New Videos Demonstrate Why Valve Masks Don’t Slow COVID-19 Spread

Matthew Stamats is testing different masks. Source: NIST

Many people wear masks in public to slow the spread of COVID-19, following the recommendations of the Centers for Disease Control and Prevention. However, masks with exhalation valves do not slow the spread of infection. New videos from the US National Institute of Standards and Technology (NIST) demonstrate why.

The video shows airflow through masks with and without exhalation valves. Materials were created by NIST Research Engineer Matthew Stamats. Videos posted along with a companion research paper in Physics of Fluids

“When you compare videos side-by-side, the difference is striking,” said Staymats. “These videos show how the valves allow air to escape from the mask without filtering it, which goes against the very essence of the mask.”

The exhalation valves make breathing through the mask easier and more comfortable. They are appropriate when the mask is intended to protect the user himself. For example, masks with valves protect workers from dust on a construction site or hospital staff from infected patients.

The masks that are recommended to slow the spread of COVID-19 are primarily designed to protect people around the wearer. They trap exhaled droplets that may contain the virus, and thus slow the spread of the infection. According to experts, even people without symptoms should wear masks because they can carry the disease asymptomatically.

This video, created using a schlieren imaging system, shows airflow dynamics for the N95 mask with exhalation valve (left) and mask without exhalation valve. The air passes through the valve unfiltered. Valve masks do not slow the spread of COVID and should not be worn for this purpose. Source: Matthew Stamats / NIST.

“I don’t wear a mask because I want to protect myself. I wear it to protect the person next to me, because I can be an asymptomatic patient and spread the virus without even knowing it, Staymats said. “But if I wear a mask with a valve, then I am not helping the situation.”

Staymats is an expert in flow imaging techniques that allow him to capture air movement on camera. He usually works on new technologies to detect explosives and drugs at airports and terminals, literally sniffing out traces of these materials in the air. Recently, Staymats has taken up masks to develop new ways to measure and improve their effectiveness.

Staymats recorded two videos and used different flow rendering techniques. The first video was created using the so-called schlieren imaging system. With it, the difference in air density is displayed in the camera as patterns of shadow and light.

The Schlieren method makes the exhaled air visible because it is warmer and therefore less dense than the surrounding air. This video only shows the movement of the air itself, without the movement of the exhaled droplets in the air. In the video on the left, Staymats is wearing an N95 respirator with a valve through which exhaled air is released into the environment without filtration. There is no valve on the right, and the air passes through the mask and most of the droplets are filtered out.

This video was created using the light scattering technique. The video shows the dynamics of airflow when wearing an N95 mask with an exhalation valve (left) and without an exhalation valve (center). Valve masks do not slow the spread of COVID and should not be worn to do so. Source: Matthew Stamats / NIST.

Staymats created a second video using light scattering.

For the second video, the researcher built a device that emits air at the speed and pace of a sleeping adult, and then connected the device to a dummy. Instead of droplets being exhaled, the air carries water droplets of various sizes, typical of droplets in human respiration when exhaling, talking and coughing. The high-intensity LED light behind the mannequin illuminates the air droplets, causing them to diffuse the light and appear as you shoot.

As opposed to shooting with the schlieren method, this video shows the movement of droplets in the air. On the left, droplets come out and are not filtered through the valve of the N95 mask. There is no valve in the middle and no breathing is visible because the mask has trapped the drops. The mask is not on the right.

Using a mannequin and mechanical breathing apparatus, Stamets observed airflow patterns while keeping respiration rate, air pressure, and other variables constant.

In addition, videos captured by light scattering can be analyzed on a computer in a way that is not possible with schlieren images. Staymats wrote code that counted the bright pixels in the video and used it to estimate the number of droplets in the air. This is not an accurate measurement of the number of droplets: 2D video cannot capture what is happening in a full 3D volume of air. However, the numbers obtained show trends that can be analyzed to better understand the dynamics of airflow through different types of masks.

This research project examined only one type of valve mask; other types of valve masks will work differently. Masks that do not fit snugly against the face allow air to pass around the mask rather than through it. It can also degrade the performance of the mask.

The main effect of the valves is still visible on these rollers. Staymats 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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