Woven cloth masks fail to filter 90% of airborne particles, according to research published in Physics of Fluids that is reportedly the first to simulate particles going directly through the gaps in woven fabric. 

Using 3D imagery produced by confocal microscopy to see the air flow channels, the scientists simulated the airflow through these channels and calculate filtration efficiency for particles a micrometer and larger in diameter. The study concludes 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 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,” said Sear. “In contrast, the filtering layer of an N95 mask is made from much smaller, 5-micrometer fibers with gaps that are 10 times smaller, making it much better for filtering nasty particles from the air, such as those containing virus.”