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In plea for face masks, researchers visualize how speech sends fluid droplets flying

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Researchers used a cardboard box with two parallel slots and a green laser to illuminate droplets projected during conversation. Image credit: NIDDK

As COVID-19 ravaged Wuhan, China, in January, scientists remained unsure how the virus was spreading so rapidly. Biophysicist Adriaan Bax, chatting over dinner with his linguist wife Ingrid Pufahl, started to suspect that the saliva we project as we speak was a big culprit. Convinced that this insight could be key to stopping the virus, Bax, who’s at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) in Bethesda, MD, made a video that reveals how speech sends respiratory droplets flying.

The findings, reported last week in The New England Journal of Medicine, are nothing new to aerobiologists. But the visualization could compel the public and scientists alike to consider homemade masks. “Scientific studies dating back to 1946 indicate that talking generates as many or more small respiratory droplets than coughing or sneezing,” says Bax. “I felt that newer technology to highlight and visualize this was needed.”

Bax recruited NIDDK colleagues Philip Anfinrud and Valentyn Stadnytskyi, as well as his daughter, Christina Bax, a fourth-year medical student at the University of Pennsylvania in Philadelphia. Anfinrud and Stadnytskyi devised a way for the team to capture speech droplets with the same high-power laser that they use to study how photons trigger proteins to change structure.

The researchers cut two parallel slots into opposite sides of a cardboard box. They directed the green laser through the slots, creating a sheet of light that illuminated anything that passed through. One of the researchers spoke through an open end of the box as an iPhone camera captured the droplets that burst forth like green fireworks. The phrase “Stay healthy,” for example, produced an explosion of droplets measuring from 20 to 500 microns. (A human hair is about 70 microns.)

The speaker then covered his mouth with a washcloth, slightly dampened to cut down on dust that would confuse results. This time when he spoke, the box remained dark.

At the time, the Centers for Disease Control and Prevention (CDC) had not yet issued its recommendation to wear face coverings in public settings. Wanting to make an impact fast, the team shared their findings in a series of YouTube videos, released anonymously because the work had not been peer reviewed yet. “As a millennial, I had to make the videos go viral,” says Christina Bax. She posted on Facebook and Twitter. It didn’t work. “We made the videos to alter people’s behavior and save lives, but it clearly wasn’t getting noticed,” she says. It was her idea to submit the work as a letter to The New England Journal of Medicine.

When researchers said "Stay healthy," a green laser illuminated an explosion of droplets measuring from 20 to 500 microns. Image credit: NIDDK

When researchers said “Stay healthy,” a green laser illuminated an explosion of droplets measuring from 20 to 500 microns. Image credit: NIDDK

“I hope [the video] convinces people that wearing a mask can help stop people from spreading droplets in the air and on surfaces, even if they’re not coughing, if they’re just talking,” says Linsey Marr, a civil and environmental engineer at Virginia Tech who studies bioaerosols. (Marr was not involved in the research.)

“Their videos were very beautiful and dramatic and really illustrated what one of the authors referred to as the oral mode for droplet production,” says William Ristenpart, a chemical engineer at the University of California, Davis, who was also not involved in the study. Ristenpart’s own work shows that respiratory droplets increase with speech volume.

But Ristenpart and Marr both note that there are also smaller respiratory droplets, too miniscule to be seen with the laser. For every droplet the researchers saw, there were probably 100 more, says Marr.

Many of these tiny droplets stem from the mucous lining of the lungs and vocal chords. They exit the mouth as aerosol particles that are small enough to move through many fabrics, light enough to drift for hours, and large enough to potentially carry a virus—though researchers studying fluid dynamics of disease haven’t yet confirmed whether coronavirus can actually spread via aerosol particles.

Marr believes that the videos demonstrate that a homemade mask, though not a complete barrier to these smaller particles, can still do a lot of good. Ristenpart isn’t so sure. “I am not confident that aerosol transmission from asymptomatic individuals will be appreciably altered by wearing a makeshift mask,” he says.

Ristenpart cites a 2010 study in The Annals of Occupational Hygiene that suggests that common household fabrics such as scarves and towels provide only limited protection against nanoparticles large enough to carry viruses in exhaled breath. “I worry that people might think it’s safe to have a long conversation with somebody else provided they’re both wearing homemade masks,” Ristenpart says. “That is not necessarily true.”

The Bax team is now exploring whether droplets smaller than 20 microns can be blocked with homemade masks made from a variety of different materials. Adriaan Bax also notes that larger speech droplets can dehydrate to form aerosol particles. “That’s why it’s so much easier to stop them [with a mask] at time of emission, rather than to block inhalation,” he says.

“This whole idea of social distancing and covering your mouth is so important. But it is such an abstract idea,” says Christina Bax. “The visualization gets people to see why it works.”

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