Face masks made using certain types of cotton may be better at preventing the spread of coronavirus particles than those featuring synthetics, according to a study.
Researchers tested 32 types of cloth for their study: 14 cottons, one wool, nine synthetics, four synthetic blends, and four synthetic/cotton blends. To mimic different methods of making face coverings, they also ran experiments with multi-layered and mixed material samples of different types of cloth. Seven materials partly made of polypropylene—which is used in healthcare worker masks such as N95 respirators— were also tested, as well as papers, including coffee filters and paper towels.
In a series of experiments, the researchers directed aerosol salt particles of varying sizes at the different fabric samples. They counted the number of particles in the air before and after they sprayed the fabric. The U.S Center for Disease Control and Prevention's National Institute for Occupational Safety and Health recommends using salt, or sodium chloride, to test masks, rather than virus particles. The findings were published in the journal ACS Nano.
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The salt particles ranged from 50 to 825 nanometers in diameter. For context, the COVID-19-causing coronavirus is around 110 nanometers in diameter.
Three of the samples that performed best were woven 100 percent cottons with high- to medium-level yarn counts. These were down proof ticking, a woven hand towel, and a lightweight flannel. The other two were woven synthetics of twill weave and polyester, respectively, with a moderate yarn count.
The lightweight flannel the team tested filtered air more efficiently and blocked particles better when it was layered, suggesting multi-layered cloth masks may offer better protection from particles.
The worst performers were a lightweight chiffon made of polyester, followed by cotton muslin, polyester knit, Rayon, and a blend of polyester and cotton. All of these fabrics were highly breathable.
By examining the best materials, the noticed the woven fabrics formed "fiber-webs," with the highest concentration found in cotton flannel samples. These webs seemed to disrupt the flow of air and provide more surfaces particles could stick to. This may explain why the cottons out-performed fabrics with a higher yarn counts or tighter weave. Therefore, the fabric's visual appearance may not be indicative of its ability to act as a filter.
The authors wrote: "The best performing cloth materials had moderate yarn counts with visible raised fibers. No measured cloth masks performed as well as an N95."
It is important to note the fabrics were tested in "idealized conditions" that minimized leaking, and other real-world factors that might affect a mask's performance, such as humidity, the moisture of the fabric, the placement of seams, and washing.
Co-author Christopher Zangmeister of the Chemical Process and Nuclear Measurements Group at the National Institute of Standards and Technology said in a statement: "It turns out that off-the-shelf materials provide some protection from aerosols if you use multiple layers of cloth and a face covering fits snugly. But none are as good as an N95 mask."
He said: "We hope this research will help manufacturers and DIYers determine the best fabrics for the job and serve as a basis for additional research."
Co-author Jamie Weaver, also of the Chemical Process and Nuclear Measurements Group of the Weaver National Institute of Standards and Technology, said: "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.
"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 U.S. Centers for Disease Control and Prevention recommends that people wear face coverings in public when social distancing is not possible.
