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Face masks are a crucial first line of defense against the spread of Covid-19, but this does not mean that they are all the same.
Cloth and N95 masks with filter valves may be more comfortable for the wearer, but a new study shows 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 visualization of fluid dynamics to demonstrate once and for all why these masks should be thrown into oblivion.
The findings were published Tuesday in the journal Fluid physics.
“If I’m wearing a mask with a valve, I’m not helping.”
In April, the United States Center for Disease Control and Prevention began recommending people to wear tissue 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, wearing the mask is a controversial issue in the United States, but the number of mask wearers has significantly 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 interacting by limiting the flow of potentially contagious particles through the surgical tissue or layers of cloth. But when it comes to valve masks, Staymates states in a statement that they only satisfy part of that equation.
“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.”
The problem with valves is that they were originally designed to be worn in situations where it was only important to protect the wearer, like when sanding wood, for example, and to limit incoming particles without filtering out air.
This isn’t exactly new: in a mask FAQ on its website, the CDC recommends re-covering N95 ventilator masks with a surgical mask to prevent the spread of the virus. But Staymates explains in his article 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 mannequin.
A mannequin with a mask – To show the difference between a standard N95 mask, an N95 mask with a valve and no mask, Staymates invented two different test scenarios.
For the first time, he took a mannequin head and mounted a clear tube through his head and out of his mouth to approximate the average exhalation airflow of a grown man. On these mannequins, Staymate simulated all three variations of the mask and recorded them while “taking” a few deep breaths.
In the second less complicated scenario, Staymates himself wore both an N95 mask with and without a valve.
In both scenarios, Staymates used fluid dynamic modeling and specialized lighting to better observe the airflow through the various mask scenarios. Videos of both scenarios show large waves of air escaping from the open vent of N95 masks with valves, while masks without it appear to emit a much smaller and more dispersed bloom of air.
“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.”
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 the non-valved N95 resulted in a 95% decrease in pixel density (analogous in this case to droplet density) while the N95 with valves reduced the pixel density by only 40%.
In the paper, Staymates also attributes this limited droplet density reduction to particles simply stuck in the valve structure itself and not necessarily due to the expected filtration.
Takeaway – While these results are surprising, a manikin and a single human participant are far from a hermetic experiment. However, Staymates argues in the paper that the point still holds and suggests that future research could incorporate other varieties of masks and 3D airflow assessment.
In the meantime, he hopes these videos will help audiences visualize and understand the impact of what may seem like just a small difference in design.
“[T]The main goal here is to create compelling images that are easy to understand and accessible to a large audience, “Staymates explains in the paper.” Furthermore, this work can help with public awareness and perception of 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 the respiration of an adult male through an N95 respirator with and without a valve. The Schlieren imaging technique showed the flow of hot air passing through these respirators, but did not provide information on droplet penetration. For this, strategic mist droplet illumination was used with a mannequin head to visualize droplet penetration through both masks. The mannequin exhaled with a realistic reach and speed that matched an adult male. Penetration of mist droplets was also visualized with a custom system that seals each respirator onto the end of a flow tube. The results of these qualitative experiments show that an N95 respirator without an exhalation valve is effective in blocking most of the droplets from penetrating through the mask material. The results also suggest that N95 respirators with exhalation valves are not appropriate as a source control strategy to reduce the proliferation of infectious diseases that spread through respiratory droplets.
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