Tiny airborne droplets produced through speech can linger in the air for longer than 10 minutes – and may be potentially significant in the spread of coronavirus, research suggests.
Scientists have found that loud speech can emit thousands of oral fluid droplets per second.
The study, 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 journal.
It is based on an experiment which used laser light to analyse the number of small respiratory droplets emitted through human speech.
In a closed, stagnant air environment, the droplets disappear from the window of view with time constants in the range of eight to 14 minutes, researchers say.
They write: "These observations confirm that there is a substantial probability that normal speaking causes airborne virus transmission in confined environments."
The research did not involve the coronavirus or any other virus, but looked at how people generate respiratory droplets when they speak.
It focused on small droplets that can linger in the air much longer.
According to the researchers, these droplets still could potentially contain enough virus particles to represent an infectious dose.
They say the probability that speech droplets pass on an infection when emitted by a virus carrier must take into account how long the droplet remains airborne.
But the study sets out that considering frequent person-to-person transmission had been reported in community and health care settings, it appears likely that it could be relevant for Covid-19 and other contagious airborne respiratory diseases, such as the flu and measles
The authors write: "Our laser light scattering method not only provides real-time visual evidence for speech droplet emission, but also assesses their airborne lifetime.
"This direct visualisation 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."
Lawrence Young, professor of molecular oncology at the University of Warwick, said: "This study measured the size and spread of oral fluid droplets using a laser. It convincingly shows that normal speech generates airborne droplets that can remain suspended in the air for tens of minutes or longer.
"This suggests that virus from an infected individual could be transmitted this way in confined spaces, however there is no direct analysis of the presence of viruses in the droplets or their ability to pass on infection.
"The work is a physical study using a laser scattering method. One of the main assumptions in this paper is that each virus particle in a droplet is equally capable of causing an infection.
"We don't know that this is the case for Sars-CoV-2.
"The study is novel and supports the view that respiratory and aerosol transmission are significant mechanisms of virus spread.
"It does not really change our understanding of how this virus is transmitted, just confirms and extends previous data.
"It adds weight to the need for social distancing and raises important issues about the potential for the virus to spread in confined spaces such as offices and factories.
"It also highlights the problem of virus transmission from infected individuals who do not have symptoms."