Face masks help prevent virus spread, but cloth is inferior

An international research team has developed a new theoretical model to better assess the risks of spreading viruses such as COVID-19 — with and without a face mask — while a separate research team has warned about the ineffectiveness of cloth masks.

Viruses, such as SARS-CoV-2, are spread from an infected individual to other susceptible individuals through virus-filled droplets that are released when talking, coughing or sneezing. Droplets emitted from the salivary glands are sprayed out through the exhaled air. Once out of the mouth, these drops can either evaporate, settle or remain floating. Larger and heavier droplets tend to fall in a ballistic motion before evaporating, while smaller droplets behave like aerosols that spray and remain airborne.

Researchers from Italy, Sweden and Austria have now developed a model to calculate the direct risk of spreading COVID-19 infection by including a number of factors, such as interpersonal distance, temperature, humidity levels, viral load and type of exhalation. They also demonstrated how these risks change with and without a face mask, with the results published in the Journal of the Royal Society Interface.

The study revealed, for example, that a person talking without a face mask can spread infected droplets 1 m away. Should the same person cough, the drops can be spread up to 3 m; if the person sneezes, the spread distance can be up to 7 m.

But using a surgical face mask or, to an even greater extent, an FFP2 mask can provide excellent protection that significantly reduces the risk of infection, the researchers found. Provided that the face mask is worn correctly, the risk of infection is negligible even at distances as short as 1 m, regardless of environmental conditions and if the person is talking, coughing or sneezing.

But what about cloth masks? A separate study from the UK, Germany and France focused on examining the efficacy of particle filtration by woven fabric, which, unlike material used in standard air filters and masks, consists of fibres twisted together into yarns. Their results were published in the journal Physics of Fluids.

Using 3D imagery produced by confocal microscopy to see the airflow channels, the scientists simulated the airflow through these channels and calculated filtration efficiency for particles a micrometre and larger in diameter. The study concluded that for particles in this size range, the filtration efficiency is low.

“Masks are air filters, and woven fabrics, such as cotton, make for good jeans, shirts and other apparel, but they are lousy air filters,” said study co-author Richard Sear, from the University of Surrey. “So, use woven fabric for clothing, and N95s or FFP2s or KF94s for masks.”

Indeed, the flow simulations suggest when a person breathes through cloth, most of the air flows through the gaps between the yarns in the woven fabric, bringing with it with more than 90% of the particles.

“In other words, these relatively large gaps are responsible for cloth being a bad material to make air filters from,” Sear said. “In contrast, the filtering layer of an N95 mask is made from much smaller, 5 µm fibres with gaps that are 10 times smaller, making it much better for filtering nasty particles from the air, such as those containing virus.”

While earlier research revealed similar findings, this study represents the first to simulate particles going directly through the gaps in woven fabric. Sear added masks should feature the “two Fs” — good filtration and good fit — and that even surgical masks don’t always pass this test.

“Surgical masks fit badly, so a lot of air goes unfiltered past the edges of the mask by the cheeks and nose,” he said.

Image credit: ©stock.adobe.com/au/Girts

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