Watch how air escapes masks with valves. Experts warn they do not slow COVID-19 spread
Masks with valves or vents do not help slow the spread of COVID-19. It’s a message long touted by health officials since the pandemic began, but now there are videos that show why.
One researcher with the National Institute of Standards and Technology, a non-regulatory agency of the United States Department of Commerce, made two side-by-side videos showing how air and respiratory droplets flow with masks with and without exhalation valves.
“When you compare the videos side by side, the difference is striking,” study author Matthew Staymates, NIST research engineer and an expert in flow visualization techniques, said in a news release. “These videos show how the valves allow air to leave the mask without filtering it, which defeats the purpose of the mask.”
In August, the Centers for Disease Control and Prevention said masks with valves do little to prevent an infected person from potentially spreading the virus to others.
Valves make it easier to breathe and are designed to protect workers from contaminated air, such as construction workers with dust. But these masks do not protect others around the wearer, as the videos show, because they cannot capture exhaled droplets that may contain the coronavirus how surgical and N95 masks do.
The first video (provided above) shows the movement of air through a filtered (no valve) and unfiltered (valve) mask. It shows a technique called “schlieren imaging system” that reveals differences in air densities as shadow and light, Staymates said.
Because exhaled breath is warmer than surrounding air, it’s visible on camera.
In a second video, Staymates tested how airborne droplets flow by using a light-scattering technique that uses a high-intensity LED light to illuminate the droplets.
A mannequin head and fog machine were used to mimic real exhaled droplets when a person speaks and coughs. The setup emitted air at the same velocity as a resting adult, Staymates said.
The video shows how a person with no mask emits complete unfiltered droplets, while a person with an N95 mask emits no droplets detectable to the naked eye. While the mannequin with a valved mask emits about the same amount of droplets as the one with no mask, the first shows air flowing down rather than straight across.
The study by Staymates was published Tuesday in the journal Physics of Fluids.
This story was originally published November 12, 2020 at 3:20 PM with the headline "Watch how air escapes masks with valves. Experts warn they do not slow COVID-19 spread."
