A new study from UC Davis found that the louder people talk, the more airborne particles they emit, making loudness a potential factor in spreading airborne diseases. The study, led by chemical engineering Ph.D. student Sima Asadi in professor William Ristenpart’s group, looked at particle emission during speech as a function of loudness, among other factors.

Every time we open our mouths and exhale, we release tiny respiratory droplets generated in our lungs and throat that can potentially carry viruses. Most people know to cover their mouth when they sneeze or cough, but many don’t know that talking produces a similar effect.

“A key result of our research is that talking is potentially as important as sneezing and coughing for influenza transmission,” said Asadi. “When you are breathing or talking, you cannot see the droplets with the naked eye because they are micron-sized, but they are still large enough to carry viruses.”

Each year, airborne illnesses such as influenza (the flu) infect millions and hospitalize anywhere from 200,000 to 800,000 people, according to the Center for Disease Control. Learning how these diseases spread is the first step toward learning how to combat them. Asadi hopes to contribute to this understanding by studying behavior of these droplets that humans and animals release as they exhale.

She began her research by realizing that not much work had been done on particle emission during speech, and the studies that had been done had inconsistent results. She decided to find out why by testing emission as a function of different speech factors and found a strong correlation with loudness.

“We showed that as you talk louder, no matter what you say, you will release more particles,” she said.

The study also identified a group of people Asadi calls, “superemitters,” who emit way more particles than their peers when they speak, regardless of volume. The team tested for age, gender and BMI as potential explanations, but did not observe a clear correlation.

Even so, this is exciting to Asadi because it potentially explains the phenomenon of superspreaders—people who are capable of infecting an unusually high number of others. Though more work needs to be done to test this hypothesis, she thinks the study lays the groundwork for future research on airborne disease transmission.

She is currently working on follow-up research that looks at how different “phonemes,” the units of sound that make up words and phrases, relate to particle emission. The theory is that speaking certain words, phrases and languages impacts how many particles a person releases, though she notes that more research needs to be done to confirm.

Asadi’s collaborators include mechanical and aerospace engineering distinguished professor Anthony Wexler, civil and environmental engineering professor Chris Cappa and linguistics assistant professor Santiago Barreda at UC Davis, along with associate professor Nicole Bouvier at the Ichan School of Medicine at Mount Sinai in New York.

The study, titled “Aerosol emission and superemission during human speech increase with voice loudness,” was published earlier this year in Scientific Reports, an open access journal published by Nature. Read the article to learn more.