New airflow videos show why face masks with exhalation valves don’t slow the spread of COVID

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 don’t slow the spread of the disease, and now new videos from the National Institute of Standards and Technology (NIST) show why.

The videos, which show the airflow patterns through masks with and without exhalation valves, were created by NIST research engineer Matthew Staymates. The videos were published, along with a research article, in the journal Fluid physics.

“When you compare the videos side by side, the difference is striking,” Staymates said. “These videos show how the valves allow air to exit 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 that the CDC recommends for slowing the spread of COVID, however, are primarily intended to protect people other than the wearer. They slow the spread of the disease by capturing exhaled droplets that may contain the virus. Even people without symptoms should wear masks, according to the CDC, because it’s possible to be infected without showing 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 wear a mask with a valve on it, I’m not helping.”

Staymates is an expert in flow visualization techniques that allow him to capture the movement of air on camera. His regular research focuses on new technologies for detecting explosives and narcotics at airports and shipping facilities by detecting traces of these 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 flow visualization techniques. The first video was created using a so-called schlieren imaging system, which shows differences in air density on the camera as patterns of light and shadow.

With a schlieren imaging system, the exhaled breath becomes visible because it is warmer, and therefore less dense, than the ambient air. This video only shows the movement of the air itself, not the movement of the exhaled droplets in the air. On the left, Staymates is wearing an N95 breathing mask with a valve, which allows exhaled air to flow into the environment without being filtered. On 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 scattering technique.

For the second video, Staymates built a device that emits air at the same speed and tempo as a resting adult, and then connected that device to a mannequin. As a replacement for exhaled droplets, air carries water droplets in a range of droplet sizes typical of the droplets 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 appear brightly on the camera.

Unlike the schlieren video, this video shows the movement of droplets in the air. On the left, droplets escape unfiltered through the valve of an N95 mask. In the middle, there is no valve and no breathing is visible because the mask has filtered the droplets. On the right, no mask is worn.

Using a manikin and mechanical breathing apparatus allowed Staymates to observe airflow patterns while maintaining respiratory rate, air pressure, and other variables.

In addition, videos produced by light scattering 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 the number of droplets in the air. This is not a true measure of the number of droplets, as two-dimensional video cannot capture what is happening in the full three-dimensional air volume. However, the resulting figures provide trends that can be analyzed to better understand the dynamics of airflow from 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 don’t fit properly will allow some air to escape around the mask rather than filter it. It can also compromise the performance of the mask.

But the main effect of the valves can be seen in these videos. Staymates hope the videos will help people understand – at a glance – why masks meant to slow the spread of COVID-19 shouldn’t have valves.