Cotton Is Best for Homemade Masks, Study Suggests

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Illustration for article titled Cotton Is Best for Homemade Masks, Study Suggests

Photo: Geoffroy Van Der Hasselt (Getty Images)

A new U.S. government-led study has found that common materials such as cotton can effectively filter particles as small as the coronavirus, especially when layered. The findings offer more evidence that homemade masks can slow the pandemic’s spread and possibly provide some personal protection against covid-19, albeit not as much as you would get from medical-grade respirators.

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It’s become obvious that wearing masks can reduce the spread of the coronavirus in the general population. At the very least, masks can block some of the largest droplets containing viral particles that a person exhales or coughs out, preventing them from reaching others. The widespread use of masks is important, because people can be contagious without ever feeling sick or shortly before symptoms start.

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But there are still questions about how effective different types of masks are, including homemade cloth masks, and whether they have any utility in preventing people from getting infected by someone else. Respirator masks, such as the N95, are specifically designed to filter out potentially infectious aerosols and prevent infection.

The new study was published this month in ACS Nano and involved researchers from the National Institute of Standards and Technology (NIST) as well as the Smithsonian Institution’s Museum Conservation Institute.

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They conducted simple experiments with 32 different types of cloth made from cotton, wool, and synthetic fibers. Particles large and small were flung into swatches of a fabric, and then the researchers measured how many particles weren’t captured by the cloth and remained in the air. Instead of using live coronavirus, though, they used particles of table salt, which are similar in size to the viral particles we breathe out.

By the end, it was clear that cotton fabrics were the best on average at blocking coronavirus-sized particles, compared to synthetic fabrics. But not all cotton fabrics were the same. Cotton that had raised layers, like the kind you’d see in flannel, performed better than other types of fabric. The more tightly woven a fabric was, the better it performed.

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“The texture turned out to be one of the more useful parameters to look at because we found that most of the cotton fabrics with raised threads tended to filter best,” said study author and NIST researcher Jamie Weaver in a press release from the agency. “Our findings suggest that a fabric’s ability to filter particles is based on a complex interplay between material type, fiber and weave structures, and yarn count.”

The researchers also found evidence that stacking multiple layers of fabric would further boost the effectiveness of cotton masks. But importantly, they also found that even the best homemade masks that fit snugly on someone’s face are unlikely to provide as much protection from infection as an N95 mask. The best single-layer cotton blocked about 20 percent of particles in the range of the virus, while N95 masks are designed to block 95 percent of particles that size, as the name implies. Some cotton fabrics that proved most effective at blocking virus-sized particles would also be harder to breathe through, highlighting a possible tradeoff between safety and accessibility.

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“The bottom line is that none of these fabrics are as good as an N95 mask. Still, cloth face coverings can help slow the spread of coronavirus,” said study author and fellow NIST researcher Christopher Zangmeister. “We hope this research will help manufacturers and DIYers determine the best fabrics for the job and serve as a basis for additional research.”

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