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Exhalation valves are intended to make the masks more comfortable and easier to breathe through. However, they're designed to protect the wearer from outside contaminants, not to protect others if the wearer has COVID-19.
The U.S. Centers for Disease Control and Prevention recommends masks that slow COVID-19 spread by containing exhaled droplets. Even someone without symptoms should wear them because some people infected with the coronavirus do not show symptoms, the CDC explained.
"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," research engineer Matthew Staymates said. "But if I'm wearing a mask with a valve on it, I'm not helping."
Staymates created two videos for the U.S. National Institute of Standards and Technology that show airflow patterns through masks with and without exhalation valves. He is an expert in flow visualization techniques that allow him to capture the movement of air on camera. The videos and a research article were published Nov. 10 in the journal Physics of Fluids.
"When you compare the videos side by side, the difference is striking," Staymates said in an institute news release. "These videos show how the valves allow air to leave the mask without filtering it, which defeats the purpose of the mask."
Staymates used different flow visualization techniques for each of the videos. The first video was created using what is known as a schlieren imaging system, which causes differences in air density to show up on camera as shadow and light. Exhaled breath becomes visible because it is warmer than the surrounding air.
In that video, Staymates wears an N95 respirator mask with a valve, which allows exhaled air to stream out unfiltered. He also wears a mask without a valve, and the mask filters out most of the droplets.
In the second video, Staymates used a light-scattering technique, building an apparatus that emits air at the same velocity and tempo as a resting adult. He connected that device to a mannequin. Water droplets were used as a stand-in for exhaled droplets, and a high-intensity LED light behind the mannequin illuminated them.
This video showed the movement of droplets in air. The droplets escaped unfiltered through the valve of the N95 mask. In the mask without a valve, the mask filtered the droplets, so no breath was visible.
The light-scattering video can be analyzed by a computer, estimating the number of droplets in the air.
The project only studied one type of valved mask, and others may perform differently. Also, masks that aren't tight-fitting can allow some air to escape.
Staymates hopes the videos will help people understand why masks meant to slow the spread of COVID-19 should not have valves.
SOURCE: U.S. National Institute of Standards and Technology, news release, Nov. 10, 2020
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