Proper fit of face masks more important than the material, study says

In order to provide the best protection against Covid-19, the fit of a face mask is more important in some cases than the material it is made of, according to a study which suggests new ways for manufacturers to test the effectiveness of masks.

While studies over the course of the Covid-19 pandemic have shown the importance of wearing face masks in slowing the spread of the coronavirus, the researchers from the University of Cambridge in the UK said there is a lack of understanding about the role that good fit plays in ensuring their effectiveness.

The study, published in the journal PLoS ONE, conducted a series of different fit tests and found that when a high-performance facemasks -- such as an N95 mask -- is not properly fitted, it performs no better than a cloth mask.

According to the researchers, minor differences in facial features, such as the amount of fat under the skin, make significant differences in how well a mask fits.

They said the fit-check routine used in many healthcare settings has high failure rates, as minor leaks may be difficult or impossible to detect by the wearer.

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The scientists hope their findings will help develop new fit tests that are quick and reliable in the case of future public health emergencies.

"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 the study's first author Eugenia O'Kelly from the University of Cambridge.

In the study, seven participants first evaluated N95 and KN95 masks by performing a fit check.

The participants then underwent quantitative fit testing -- which uses 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.

According to the scientists, N95 masks offered higher degrees of protection than the other categories of masks tested.

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However, they said most N95 masks failed to fit the participants adequately.

The scientists found that when fitted properly, N95 masks filtered more than 95 per cent of airborne particles, offering superior protection.

But they said 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. The most widely-fitting mask we looked at, the 8511 N95, fit only three out of the seven participants in our study," said O'Kelly.

The researchers said the width of the flange of the mask -- the area of the material which comes in contact with the skin -- may be a critical feature to fit.

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Masks which fit the greatest number of participants tended to have wider, more flexible flanges around the border, they added.

Small facial differences also have a significant impact on quantitative fit, the study noted.

"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 O'Kelly.

The scientists believe their results will be of use for those who are working on new technologies and programmes to assess fit.

They hope the findings would bring attention to the importance of fit in clinical-grade masks, especially if such masks are to be widely used by the public.