Why valved masks will not slow Covid-19 spread

Masks are an important tool in slowing the spread of Covid-19, but make sure you wear the correct one!

As we close in on the end of 2020 it is clear widespread face mask use is recommended in most parts of the world as a vital tool to prevent COVID-19 infection and slow viral spread across a community. While researchers are investigating what materials in a face mask are most protective, other experts are suggesting we need to avoid some specific kinds of masks.

Face masks with exhaust valves have been flagged for months as potentially ineffective in stifling exhalations and NIST engineer Matthew Staymates set out to visualize exactly how these kinds of masks can project droplets into the air.

“I don’t wear a mask to protect myself," says Staymates. "I wear it to protect my neighbor, because I might be asymptomatic and spread the virus without even knowing it. But if I’m wearing a mask with a valve on it, I’m not helping.”

Staymates’ latest experiments, published in the journal Physics of Fluids, use two flow visualization techniques to illustrate exactly how these kinds of masks allow exhaled air to disperse into the surrounding environment. Across two videos Staymates demonstrates why these masks are fundamentally useless at controlling respiratory droplet transmission during a pandemic.

"When you compare the videos side by side, the difference is striking," he notes. "These videos show how the valves allow air to leave the mask without filtering it, which defeats the purpose of the mask."

These kinds of N95 masks have grown in popularity as they are much easier to breathe through compared to a regular face covering. However, they were primarily designed for uses such as protecting construction workers from dust or hospital staff from infected patients.

It is important to note these videos do not demonstrate the movement of viral particles. Instead these kinds of airflow visualizations offer easy-to-understand depictions of how exhalations can escape a seemingly secure mask. Staymates hopes his work will help the general public understand the importance of wearing the right kind of face masks.

“My hope is that this work will help inform a large audience that valves on a mask are not helpful as we fight this global pandemic together,” he concludes.

The new study was published in the journal Physics of Fluids.