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Many people wear masks in public to slow the spread of COVID-19, as recommended by the Centers for Disease Control and Prevention (CDC). However, masks with exhalation valves do not slow the spread of the disease, and now new videos from the National Institute of Standards and Technology (NIST) show why.
The videos, showing patterns of airflow through masks with and without exhalation valves, were created by NIST research engineer Matthew Staymates. The videos were published, along with an accompanying research article, in the journal Fluid physics.
“When comparing the videos side by side, the difference is astonishing,” Staymates said. “These videos show how valves allow air to escape from the mask without filtering it, which defeats the purpose of the mask.”
Exhalation valves, which make masks easier to breathe and more comfortable, are appropriate when the mask is intended to protect the wearer. For example, valve masks can protect workers from dust on a construction site or hospital workers from infected patients.
The masks the CDC recommends to slow the spread of COVID, however, are primarily meant to protect people other than the wearer. They slow the spread of the disease by capturing exhaled droplets that may contain the virus. People without symptoms should also wear masks, according to the CDC, because it’s possible to get infected but show no symptoms.
“I don’t wear a mask to protect myself. I wear it to protect my neighbor, because I could be asymptomatic and spread the virus without even knowing it,” Staymates said. “But if I’m wearing a mask with a valve, I’m not helping.”
Staymates is an expert in flow visualization techniques that allow him to capture the movement of air on the camera. His habitual research involves new technologies to detect explosives and narcotics in airports and shipping facilities by sniffing the traces of those materials in the air. He recently turned his expertise to masks to help develop new ways to measure and improve their performance.
Staymates created two videos using different stream visualization techniques. The first video was created using what is known as the Schlieren imaging system, which causes differences in air density to appear on the camera as patterns of light and shadow.
With a Schlieren imaging system, exhaled breath becomes visible because it is warmer, and therefore less dense, than the surrounding air. This video only shows the movement of the air itself, not the movement of the droplets exhaled into the air. On the left, Staymates wears an N95 breathing mask with a valve, which allows exhaled air to flow into the unfiltered environment. To the right there is no valve and air passes through the mask, which filters out most of the droplets.
Staymates created the second video using a light diffusion technique.
For the second video, Staymates built an apparatus that emits air at the same speed and pace as a resting adult, then connected that device to a mannequin. In place of exhaled droplets, air carries water droplets in a range of droplet sizes that people emit in their breath when they exhale, speak and cough. A high intensity LED light behind the mannequin illuminates the airborne droplets, causing them to scatter the light and display them brightly on the camera.
Contrary to Schlieren’s video, this video shows the movement of droplets in the air. On the left, the droplets escape unfiltered through the valve of an N95 mask. In the center, there is no valve and no breath is visible because the mask has filtered the droplets. Right, no mask is worn.
The use of a manikin and a mechanical respirator allowed Staymates to observe airflow patterns while keeping respiratory rate, air pressure and other variables constant.
Furthermore, the videos produced by the scattering of light can be analyzed by a computer in a way that schlieren images cannot. Staymates wrote computer code that calculated the number of bright pixels in the video and used it to estimate how many droplets there were in the air. This is not a true measure of the number of droplets because the two-dimensional video is unable to capture what is happening in the full three-dimensional volume of air. However, the resulting numbers provide trends that can be analyzed to better understand the airflow dynamics of different types of masks.
This research project examined only one type of valve mask; different types of valve masks will work differently. Also, masks that are not tight will allow air to escape around the mask rather than filter it through it. This can also affect the performance of the mask.
But the main effect of the tubes is visible in these videos. Staymates hopes the videos will help people understand, at a glance, why masks designed to slow the spread of COVID-19 shouldn’t have valves.
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