Ordinary speech can emit small respiratory droplets that linger in the air for at least eight minutes and potentially much longer, according to a study published Wednesday that could help explain why infections of the coronavirus so often cluster in nursing homes, households, conferences, cruise ships and other confined spaces with limited air circulation.
The report, from researchers at the National Institute of Diabetes and Digestive and Kidney Diseases and the University of Pennsylvania, was published in the Proceedings of the National Academy of Sciences, a peer-reviewed journal. It is based on an experiment that used laser light to study the number of small respiratory droplets emitted through human speech.
The answer: a lot.
"Highly sensitive laser light scattering observations have revealed that loud speech can emit thousands of oral fluid droplets per second," the report states.
Previous research has shown large outbreaks of coronavirus infections in a call center in South Korea where workers were in proximity and in a crowded restaurant in China, and such events have led some experts to suspect that the highly contagious virus can spread through small aerosol droplets. That remains the subject of research and debate, and for now, the consensus among infectious disease experts is the virus is typically spread through large respiratory droplets.
This new study did not involve the coronavirus or any other virus, but instead looked at how people generate respiratory droplets when they speak. The experiment did not look at large droplets but instead focused on small droplets that can linger in the air much longer. These droplets still could potentially contain enough virus particles to represent an infectious dose, the authors said.
Louder speech produces more droplets, they note. The paper estimates that one minute of "loud speaking" generates "at least 1,000 virion-containing droplet nuclei that remain airborne" for more than eight minutes.
"This direct visualization demonstrates how normal speech generates airborne droplets that can remain suspended for tens of minutes or longer and are eminently capable of transmitting disease in confined spaces," the authors write.
A video showing the laser experiment was circulating early last month through social media even as public health officials were weighing whether to recommend that people wear facial coverings. At the time, the National Institutes of Health cautioned that the research was "very preliminary" and should not be relied upon as a basis for public health measures.
Soon thereafter, however, the Centers for Disease Control and Prevention recommended facial coverings in public places where social distancing could not easily be maintained.
"This study is the most accurate measure of the size, number and frequency of droplets that leave the mouth during a normal conversation and shower any listeners within range," said Benjamin Neuman, a virologist at Texas A&M University-Texarkana who was not involved in the research.
"This study doesn't directly test whether the virus can be transmitted by talking, but it builds a strong circumstantial case that droplets produced in a normal close conversation would be large enough and frequent enough to create a high risk of spreading SARS-CoV-2 or any other respiratory virus between people who are not wearing face masks," Neuman said.