“We know that unless there is a good seal between the mask and the wearer’s face, many aerosols and droplets will leak through the top and sides of the mask, as many people who wear glasses will be well aware of,” said Eugenia O’Kelly from Cambridge’s Department of Engineering, the paper’s first author. “We wanted to quantitatively evaluate the level of fit offered by various types of masks, and most importantly, assess the accuracy of implementing fit checks by comparing fit check results to quantitative fit testing results.”
The participants underwent quantitative fit testing — which uses a counter to measure the concentration of particles inside and outside the mask — while wearing N95 and KN95 masks, surgical masks and fabric masks.
The researchers found that properly fitted N95 masks filtered more than 95% of airborne particles, offering superior protection. Poorly fitted N95 masks, however, were only comparable with surgical or cloth masks.
“It’s not enough to assume that any single N95 model will fit the majority of a population,” O’Kelly said. “The most widely fitting mask we looked at, the 8511 N95, fit only three out of the seven participants in our study.”
The researchers said the width of the flange of the mask —the area of the material that touches the skin — might be a critical feature to fit. Masks that fit the most participants had wider, more flexible flanges.
They also found that small facial differences had a significant impact on quantitative fit.
“Fitting the face perfectly is a difficult technical challenge and, as our research showed, small differences such as a centimetre wider nose or slightly fuller cheeks can make or break the fit of a mask,” O’Kelly said.
The study was published in the journal Plos One.