A number of studies so far have looked into the efficacy of masks made of various materials in keeping out airborne particles that may be carrying the novel coronavirus. Most of them, however, have only looked at a small selection of fabrics — and when the wearer is breathing normally, when particles are expelled at lower speed.
Now, a new study has now tested a wider range of materials — from T-shirts and socks to jeans and vacuum bags — and looked at their effectiveness in filtering out virus-sized particles (0.02-0.1 micrometres) at high speeds, comparable to coughing or heavy breathing. It also tested N95 and surgical masks.
The study, by researchers from the University of Cambridge and Northwestern University, is published in BMJ Open. 📣 Express Explained is now on Telegram
The broad finding: most of the fabrics commonly used for non-clinical masks are effective at filtering ultrafine particles. N95 masks were highly effective. A reusable HEPA (high-efficiency particulate air) vacuum bag, in fact, exceeded the N95 performance in some respects.
Homemade masks are more effective when made of multiple layers of fabric. When they incorporated interfacing (normally used to stiffen collars), their performance improved significantly, but this improvement also made them more difficult to breathe through.
The researchers also studied the performance of different fabrics when damp, and after they had gone through a washing and drying cycle. The fabrics worked well while damp and worked sufficiently after one laundry cycle. But previous studies have shown that repeated washing degrades the fabrics, and the new study cautions that masks should not be reused indefinitely.
Eugenia O’Kelly, from Cambridge’s Department of Engineering, and her colleagues built an apparatus with a fabric sample in the middle. Aerosolised particles were generated at one end of the apparatus, and made to pass through the fabric sample at a speed similar to coughing. Their levels were measured before and after.
“A mask which blocks particles really well but restricts your breathing isn’t an effective mask. 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,” O’Reilly said in a statement.
The researchers acknowledge several limitations: they did not look at the role which fit plays in filtering particles. In addition, many viruses are carried on droplets larger than those looked at in the current study. However, O’Kelly said the results may be useful for mask makers.
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Source: University of Cambridge
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