Are masks effective against airborne viruses? A study examining the efficacy of particulate filtration by woven fabric shows that cloth masks are “inferior” in protection against airborne virals.
The study, published in ‘Physics of Fluids’ by AIP Publishing, seeks to understand how a knitwear filters small particles from the air, and concludes that due to the hierarchical structure of woven fabrics, they ” Bad filter”.
During the COVID-19 pandemic, many millions of people have worn masks made of woven fabric to reduce the risk of transmission of COVID-19. Masks are essentially face-worn air filters that filter out as many dangerous particles as possible. Here, the dangerous particles are virus-containing droplets that are exhaled by an infected person.
Surgical masks and respirators are made of non-woven materials, while cloth masks are made of woven materials. The filtration of air by non-woven material is well studied. However, pre-pandemic, little research had been done into filtration by woven materials, which have a different composition to non-woven materials, the study found.
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It said that the structure of woven fabrics is very different from that of surgical masks. “We compared the structures of woven fabrics and surgical masks. Surgical masks are long, thin meshes of fibers, with diameters ranging from a few micrometers to tens of micrometres. However, fabrics are different; they are cotton (or polyester, silk, etc.) yarns.”
Cotton yarn is a few hundred micrometers thick and is made of cotton fibers, each ten micrometers thick. These fibers are spun into yarn, which in turn are woven into the fabric.
“This two-length scale (fiber and yarn) hierarchical structure of fabrics is known to influence fluid flow through them, which has been studied in the context of laundry. Very little effort has been made for particulate filtration,” the study says.
The researchers found that for particles with a diameter of 1.5 microns, their estimated efficiency is in the range of 2.5 percent to 10 percent. The low efficiency is due to most of the airflow circulating through the relatively large (tens of micrometers across) inter-yarn pores in the woven mask.
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