'Striking' videos show why to avoid one common face mask design

Face masks are a crucial first line of defense against the spread of Covid-19, but that doesn't mean they're all created equal.

Cloth and N95 masks with filtration valves may be more comfortable for the wearer, but a new study demonstrates just how weak their defense is when it comes to preventing airflow and the spread of respiratory particles.

Using a mannequin and himself as a model, NIST research engineer Matthew Staymates uses fluid dynamics visualization to prove once and for all why these masks should be tossed to the wayside.

The findings were published Tuesday in the journal Physics of Fluids.

"If I'm wearing a mask with a valve on it, I'm not helping."

Back in April, the U.S.'s Center for Disease Control and Prevention began recommending that people wear cloth or surgical face masks to slow the spread of Covid-19. This came a month after the World Health Organization declared the virus a pandemic.

Seven months later, mask-wearing is a contentious issue in the U.S., but the number of mask wearers has substantially increased since the CDC's initial recommendation.

The beauty of wearing a face mask is that it's a one-two-punch that protects both the wearer and anyone they interact with by limiting the flow of potentially contagious particles through surgical fabric or cloth layers. But when it comes to masks with valves, Staymates says in a statement that they fulfill only one part of that equation.

"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," Staymates said. "But if I'm wearing a mask with a valve on it, I'm not helping."

The problem with valves is that they were originally designed to be worn in situations where only protecting the wearer was important — like when sanding wood, for example — and limit incoming particles without filtering outgoing air.

This isn't exactly news — in a mask-wearing FAQ on its website, the CDC recommends that N95 masks with ventilators be covered again with a surgical mask to prevent virus spread. But Staymates explains in his paper that he wanted to find a way to translate this crucial information to the public that was clear and palatable.

So, he decided to build a dummy.

A dummy in a mask — To show the difference between a standard N95 mask, an N95 mask with a valve, and no mask at all, Staymates concocted two different trial scenarios.

For the first, he took a mannequin head and rigged clear tubing through its head and out its mouth to approximate the average exhalation airflow of an adult man. On these dummies, Staymate simulated all three mask variations and recorded them as they 'took' a few deep breaths.

In the second, less involved scenario, Staymates himself donned both an N95 mask with and without a valve.

In both scenarios, Staymates used fluid dynamic modeling and specialized lighting to take a better look at the flow of air through the various mask scenarios. Videos of both scenarios show large billows of air spewing down from the open vent of the N95 masks with valves, while the masks without it appear to emit a much smaller and dispersed bloom of air.

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

With computer modeling, Staymates was able to estimate how many droplets were in the air based on the density of bright pixels in the video and found that non-valved N95s resulted in a 95 percent decreased in pixel density (analogous in this case to droplet density) whereas valved N95s only decreased pixel density by 40 percent.

In the paper, Staymates attributes even this limited reduction in droplet density to particles simply being stuck in the structure of the valve itself and not necessarily because of intended filtration.

Takeaways — While these results are striking, a mannequin and a single human participant is far from an airtight experiment. Nevertheless, Staymates argues in the paper that the point still stands and suggests that future research could incorporate other varieties of masks as well as 3D evaluation of the airflow.

In the meantime, he hopes that these videos help the public visualize and understand the impact of what may seem like only a small difference in design.

"[T]he primary objective here is to create compelling visuals that are easy to understand and accessible to a broad audience," explains Staymates in the paper. "Additionally, this work may help with public awareness and perceptions about the usefulness of face coverings and masks."

Abstract: This work demonstrates the qualitative fluid flow characteristics of a standard N95 respirator with and without an exhalation valve. Schlieren imaging was used to compare an adult male breathing through an N95 respirator with and without a valve. The schlieren imaging technique showed the flow of warm air passing through these respirators but did not provide information about droplet penetration. For this, strategic lighting of fog droplets was used with a mannequin head to visualize the penetration of droplets through both masks. The mannequin exhaled with a realistic flow rate and velocity that matched an adult male. The penetration of fog droplets was also visualized with a custom system that seals each respirator onto the end of a flow tube. Results of these qualitative experiments show that an N95 respirator without an exhalation valve is effective at blocking most droplets from penetrating through the mask material. Results also suggest that N95 respirators with exhalation valves are not appropriate as a source control strategy for reducing the proliferation of infectious diseases that spread via respiratory droplets.