Explained: Videos show how masks with valves fail to filter exhaled air

Staymates, whose work involves flow visualisation techniques that capture the movement of air on camera, turned this expertise toward masks.

It is widely understood why health authorities around the world, including in India, recommend against using a mask with an exhalation valve. Now, a study has used videos to show how masks with valves fail to filter exhaled air, thus protecting the wearer but not others.

The study is published in Physics of Fluids. The author, Matthew Staymates, is a research physicist with the US National Institute of Standards and Technology. Staymates, whose work involves flow visualisation techniques that capture the movement of air on camera, turned this expertise toward masks.

“Our intuition was that these valve masks are not appropriate for source control during this pandemic,” Staymates told The Indian Express, by email. “This new work was designed to visually show why this is the case. The videos are powerful and easy to understand for a broad audience, and our hope is that many people get a chance to see this and realise why valves are not a good idea right now.”

The videos

Staymates created two videos with mannequins, using different techniques. One video was created using the “schlieren imaging system”, which makes exhaled breath more visible because it is warmer and less dense than the surrounding air. “The optical components are fairly simple, except for the large mirror that I use,” he said.

The schlieren imaging system shows the movement of the air itself, while the “fog visualisation system”, with which the other video was made, shows the movement of exhaled particles. “A fog droplet generator is used to produce the fog within the ‘throat’ of the mannequin. A bright LED light is positioned behind the head of the mannequin, and this helps to brightly illuminate the droplets as they exit the mouth of the mannequin,” Staymates said. 📣 Express Explained is now on Telegram

The second video shows droplets escape unfiltered through the valve of an N95 mask. When there is no valve, no breath is visible because the mask has filtered out the droplets.

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