Masks block 99.9% of large COVID-linked droplets

“If you wear a mask, you are mitigating the virus transmission by an order of magnitude—10 times less,” one study author said. For larger droplets, which are more likely to carry viral particles, it’s 1000 times less.

In what seems like the millionth study that confirms mask effectiveness in the fight against COVID-19, researchers working in Scotland have shown that masks filter the vast majority of viral-carrying particles — even homemade ones.

“There is no more doubt whatsoever that face masks can dramatically reduce the dispersion of potentially virus-laden droplets,” senior author Ignazio Maria Viola, an expert in applied fluid dynamics at the University of Edinburgh’s School of Engineering, told AFP.

Viola and colleagues carried out a lab experiment with mechanical mannequins and human subjects, focusing especially on particles larger than 170 microns (roughly two times the width of human hair). Larger particles are more likely to carry viruses, but spend less time in suspension, whereas smaller particles (aerosols) can stay suspended in the air for longer periods of time, but their role in spreading the novel coronavirus is still not that well understood.

Laser imaging of respiratory droplets in flight. (a) The experimental set-up. Boxes indicate imaging windows. (b) Examples of images captured at position A (directly in front of the mouth) for speaking (i, ii, iii) and coughing (iv, v, vi), without mask (i, iv), with the surgical mask (ii, v) and with the handmade mask (iii, vi). 

A realistic scenario was modelled by researchers, and two types of masks were trialed, measuring the number of droplets at different distances, up to 2.15 m.

They found that when it comes to larger particles, at least, all masks do an excellent job at filtering out particles, up to the point where they block virtually all large (non-aerosol) particles. Both cotton and surgical face masks were comparably effective at filtering particles.

Droplet deposition rate versus horizontal distance from the manikin’s mouth in (a,c) speaking or (b,d) coughing conditions. Image credits:

“Whether manikin or human, wearing a face covering decreased the number of projected droplets by less than 1000-fold. We estimated that a person standing 2 m from someone coughing without a mask is exposed to over 10 000 times more respiratory droplets than from someone standing 0.5 m away wearing a basic single-layer mask.”

While different studies have produced somewhat different results when it comes to the filtration of masks, there is a clear consensus showing that masks really do work when it comes to filtering and they are no real substitutes for masks.

Vaccination campaigns have started in several countries, but it will still be a long time before we can reach some level of herd immunity. Meanwhile cases continue to surge in many parts of the world and concerns about potentially threatening viral mutations still loom. For the time being, face masks remain one of our main lines of defense against the virus.

The study was published in Royal Society Open Science.

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