The team’s opinion was published in the Annals of Internal Medicine on May 22. It can be found at https://www.acpjournals.org/doi/10.7326/M20-2567.

Whether cloth masks protect others from the wearer was studied in the 1960s and 1970s. A mask made of three layers (muslin-flannel-muslin) reduced surface contamination by 99 per cent, total airborne microorganisms by 99 percent, and bacteria recovered from the smaller particles, aerosols, by 88 per cent to 99 per cent. A commercial mask made of four-layer cotton muslin was shown to reduce all particles by 99 per cent, compared with 96 per cent to 99 per cent for contemporary disposable medical masks. Even for aerosols, the cloth mask was comparable with the medical masks of the day, the researchers say.

The filtration of cloth is quite variable and single layers of scarf, sweatshirt and t-shirt may be in the 10 to 40 per cent range. But multiple layers increase efficiency, and modern studies have confirmed that some combinations of cloth, for example, cotton-flannel, block more than 90 per cent of particles.

“Our work is just one part of a complex puzzle,” said Clase. “In terms of making masks, it is important to realise that more layers will give more protection, both inward and outward, but will make it harder to breathe. For this reason, it is not recommended that children under two and people with breathing difficulties wear masks.”

She added that more research is also needed into the best materials and design of cloth masks, to help the many people who are sewing masks to protect people in their community.

“Given the severity of this pandemic and the difficulty of control, we suggest that the possible benefit of a modest reduction in transmission likely outweighs the possibility of harm,” said Clase.