Proper fit of face masks is more important than material, study suggests
Fitting the face perfectly is a difficult technical challenge and small differences, such as a centimetre wider nose or slightly fuller cheeks, can make or break the fit of a mask
Eugenia O'Kelly
N95 and KN95 respirators tested. Top row, from left to right: 3M 8511, 3M 8200, Aero Pro AP0028. Bottom row, from left to right: Makrite 9500, Xiantao Zong ZYB-11, Zhong Jian Le KN95.
A team of researchers studying the effectiveness of different types of face masks has found that in order to provide the best protection against COVID-19, the fit of a mask is as important, or more important, than the material it is made of.
The results, published in the journal PLoS ONE, also suggest that 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.
While the sample size was small, the researchers hope their findings will help develop new fit tests that are quick and reliable, in the case of future public health emergencies. The current study only evaluated the impact of fit on the wearer of the mask – the team will evaluate how fit impacts the protection of others in future research.
The researchers, from the University of Cambridge, carried out a series of different fit tests, and found that when a high-performance mask – such as an N95, KN95 or FFP2 mask – is not properly fitted, it performs no better than a cloth mask. Minor differences in facial features, such as the amount of fat under the skin, make significant differences in how well a mask fits.
The COVID-19 pandemic has made
well-fitting face masks a vital piece of protective equipment for healthcare
workers and civilians. While the importance of wearing face masks in slowing
the spread of the virus has been demonstrated, there remains a lack of
understanding about the role that good fit plays in ensuring their
effectiveness.
“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.”
For the study, seven participants
first evaluated N95 and KN95 masks by performing a fit check, according to NHS
guidelines. 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.
The results assessed the protection to the mask wearer, which is important in
clinical settings.
N95 masks – which are a similar
standard to the FFP3 masks available in the UK and the rest of Europe – offered
higher degrees of protection than the other categories of masks tested;
however, most N95 masks failed to fit the participants adequately.
In their study, the researchers
found that when fitted properly, N95 masks filtered more than 95% of airborne
particles, offering superior protection. However, 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 O’Kelly. “The
most widely-fitting mask we looked at, the 8511 N95, fit only three out of the
seven participants in our study.”
One observation the researchers made
during their study was 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. Masks which fit the greatest number of participants tended to have wider,
more flexible flanges around the border.
In addition, small facial
differences were observed to have 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,” said O’Kelly.
Self-performed fit-checks are
attractive because they save on time and resources, and are often the only
method of fit testing available. However, this study, and studies of fit-check
systems in other countries, indicate that such fit-check systems are not
reliable.
The researchers hope that their results will be of use for those who are working on new technologies and programmes to assess fit, so that healthcare and other frontline workers are adequately protected in the case of any future pandemics. Additionally, they hope these results will bring attention to the importance of fit in clinical-grade masks, especially if such masks are to be widely used by the public. This study did not evaluate the impact of fit on protecting others, which is a future area of research.
Reference:
Eugenia O’Kelly et al. ‘Comparing the
fit of N95, KN95, surgical, and cloth face masks and assessing the accuracy of
fit checking.’ PLoS ONE (2021). DOI:
10.1371/journal.pone.0245688
