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Fit of face masks is as important as material they are made of, say University of Cambridge researchers

University of Cambridge researchers studying the effectiveness of face masks found the fit is as important, or more important, than the material of which it is made.

They discovered that even a high-performance mask – such as an N95, KN95 or FFP2 mask – performs no better than a cloth mask if it is not properly fitted. Minor differences in facial features can make significant differences in how well a mask fits.

The fit of face masks is critical, a University of Cambridge study has shown
The fit of face masks is critical, a University of Cambridge study has shown

The results, published in PLoS ONE, also suggest that the fit-check routine used in many healthcare settings has high failure rates.

Eugenia O’Kelly from the Department of Engineering, from the Department of Engineering, said: “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.

“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 researchers say their sample size was small, but they hope their findings will help develop new fit tests that are quick and reliable.

For the study, seven participants evaluated N95 and KN95 masks by performing a fit check using NHS guidelines. They then underwent quantitative fit testing, using a particle counter to measure the concentration of particles inside and outside the mask, while wearing N95 and KN95 masks, surgical masks, and fabric masks.

N95 masks, which are a similar standard to the FFP3 masks available in the UK and Europe, offered higher degrees of protection than the other categories of masks tested, but most of the N95 masks failed to fit the wearers adequately.

When fitted properly, N95 masks filtered more than 95 per cent of airborne particles. But in some cases, poorly-fitted N95 masks 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,” said Eugenia. “The most widely-fitting mask we looked at, the 8511 N95, fit only three out of the seven participants in our study.”

The width of the flange of the mask, which is the area of the material which comes in contact with the skin, could be critical to fit, the study suggests. Masks that fit more participants in the research tended to have wider, more flexible flanges around the border.

“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,” said Eugenia.

While self-performed fit checks save on time and resources - and are sometimes the only viable option - the study, and others of systems in other countries, such such checks are not reliable.

Having so far studied the impact of fit on the wearer so far, the team now intend to evaluate how fit impacts the protection of others in future research.

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