face mask in sunlight

‘Double masking’ has little effect; it’s the fit that matters

Image credit: Dreamstime

A Covid-19 study has found that efforts to avoid contracting the disease by double-masking with improperly fitted masks may not significantly improve efficiency, while producing a false sense of security for the wearer.

US health authorities already recommend people wear N95 and KN95 masks, as they offer the most protection against the disease, while loosely woven cloth masks have been found to offer the least protection.

But researchers at Florida State University (FSU) said there is still not a full understanding of mask characteristics for the most optimal protection, over two years since the pandemic began.

They used a technique called principal component analysis (PCA) along with fluid dynamics simulation models to show the importance of proper fit for all types of masks and how face shape influences the ideal fit.

More layers mean a less porous face covering, leading to more flow forced out of the perimeter gaps (sides, top, and bottom) in masks with a less secure fit. While double layers do increase the filtering efficiency, they only do so with a good mask, the researchers said. The downside is that a tight fit can also lead to breathing difficulties.

The researchers modelled a moderate cough jet from a mouth of an adult male wearing a cloth mask over the nose and mouth with elastic bands wrapped around the ears. They calculated the maximum volume flow rates through the front of mask and peripheral gaps at different material porosity levels.

For a more realistic 3D face shape and size, the researchers used PCA that integrated 100 adult male and 100 adult female heads retrieved from head scan data at Basel University in Switzerland.

Their model showed how the slight asymmetry typical in all facial structures can affect proper mask fitting. For example, a mask can have a tighter fit on the left side of the face than on the right side.

“Facial asymmetry is almost imperceivable to the eye but is made obvious by the cough flow through the mask,” said co-author Tomas Solano, from FSU.

“For this particular case, the only unfiltered leakage observed is through the top. However, for different face shapes, leakage through the bottom and sides of the mask is also possible.”

Creating 'designer masks' customised to each person’s face is not practical at scale. Still, PCA-based simulations can be used to design better masks for different populations by revealing general differences between male and female or child versus elderly facial structures and the associated air flow through masks.

A University of Cambridge study last year found that a closely fitting face mask is just as important as the material it is made from to ensure the best protection against Covid-19.

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