New York: Scientists now report that wearing two face coverings can nearly double the effectiveness of filtering out SARS-CoV-2-sized particles, preventing them from reaching the wearer’s nose and mouth and causing Covid-19.

The reason for the enhanced filtration isn’t so much adding layers of cloth but eliminating any gaps or poor-fitting areas of a mask, according to the study published in JAMA Internal Medicine.

“The medical procedure masks are designed to have very good filtration potential based on their material, but the way they fit our faces isn’t perfect,” said Emily Sickbert-Bennett, associate professor of infectious diseases at the University of North Carolina (UNC) School of Medicine and lead author of the study.

To test the fitted filtration efficiency (FFE) of a range of masks, the researchers worked with James Samet and colleagues in the USEPA Human Studies Facility on the campus of UNC-Chapel Hill.

There they filled a 10-foot by 10-foot stainless-steel exposure chamber with small salt particle aerosols, and had researchers don combinations of masks to test how effective they were at keeping particles out of their breathing space.

By measuring particle concentration in the breathing space underneath the mask compared to that in the chamber, researchers determined the FFE.

“We also had the researchers in the chamber undergo a series of range-of-motion activities to simulate the typical motions a person may do throughout their day, bending at the waist, talking, and looking left, right, up and down,” said Phillip Clapp, an inhalation toxicologist in the UNC School of Medicine.

According to their findings, the baseline fitted filtration efficiency (FFE) of a mask differs person to person, due to each person’s unique face and mask fit.

Generally, a procedure mask without altering the fit, is about 40-60 per cent effective at keeping Covid-19-sized particles out. A cloth mask is about 40 per cent effective.

When layered over procedure masks, cloth masks improve fit by eliminating gaps and holding the procedure mask closer to the face, consistently covering the nose and mouth.

When a procedure mask is worn over a cloth mask, FFE improved by 16 per cent.

“We’ve found that wearing two loosely fitted masks will not give you the filtration benefit that one, snug-fitting procedure mask will,” Sickbert-Bennett said.