People infected with coronavirus could spread the potentially deadly disease simply by talking, new research has suggested.
Speaking loudly can leave droplets lingering in the air for as long as 14 minutes, and if inhaled they could lead to a new infection, according to scientists.
They used a highly sensitive laser light to demonstrate how loud speech can emit thousands of airborne droplets per second.
In a confined space, the droplets hang around in the air for eight to 14 minutes, according to the research, which highlights the importance of wearing masks to prevent the spread of Covid-19.
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The research was carried out by scientists from the University of Pennsylvania and the National Institutes of Health in the US.
It was published in the Proceedings of the National Academy of Sciences of the United States of America.
In a report, the researchers wrote: "Speech droplets generated by asymptomatic carriers of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are increasingly considered to be a likely mode of disease transmission.
"Highly sensitive laser light scattering observations have revealed that loud speech can emit thousands of oral fluid droplets per second.
"In a closed, stagnant air environment, they disappear from the window of view with time constants in the range of 8 to 14 min.
"These observations confirm that there is a substantial probability that normal speaking causes airborne virus transmission in confined environments."
The report added: "We estimate that 1 min of loud speaking generates at least 1,000 virion-containing droplet nuclei that remain airborne for more than 8 min. These therefore could be inhaled by others and... trigger a new SARS-CoV-2 infection."
During the experiment, the researchers used a laser to observe the spread of droplets as a person repeated the phrase "stay healthy" in a confined space for 25 seconds.
The researchers wrote: "This phrase was chosen because the 'th' phonation in the word 'healthy' was found to be an efficient generator of oral fluid speech droplets."
They concluded in their report: "Our laser light scattering method not only provides real-time visual evidence for speech droplet emission, but also assesses their airborne lifetime.
"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."
