Small airborne droplets produced by speech can linger in the air for more than 10 minutes – and could potentially be significant in the spread of coronavirus, research suggests.
Scientists have discovered that loud speech can emit thousands of drops per second in the mouth.
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 that uses laser light to analyze the number of small breath droplets emitted by human speech.
In a closed, stationary air environment, the drops disappear from the viewing window with time constants in the range of eight to 14 minutes, researchers say.
They write, “These observations confirm that there is a high probability that normally causes airborne virus transmission in confined spaces.”
The research did not involve the coronavirus or any other virus, but it looked at how people generate breathing drops when speaking.
It focused on small droplets that can linger in the air for much longer.
According to the researchers, these drops may still contain enough virus particles to represent an infectious dose.
They say that the likelihood that speech drops will transmit an infection when emitted from a virus carrier should take into account how long the drop remains in the air.
But the study shows that, given that frequent person-to-person transmission has been reported in communities and in health care, it seems likely that it could be relevant to Covid-19 and other infectious airborne diseases, such as flu and measles
The authors write, “Our laser light scattering method not only provides real-time visual evidence for speech droplet emissions, but also assesses their longevity in the air.
“This direct visualization shows how normal speech generates airborne droplets that can linger for tens of minutes or longer and are ideally suited to transmit diseases in confined spaces.”
Lawrence Young, professor of molecular oncology at the University of Warwick, said, “This study measured the size and distribution of drops for oral fluid using a laser. It convincingly demonstrates that normal speech generates airborne droplets that can linger in the air for tens of minutes or more.
“This suggests that an infected person’s virus can be transmitted in confined spaces in this way, but there is no direct analysis of the presence of viruses in the drops or their ability to transmit infection.
“The work is a physical study using a laser scattering method. One of the main assumptions in this article is that any virus particle in a drop can cause infection as well.
“We do not know that this is the case for Sars-CoV-2 (Covid-19).
“The study is new and supports the view that the transmission of airways and aerosols are important mechanisms for the spread of viruses.
“It doesn’t really change our understanding of how this virus is transmitted, it only confirms and extends previous data.
“It adds weight to the need for social distance and raises significant concerns 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 persons who have no symptoms.”