Researchers have been testing various materials to find which are most effective at trapping the fine particles which can carry SARS-CoV-2.
Materials from T-shirts, socks, jeans, and vacuum bags have been tested to determine their effectiveness at filtering particles between 0.02 and 0.1 micrometres – around the size of most viruses – at high speeds, comparable to coughing or heavy breathing.
They compared the performance of these materials with N95 respirators and surgical masks.
Previous studies have only examined a small selection of materials when the wearer is breathing normally and particles are expelled at lower speed. Studying a wider range of fabrics and testing them at higher speeds provides a more robust evidence base for the effectiveness of fabric masks.
The results 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 exceeded the N95 performance in some respects.
As for homemade fabric masks, those made of multiple layers of fabric were more effective. Those which also incorporated interfacing – which is used to stiffen and strengthen fabric, such as in collars – showed a significant improvement in performance although were 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.
“Fabric masks have become a new necessity for many of us since the start of the Covid-19 pandemic,” said first author Eugenia O’Kelly from Cambridge University. “In the early stages of the pandemic, when N95 masks were in extremely short supply, many sewers and makers started making their own fabric masks, meeting the demands that couldn’t be met by supply chains, or to provide a more affordable option.”
While there are numerous online resources which help people make their own masks, there is little scientific evidence on what the most suitable materials are.
“There was an initial panic around PPE and other types of face masks, and how effective they were,” said O’Kelly. “As an engineer, I wanted to learn more about them, how well different materials worked under different conditions, and what made for the most effective fit.”
The researchers built an apparatus consisting of sections of tubing, with a fabric sample in the middle. Aerosolised particles were generated at one end of the apparatus, and their levels were measured before and after they passed through the fabric sample at a speed similar to coughing.
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,” O’Kelly said. “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.”
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.