Investigating Role of Aerosoles in Disease Transmission Could Lead to Better Indoor Air Quality
In the time it takes to type this sentence, I will have taken three breaths. Twelve in a minute, 20,000 a day, 7.5 million a year, most of them indoors. Yet until quite recently, researchers have not paid much attention to the quality of indoor air.
“It used to a be a pretty robust field, dominated by airborne infectious disease transmission, up until the 1960s,” said Richard Corsi, dean of the University of California, Davis, College of Engineering. Ironically, the growing environmental movement, passage of the Clean Air and Clean Water Acts and the formation of the Environmental Protection Agency, all in the 1970s, moved the focus to outdoor pollution.
“The Clean Air Act was focused entirely on outdoor air and trying to reduce the amount of smog and other air pollutants in cities,” Corsi said.
The COVID-19 pandemic marked a significant change, prompting scientists and engineers to unravel the complex factors that go into indoor air quality. UC Davis researchers are studying ways to optimize the flow of outdoor air into buildings for both energy efficiency and reducing exposure to pollutants and are taking a close look at how viruses and other pathogens travel from person to person via indoor air currents. Their work could mean better health at home and increased productivity at work.
Small Droplets, Big Impact
At the beginning of the COVID-19 pandemic, the transmission of the virus was not well-understood, leading to recommendations for social distancing and other restrictions that assumed the virus spread through contaminated surfaces and large droplets from sneezes and coughs
Researchers at the UC Davis College of Engineering had some other ideas. A year before COVID-19 broke out, Sima Asadi, then a graduate student working with Professor William Ristenpart in the Department of Chemical Engineering, published a paper showing that when people talk they emit aerosols, invisible particles so tiny that they float in the air instead of falling to the ground.
Since then, Asadi (now the Clare Booth Luce assistant professor of chemical engineering at the University of Notre Dame), Ristenpart, Cappa and colleagues at the Icahn School of Medicine at Mt. Sinai, have explored how these aerosols are involved in disease transmission. Intriguingly, a subset of people, known as “supe remitters,” consistently emit far more aerosols than others.
It's easy to see how coughs and sneezes can spread droplets. But how does just regular speech expel tiny aerosols?
Harishankar Manikantan, assistant professor in the Department of Chemical Engineering, studies fluid mechanics at the microscale, especially when fluids interact with solid surfaces or other fluids. When fluid flows become unstable, they can break into droplets, he said.
You can see this for yourself with a kitchen faucet.
“When you turn on your faucet, there's a nice laminar stream that looks as if it's not moving at all, but then you try to decrease it more and more, and at some point, that becomes unstable and it starts to break into droplets,” Manikantan said.
The team’s hypothesis is that these aerosols are formed from saliva on the vocal cords. These membranes vibrate hundreds of times a second to create speech. Unlike water, saliva has elastic properties – it can stretch and return. As it stretches, it forms strings and then beads that could break into droplets.
Ristenpart and Manikantan contacted Daniel Cates, an otolaryngologist at the UC Davis School of Medicine, to use some of his equipment to look at the larynx.
“We showed up one day and put cameras up our noses, and it's amazing what you see,” Manikantan said. Using stroboscopic light to slow the apparent movement, they could see sheets and strings of fluid on the vocal cords. But are they forming into tiny droplets?
That initial work led to a National Science Foundation grant, led by Ristenpart with Manikantan and Cates (now at UC San Diego) as co-principal investigators, to test the hypothesis that aerosol droplets are forming on the vocal cords. They will use fiber-optic cameras to image the vocal cords of volunteers while also measuring aerosols being exhaled and collecting saliva to measure their viscoelastic properties.
“People in the medical community have seen these salivary filaments, but they never thought beyond that. But for us, we know that those things have to make droplets,” Manikantan said. “These are very much candidates for what would be aerosols.”
The engineers are also building a model larynx. This consists of two plates, a few millimeters apart and wetted with volunteered saliva, sitting on a subwoofer that can vibrate them by a tiny but consistent amount at a few hundred cycles a second.
The team hopes to get insights into how these aerosols form and why some “super emitters” make so many more of them than other people.