Coughs or sneezes may not be the only way people transmit infectious pathogens like the novel coronavirus to one another. Talking can also launch thousands of droplets so small they can remain suspended in the air for eight to 14 minutes, according to a new study.
The research, published Wednesday in the Proceedings of the National Academy of Sciences, could help explain how people with mild or no symptoms may infect others in close quarters such as offices, nursing homes, cruise ships and other confined spaces.
The study’s experimental conditions will need to be replicated in more real-world circumstances, and researchers still don’t know how much virus has to be transmitted from one person to another to cause infection.
But its findings strengthen the case for wearing masks and taking other precautions in such environments to reduce the spread of the coronavirus.
Elaborate experiments have revealed how coughing or sneezing can produce a crackling burst of air mixed with saliva or mucus that can force hundreds of millions of influenza and other virus particles into the air if a person is sick. A single cough can propel about 3,000 respiratory droplets, while sneezing can generate as many as 40,000.
To see how many droplets were produced during normal conversation, researchers at the National Institute of Diabetes and Digestive and Kidney Diseases and the University of Pennsylvania, who study the kinetics of biological molecules inside the human body, asked volunteers to repeat the words “stay healthy” several times. While the participants spoke into the open end of a cardboard box, the researchers illuminated its inside with green lasers, and tracked bursts of droplets produced by the speaker.
The laser scans showed that about 2,600 small droplets were produced per second while talking. When researchers projected the amount and size of droplets produced at different volumes based on previous studies, they found that speaking louder could generate larger droplets, as well as greater quantities of them.
The scientists also found that while droplets start shrinking from dehydration as soon as they leave a person’s mouth, they can still float in the air for eight to 14 minutes.
“These observations confirm that there is a substantial probability that normal speaking causes airborne virus transmission in confined environments,” the authors wrote in the study.