Cloth masks ineffective for protection against airborne viral, says study
Washington, March 7: Masks have long been thought to be an important first line of defence against airborne transmission of the disease. But, is every mask effective against the infection that spreads through the air?
A recent study on the topic has been published in 'Physics of Fluids.' Like many other viruses, Covid-19 too is transmitted predominantly via particles carried in the air. Infected people exhale virus-containing particles into the air, which can be inhaled by someone else, who subsequently becomes infected.
Since not all masks are made equal, the newest wave of the pandemic, fueled by the omicron strain, prompted public health officials to recommend more protective face coverings.
Researchers from England, Germany and France used their expertise and microscopes to examine the efficacy of particle filtration by woven fabric, which, unlike ordinary air filters and masks, is made up of twisted fibres woven into yarns. Thus, there are two length scales, the diameters of the fibre and the yarn.
The researchers simulated airflow through these channels and calculated filtration effectiveness for particles a micrometre and larger in diameter using 3D imagery captured by confocal microscopy to view the airflow channels. The study concluded that filtering effectiveness is low for particles in this size range.
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"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 co-author Richard Sear, from the University of Surrey adding, "use woven fabric for clothing and N95s or FFP2s or KF94s for masks."
According to the flow simulations, when a person breathes through cloth, the majority of the air passes via the spaces between the threads in the woven fabric, carrying more than 90% of the particles with it.
"In other words, these relatively big gaps are the reason cloth is a poor material for making air filters," Sear explained. "In contrast, the filtering layer of an N95 mask is made from much smaller, 5-micrometre fibres with gaps that are 10 times smaller, making it much better for filtering nasty particles from the air, such as those containing virus," the team of researchers said.
Good masks should feature the "two Fs: good filtration and good fit."
"Surgical masks fit badly, so a lot of air goes unfiltered past the edges of the mask by the cheeks and nose," they concluded.
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-PTC News
