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Researchers from Cambridge University and Northwestern University tested the effectiveness of different tissues in filtering particles between 0.02 and 0.1 micrometers – about the size of most viruses – at high speeds, comparable to coughing. or heavy breathing. They also tested the N95 and surgical masks, which are more commonly used in healthcare settings.
Previous studies have looked at only a small selection of fabrics when the wearer breathes normally, when particles are expelled at a slower rate. Studying more fabrics and testing them at higher speeds provides a firmer evidence base for the effectiveness of fabric masks.
The results, reported in the journal BMJ Open, show that most tissues commonly used for non-clinical face masks are effective in filtering out ultrafine particles. The N95 masks were very effective, although a reusable HEPA vacuum cleaner bag actually outperformed the N95 in some respects.
As for the homemade masks, those made with multiple layers of fabric were more effective and those that also incorporated the interface, which is normally used to stiffen the collars, showed a significant improvement in performance. However, this performance improvement also made them harder to breathe than an N95 mask.
The researchers also studied the performance of different fabrics when they are wet and after running a normal wash and dry cycle. They found that fabrics worked well when wet and worked sufficiently after a laundry cycle, however previous studies have shown that repeated washing degrades the fabrics, and researchers warn that masks should not be reused indefinitely.
“Cloth masks have become a new necessity for many of us since the start of the COVID-19 pandemic,” said first author Eugenia O’Kelly of the Cambridge Department of Engineering. “In the early stages of the pandemic, when N95 masks were in short supply, many sewers and manufacturers began creating their own cloth masks, meeting demands that could not be met by supply chains or to provide a more affordable option. “
While there are numerous online resources that help people make their own masks, there is little scientific evidence as to which materials are best suited.
“There was a first panic around PPE and other types of face masks, and how effective they were,” O’Kelly said. “As an engineer, I wanted to learn more about them, how the different materials worked well in different conditions and what made the fit more effective.”
For the current study, O’Kelly and his colleagues built an apparatus consisting of sections of tubes, with a tissue sample in the middle. Aerosolized particles were generated at one end of the apparatus and their levels were measured before and after passing through the tissue sample at a rate similar to that of coughing.
The researchers also tested each tissue’s performance in terms of breathing resistance, based on the users’ qualitative feedback. “A mask that blocks particles very well but restricts breathing is not an effective mask,” O’Kelly said. “Denim, for example, has been pretty effective at blocking particles, but it’s hard to breathe, 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 than any combination of fabric with similar filtration levels. “
In preparation for the study, the researchers consulted with online sewing communities to find out what types of fabric they were using to make masks. Due to the severe shortage of N95 masks at the time, many of the sewers reported that they were experimenting with inserting vacuum bags with HEPA filters into the masks.
Researchers found that single-use and reusable vacuum bags were effective in blocking particles, but beware that single-use bags should not be used in face masks, as they fall apart when cut and may contain components that are not safe to inhale. .
“It’s about finding the right balance: we want materials to be effective at filtering out particles, but we also need to know that they don’t put users at risk of inhaling fibers or lint, which can be harmful,” said O ‘Kelly.
The researchers caution that their study has several limitations: namely, they did not examine the role that adaptation plays in filtering out particles. In a related project, O’Kelly investigated how to improve the fit of masks in healthcare settings. Additionally, many viruses are carried on droplets larger than those examined in the current study.
However, O’Kelly says the findings can be useful for sewers and manufacturers when choosing which fabric to use to make masks. “We have shown that in an emergency situation where N95 masks are not available, such as in the early days of this pandemic, cloth masks are surprisingly effective at filtering out particles that may contain viruses, even at high speed.”
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