“Our work is just one part of a complex puzzle.” Catherine Clase, a researcher at McMaster University in Canada, says that about the review she led, and she is not being modest for show. Her team went looking for what a century of evidence says about cloth masks, and the answer is better than many people expected.
Published in the Annals of Internal Medicine, the review found strong evidence that cloth, including cloth masks, can cut contamination of air and surfaces. Masks with several layers of cotton blocked up to 99 per cent of infectious particles, the researchers reported. The finding backs Canadian public health policy on masks.
Old experiments, still useful
Whether cloth masks protect other people from the wearer was studied back in the 1960s and 1970s, the researchers said. One mask from that work had three layers, muslin then flannel then muslin. It brought surface contamination down by 99 per cent and total airborne microorganisms down by 99 per cent. For bacteria recovered from smaller particles, the cut ranged from 88 per cent to 99 per cent.
A commercial four-layer cotton muslin mask reduced all particles by 99 per cent. Contemporary disposable medical masks managed 96 per cent to 99 per cent. Against aerosols, too, the cloth mask matched the medical masks of its day, the researchers said.
Not every fabric works
Cloth varies a great deal. A single layer of scarf, sweatshirt or T-shirt may filter only 10 to 40 per cent. Stacking layers lifts that figure, and modern studies confirmed that some combinations, cotton and flannel for one, block more than 90 per cent of particles.
There is a catch, and Clase named it. “More layers will give more protection, both inward and outward,” she said, “but will make it harder to breathe.”
Protecting the people around you
The goal, as she put it, is a mask that works in both directions: it shields the wearer from the environment and it limits what the wearer puts into the air and onto surfaces. “The point is not that some particles can penetrate the mask, but that some particles are stopped, particularly outwardly, from the wearer,” Clase said.
Plenty of people were sewing masks at home for neighbors and relatives when the review appeared. The researchers noted that more work is needed on the best materials and designs to guide them.
A separate study published the month before in ACS Nano found that cotton paired with natural silk or chiffon can filter out aerosol particles effectively.
Sources
Sources
- Evidence shows cloth masks may help against… – McMaster News
- Homemade cloth masks can reduce COVID-19 spread: Study
