Evidence shows that fabric masks, especially those that have several layers of cotton cloth, droplet blocks and aerosol contamination to the environment, can reduce transmission of COVID-19.
“The point is not that some particles can penetrate the mask, but that some particles are stopped, especially from the outside, from the wearer,” said first author Catherine Clase, professor of medicine at McMaster University and nephrologist from St. Joseph’s Healthcare Hamilton.
“Ideally, we want the mask to work in both directions, protecting the wearer from the environment and reducing environmental contamination – air and surface – by the wearer.”
His international research team examined a century of evidence including the latest data, and found strong evidence showing that cloth and fabric masks can reduce air and surface contamination.
“Direct evidence about whether wearing any mask outside of health care settings reduces actual COVID-19 transmission.
This is why public health decisions about wearing public masks are difficult to make, and why they differ across the globe, “Clase said.” Our review shows that the fabric can block particles, even aerosol-sized particles, and this supports the Canadian public’s health policy on this issue. “
Team opinions are published at Annals of Internal Medicine on the 22nd of May.
Whether cloth masks protect others from wearers was studied in the 1960s and 1970s. Masks made of three layers (muslin-flannel-muslin) reduce surface contamination by 99 percent, total microorganisms in the air by 99 percent, and bacteria recover from smaller particles, aerosols, by 88 percent to 99 percent.
Commercial masks made of four-layer muslin cotton have been shown to reduce all particles by 99 percent, compared with 96 percent to 99 percent for contemporary disposable medical masks. Even for aerosols, the cloth mask was comparable to medical masks at the time, the researchers said.
Fabric filtering varies greatly and a single layer of scarves, shirts and t-shirts may be in the range of 10 to 40 percent. But multiple layers increase efficiency, and modern studies have confirmed that some fabric combinations, such as cotton fabrics, block more than 90 percent of particles.
Our work is only one part of a complex puzzle. When it comes to making a mask, it’s important to realize that more layers will provide more protection, both inside and outside, but will make it harder to breathe. For this reason, it is not recommended for children under two years and people with breathing difficulties to wear masks. “
Catherine Clase, Study of First Author and Associate Professor, Department of medicine, McMaster University
He added that further research was also needed in the best material and fabric mask design, to help many people who sew masks to protect people in their communities.
“Given the severity of this pandemic and the difficulty of controlling it, we suggest that the possible benefits of a simple reduction in transmission are likely to be greater than the possibility of danger,” Clase said.
Clase, C.M., et al. (2020) Fabric Masks Can Prevent COVID-19 Transmission: Evidence-Based and Risk-Based Approaches. ACP Journal. doi.org/10.7326/M20-2567.
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