Speaking loudly can leave coronavirus lingering in the air for up to 14 minutes, research has shown.
Scientists found that thousands of tiny droplets sprayed from the mouths of people talking noisily can remain in the air far longer than expected.
The research - published in the Proceedings of the National Academy of Sciences of the United States of America - used laser light to study spatter from someone repeating the words "stay healthy" at volume for 25 seconds.
The phrase was chosen because the "th" sound in the word "healthy" was found to be "an efficient generator of oral fluid speech droplets".
The "enclosed, stagnant air environment" was filmed for 80 minutes and each frame analysed to discover how many scattered particles hung in the air before disappearing.
The authors said: "It has long been recognised that respiratory viruses can be transmitted via droplets generated by coughing or sneezing. It is less widely known that normal speaking produces thousands of oral fluid droplets with a broad size distribution.
"Droplets can harbour a variety of respiratory pathogens, including measles and influenza virus, as well as Mycobacterium tuberculosis. High viral loads of [SARS-CoV-2] have been detected in oral fluids of Covid-19 positive patients, including asymptomatic ones. However, the possible role of small speech droplet nuclei with diameters, which potentially could remain airborne for extended periods of time, has not been widely appreciated."
The scientists concluded that "loud speech can emit thousands of oral fluid droplets per second". They added: "These observations confirm that there is a substantial probability that normal speaking causes airborne virus transmission in confined environments."
The US study was conducted by the National Institute of Diabetes and Digestive and Kidney Diseases, Maryland.