A study published today in JAMA Internal Medicine shows that wearing two face coverings can almost double the filtration efficiency of SARS-CoV-2-sized particles, preventing them from reaching the wearer’s nose and mouth and causing COVID-19. The reason for the improved filtration is not so much the addition of layers of fabric, but the elimination of gaps or ill-fitting areas of a mask.
“Medical procedure masks are designed to have very good filtration potential depending on their material, but the way they adapt to our faces is not perfect,” said Emily Sickbert-Bennett, PhD, professor. associate professor of infectious diseases in the faculty of medicine of the UNC. and lead author of the study.
To test the adjusted filtration efficiency (FFE) of a range of masks, UNC researchers worked with James Samet, PhD, and colleagues at the EPA Human Studies Facility on the UNC campus. -Chapel Hill. There, they filled a 10-by-10-foot stainless steel exposure chamber with small aerosols of the salt particles, and asked the researchers to wear coveralls to test their effectiveness in preventing the particles from entering. their breathing space.
Each individual mask or layered mask combination was fitted with a metal sample port, which was attached to a tube in the exposure chamber that measured the concentration of particles entering the breathing space under the researcher’s mask. . A second tube measured the ambient concentration of particles in the chamber. By measuring the concentration of particles in the breathing space under the mask relative to that in the chamber, the researchers determined the FFE.
“We also asked the researchers in the chamber to undergo a series of range of motion activities to simulate the typical movements that a person can do throughout the day – bending at the waist, talking and looking up. left, right, top to bottom, “said Phillip Clapp, PhD, an inhalation toxicologist at UNC Medical School who has been testing the FFE mask with Sickbert-Bennett since the start of the pandemic.
Based on their results, the basic adjusted filtration efficiency (FFE) of a mask differs from person to person, due to the unique fit of each person’s face and mask. But generally, a procedural mask without changing the fit is around 40-60% effective at keeping COVID-19-sized particles from entering. A cloth mask is about 40% effective.
Their recent findings on doubling face masks show that when a fabric mask is placed over a surgical mask, the FFE improves by about 20% and improves even more with a properly fitted sleeve-type mask, such as a gaiter. When layered over procedural masks, cloth masks improve the fit by eliminating gaps and keeping the procedural mask closer to the face, consistently covering the nose and mouth. When a procedural mask is worn over a cloth mask, FFE improved by 16%.
“We have found that wearing two loosely fitting masks will not give you the filtration advantage of only one properly fitted procedural mask,” Sickbert-Bennett said. “And with the current data confirming the effectiveness of mask wear in preventing the spread of COVID-19, the best type of double-masking is when you and the person you interact with are each properly wearing a tight-fitting mask. “
This study was partially funded by a grant from the Centers for Disease Control and Prevention (CDC), and expands research conducted by the agency earlier this year, which supported the CDC’s recommendation of double-masking to the public. Sickbert-Bennett and Clapp previously discussed this recommendation and their research on the importance of mask fitting with the media in a recorded conversation (https://news.unchealthcare.org/2021/02/unc-health -media-briefing-double -masking-importance-of-mask-fit /).
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