with those that repel it.

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