Masks with exhalation valves don't help slow the spread of COVID-19, according to new videos captured using the schlieren imaging system and a technique called light-scattering.
Masks aren't so much a tool of self-preservation and individual protection as they are a social project. Their effectiveness relies on community-wide adoption and their benefits are shared.
Studies have shown that any one mask is only partially effective at blocking virus particles, but there is a synergistic effect when multiple people wear masks.
However, new research published Tuesday in the journal Physics of Fluids suggests that synergy is undermined — and the project to slow COVID-19 transmission through mask-wearing jeopardized — when people sport masks with exhalation valves.
The materials used in N95 masks filter particles from the air more effectively than those used in surgical or cloth masks. But that filtering ability comes at a price.
N95 mask materials make breathing out a bit harder, and some N95 masks use a release valve to minimize filtration resistance during exhalation.
"It's basically a little flap that opens up when exhaling, which lets air out without being filtered through the mask material," Matthew Staymates, a mechanical engineer and fluid dynamicist at the National Institute of Standards and Technology, said in a news release.
To illustrate the risk such a mask poses to others, Staymates used a schlieren imaging system and light-scattering to record two different types of videos of the release of droplets from masks with and without exhalation valves.
Staymates used a special apparatus to create a range of water droplets similar to those released by an adult breathing at a normal rate. Staymates hooked the device to a mannequin and covered it with different masks.
The schlieren imaging system allowed Staymates to identify differences in air density and retrace airflow patterns created by the two masks.
When Staymates deployed the high-intensity LED light, the droplets scattered the light, revealing the abundance and distribution of different sized droplets escaping from the different masks.
The two types of video confirmed that dense pockets of droplet-filled air easily escape from N95 masks with exhalation valves, while N95 masks without valves effectively block the majority of exhaled droplets.
"When you compare the videos side by side, the difference is striking," Staymates said. "These videos show how the valves allow air to leave the mask without filtering it, which defeats the purpose of the mask."
The study's takeaway is clear, masks with valves are both selfish and dangerous.
"I don't wear a mask to protect myself. I wear it to protect my neighbor, because I might be asymptomatic and spread the virus without even knowing it," Staymates said. "But if I'm wearing a mask with a valve on it, I'm not helping."
