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 this week.
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, US, was published in the Proceedings of the National Academy of Sciences.
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.
According to the report, “loud speech can emit thousands of oral fluid droplets
per second”.
Previous research has shown large outbreaks of coronavirus infections in a call centre in South Korea and a crowded restaurant in China. Such events have led some experts to suspect that the highly contagious virus can be 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, but instead looked at how people generate respiratory droplets when they speak. The experiment focused on small droplets that can linger in the air much longer. These 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 1000 virion-containing droplet nuclei that remain airborne” for more than eight minutes.
A video showing the laser experiment was circulating early last month on social media even as health officials were weighing whether to recommend masks. At the time, the National Institutes of Health cautioned that the research was “very preliminary”.
Soon thereafter, however, the Centers for Disease Control and Prevention recommended the use of masks in public places.
Virologist Benjamin Neuman, who was not involved in the research, said: “This study builds a strong circumstantial case that droplets produced in a normal close conversation could create a high risk of spreading Sars-CoV-2 between people who are not wearing masks.”
“Speech creates droplets that breathing alone does not,” said epidemiologist
Andrew Noymer. “Big mouths of the
world, beware. You’re putting the rest of
us at risk.”