https://www.eurekalert.org/pub_releases/2020-10/uoc-sme102820.php
News Release 29-Oct-2020
University of Cambridge
A team of researchers have tested everything from t-shirts and socks to jeans and vacuum bags to determine what type of mask material is most effective at trapping the ultrafine particles which may contain viruses such as SARS-CoV-2, the virus which causes COVID-19.
The researchers, from the University of Cambridge and Northwestern University, tested the effectiveness of different fabrics at filtering particles between 0.02 and 0.1 micrometres - about the size of most viruses - at high speeds, comparable to coughing or heavy breathing. They also tested N95 and surgical masks, which are more commonly used in healthcare settings.
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The results, reported in the journal BMJ Open, show that most of the fabrics commonly used for non-clinical face masks are effective at filtering ultrafine particles. N95 masks were highly effective, although a reusable HEPA vacuum bag actually exceeded the N95 performance in some respects.
As for homemade masks, those made of multiple layers of fabric were more effective, and those which also incorporated interfacing, which is normally used to stiffen collars, showed a significant improvement in performance. However, this improvement in performance also made them more difficult to breathe through than an N95 mask.
The researchers also studied the performance of different fabrics when damp, and after they had gone through a normal washing and drying cycle. They found that the fabrics worked well while damp and worked sufficiently after one laundry cycle, however previous studies have shown that repeated washing degrades the fabrics, and the researchers caution that masks should not be reused indefinitely.
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The researchers also tested how well each fabric performed in terms of breathing resistance, based on qualitative feedback from users. "A mask which blocks particles really well but restricts your breathing isn't an effective mask," said O'Kelly. "Denim, for example, was quite effective at blocking particles, but it's difficult to breathe through, so it's probably not a good idea to make a mask out of an old pair of jeans. N95 masks are much easier to breathe through than any fabric combinations with similar levels of filtration."
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The researchers found that single-use and reusable vacuum bags were effective at blocking particles, but caution that the single-use bags should not be used in face masks, as they fall apart when cut, and may contain component materials which are unsafe to inhale.
"It's a matter of finding the right balance - we want the materials to be effective at filtering particles, but we also need to know they don't put users at risk of inhaling fibres or lint, which can be harmful," said O'Kelly.
The researchers caution that their study has several limitations: namely, that they did not look at the role which fit plays in filtering particles. In a related project, O'Kelly has been studying how the fit of masks in healthcare settings can be improved. In addition, many viruses are carried on droplets which are larger than those looked at in the current study.
However, O'Kelly says the results may be useful for sewers and makers when choosing which fabric to use for making masks. "We've shown that in an emergency situation where N95 masks are not available, such as in the early days of this pandemic, fabric masks are surprisingly effective at filtering particles which may contain viruses, even at high speeds."
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