In a new review study, researchers found that cloth masks, particularly those with several layers of cotton cloth, block droplet, and aerosol contamination of the environment, which may reduce transmission of COVID-19.
They examined a century of evidence including recent data and found strong evidence showing that cloth and cloth masks can reduce contamination of air and surfaces.
This review suggests that cloth can block particles, even aerosol-sized particles, and this supports Canadian public health policy on the issue.
The research was conducted by a team at McMaster University.
Direct evidence about whether wearing a mask of any sort outside a health-care setting reduces actual transmission of COVID-19 is lacking.
This is why public-health decisions about public mask-wearing have been difficult to make, and why they differ around the world.
Previous research has shown that a mask made of three layers (muslin-flannel-muslin) reduced surface contamination by 99%, total airborne microorganisms by 99%, and bacteria recovered from the smaller particles, aerosols, by 88% to 99%.
A commercial mask made of four-layer cotton muslin was shown to reduce all particles by 99%, compared with 96% to 99% for contemporary disposable medical masks.
Even for aerosols, the cloth mask was comparable with the medical masks of the day.
The filtration of cloth is quite variable and single layers of a scarf, sweatshirt and t-shirt may be in the 10 to 40 percent range.
But multiple layers increase efficiency, and modern studies have confirmed that some combinations of cloth, for example, cotton-flannel, block more than 90 percent of particles.
The team says in terms of making masks, it is important to realize 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.
The team 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.
The lead author of the study is Catherine Clase, associate professor of medicine at McMaster University.
The study is published in the Annals of Internal Medicine.
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