Researchers from the Universities of Bristol and Surrey have found that well-fitting, three-layered cloth masks can be as effective at reducing the transmission of COVID-19 as surgical masks.
At the height of the COVID-19 pandemic, 139 countries mandated the use of face coverings in public space such as supermarkets and public transports. The World Health Organization also advises the use of face coverings and offers guidance on their effective features. Face coverings suppress the onward transmission of COVID-19 through exhalation and protect the wearer on inhalation.
In a paper published by the Physics of Fluids journal, the researchers detail how they looked at how liquid droplets are captured and filtered out in cloth masks by reviewing and modelling filtration processes, including inertial impaction.
Inertial impaction does not filter as a sieve or colander does – it works by forcing the air in your breath to twist and turn inside the mask so much that the droplets can’t follow the path of the air. Instead, the droplets crash into fibres inside the mask to prevent inhalation.
The team found that, under ideal conditions and dependent on the fit, three-layered cloth masks can perform similarly to surgical masks for filtering droplets – with both reducing exposure by around 50 to 75 per cent. For example, if an infected person and a healthy individual are both wearing masks, scientists believe this could result in up to 94 per cent less exposure.
Dr Richard Sear, co-author of the study and Leader of the Soft Matter Group at the University of Surrey, said:
“While wearing a simple and relatively inexpensive cloth face mask cannot eliminate the risk of contracting COVID-19, measurements and our theoretical model suggests they are highly effective in reducing transmission. We hope that our work inspires mask designs to be optimised in the future and we hope it helps to remind people of the importance of continuing to wear masks while COVID-19 remains present in the community.”
Featured image: Summary of fabrics comprising masks considered here. (a) Knitted fabrics formed by looping yarn through previous layers (layers colored differently for clarity). (b) Woven fabrics formed by intersecting perpendicular yarns (the “warp” and “weft”). (c) Nonwoven fabrics are formed by entangling fibers through other means, resulting in less ordered arrangements. Scanning electron microscope images of example fabrics in figures (a)–(c) share a scale bar of 100 lm. (d) Geometric properties measured for sample fabric layers, with region of interest marked with a dashed circle (discussed in text). Respirators and surgical masks are comprised of multiple layers, with individual layers plotted separately within this panel. (e) Distribution of fiber diameters in cotton fabric samples, which loosely follow a lognormal distribution. Inset: the 60% cotton 40% polyester t-shirt shows a second peak at larger fiber diameter corresponding to the second material, which can also be modeled as a lognormal (pink dashed). © Robinson et al.
Reference: Joshua F. Robinson, Ioatzin Rios de Anda, Fergus J. Moore, Jonathan P. Reid, Richard P. Sear, and C. Patrick Royall , “Efficacy of face coverings in reducing transmission of COVID-19: Calculations based on models of droplet capture” , Physics of Fluids 33, 043112 (2021) https://doi.org/10.1063/5.0047622
Provided by University of Surrey