Coronavirus through speech:  Microdroplets generated by speech can remain suspended in the air in an enclosed space for more than ten minutes, a study published Wednesday showed, underscoring their likely role in spreading COVID-19.   

Researchers at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) had a person loudly repeat the phrase “Stay healthy” for 25 seconds inside a closed box.

A laser projected into the box illuminated droplets, allowing them to be seen and counted. 

They stayed in the air for an average of 12 minutes, the study published in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS) showed.

Now, scientists from the National Institutes of Health (NIH), US, have come up with troubling new evidence of transmission in confined spaces. They used an intense sheet of highly sensitive laser light to visualize bursts of small-sized droplets produced during repeated spoken phrases. The droplets are too small to be seen but are large enough to carry pathogens that can linger in the air “for tens of minutes or longer” before shrinking and eventually disappearing.

According to scientists, speaking produces about 2,600 droplets per second, which corresponds to an estimated 2.4-12 nanolitres of airborne oral fluid. In the case of COVID-19, the oral fluid has a high viral load, even in asymptomatic cases. “Now assuming that oral fluid contains approximately 7 million SARS-CoV-2 copies per mL (milliliter) in COVID-19 infected persons, then that 1 minute of loud-speaking could generate more than 1,000 virus-containing droplets that will remain airborne for 8 minutes or longer,” the study said.

“These observations confirm that normal speaking in enclosed environments might carry a substantial risk of virus transmission.” The findings were published in the journal Proceedings Of National Academy Of Sciences.

Once airborne, these speech-generated droplets rapidly dehydrate due to evaporation and start decreasing in size which delays their fall. “Our laser light scattering method not only provides real-time visual evidence for speech droplet emission but also assesses their airborne lifetime. This demonstrates how speech generates airborne droplets that can remain suspended and are capable of transmitting disease in confined spaces,” said the team.

If the level of infectiousness of COVID-19 through speech can be confirmed, it could give a scientific boost to recommendations in many countries to wear a face mask, and help explain the virus’s rapid spread.