WASHINGTON • Health experts have determined that face coverings are a vital tool in reducing the spread of the coronavirus - but little research has been done into how different kinds of masks compare.
A new study has ranked 14 types of commonly available masks, finding that medical masks offer significantly more protection against droplet spread than cotton alternatives - while bandanas and balaclavas do not do much at all.
The findings have public policy implications, particularly in the United States where the authorities have encouraged the public to use textile masks and leave the medical products to healthcare workers because they are in short supply.
"We need to scale up surgical mask production and distribution," tweeted Dr Tom Frieden, former director of the Centres for Disease Control and Prevention under then President Barack Obama, in response to the study that appeared in Science Advances.
Masks are important because some 30 per cent to 40 per cent of people who are infected may not show symptoms but still unwittingly spread the virus when they cough, sneeze or just talk.
Scientists at Duke University created an inexpensive set-up: People stood in a dark room and spoke the words "Stay healthy, people" five times into the direction of an expanded laser beam, which was recorded with a cellphone camera.
A computer algorithm was then used to calculate the number of droplets. The laser and lens can be bought for US$200 (S$275) and the experimental design is easy to replicate by non-experts.
"This sort of test could easily be conducted by businesses and others that are providing masks to their employees or patrons," said the study's co-author Martin Fischer, a chemist and physicist.
Professionally fitted N95 masks - hospital-grade protection worn by front-line workers in hospitals - reduced droplet transmission to less than 0.1 per cent.
Surgical or polypropylene masks were not far behind, cutting droplet transmission by 90 per cent or more compared with no face coverings.
Handmade cotton face coverings provided good coverage, eliminating 70 per cent to 90 per cent of the spray from normal speech, depending on the number of layers and pleating.
But bandanas reduced the droplets by only about 50 per cent and neck fleeces actually increased the amount of spray, probably by dispersing the largest droplets into many smaller droplets.
N95 masks with valves - designed for industrial settings where the user's exhalation was less important than what they inhaled - performed roughly on a par with cotton masks in terms of the amount of spray transmitted. The health authorities have discouraged the use of valved N95 masks.
Co-author Eric Westman said he had already put the information to use, avoiding the bulk purchase of a type of mask he and a local non-profit group had planned to distribute to the public in Durham, North Carolina, where the university is based. "The notion that 'anything is better than nothing' didn't hold true," he said.
AGENCE FRANCE-PRESSE
