Can I speak to someone now?

Corona: how contagious is speaking?

Invisible transmission: Even talking to each other can be enough to transmit the SARS-CoV-2 coronavirus. An experiment has now revealed how many potentially virus-carrying micro-droplets are formed when speaking and how long they remain in suspension. According to this, we generate around 2,600 droplets per second of speaking and these float around in the air for an average of eight minutes. Around 1,000 droplets per minute could contain viruses.

One factor that makes the corona pandemic so difficult to contain is the easy transferability of the virus: A person infected with SARS-CoV-2 does not only become contagious when the symptoms start, but rather a few days before that. And asymptomatic cases can also transmit the pathogen - without coughing or sneezing. For this reason, keeping your distance is considered one of the most important measures against the spread of the virus.

A cloud of slowly sinking droplets

But how big is the risk of getting infected while talking? It is known that when a person speaks, he expels many tiny droplets of liquid. But how long these droplets stay in the air and how much virus they can contain is only partially known. Studies show that these droplets dry out relatively quickly and lose 20 to 34 percent in size as a result. As a result, however, they now sink more slowly to the ground.

"In an environment with stagnant air, the nuclei of these droplets remain as a slowly sinking cloud that emerges from the speaker's mouth like a cloud," explains Valentyn Stadnytskyi from the US National Institutes of Health in Bethesda and his colleagues. "The rate at which this cloud sinks depends on the diameter of the dehydrated speech droplets."

2,600 droplets per second

To find out how many droplets are created when speaking and how long they stay in the air, Stadnytskyi and his team have used laser scattering to help. To do this, they generated a thin, vertical light field from a green laser beam in a special test chamber. If droplets get into the laser light, they scatter it and can thus be observed as bright dots. For the tests, a test person repeated the phrase “Stay Healthy” for 25 seconds by opening the chamber. The temperature in the test room was 23 degrees and 27 percent humidity.

The result: video recordings revealed that around 66,000 small droplets passed the light field in each test run. “That corresponds to around 2,600 such micro-droplets per second of speaking,” report the researchers. From the behavior of the droplets, they conclude that they were twelve to 21 micrometers in size when they were ejected. That corresponds to a fluid volume of 60 to 320 nanoliters per 25 seconds of speaking, according to Stadnytskyi and his team.

Suspended for eight minutes - at least

But how long do these droplets stay in the air? The laser scattering showed that the droplets sank at just 0.06 centimeters per second. You therefore needed around eight minutes to cover a distance of around 30 centimeters. The researchers conclude that the speech droplets dried up very quickly after leaving the mouth and shrunk to around four micrometers in diameter.

This means that even a short speech creates a cloud of droplets that remain in suspension for more than eight minutes - and could therefore be inhaled by people standing nearby. "If we assume an average viral load of seven million virions (individual virus particles) per milliliter, then one minute of loud speaking generates at least 1,000 virion-containing microdroplets that remain in the air for more than eight minutes," the researchers report.

Confirmation for keeping your distance?

According to the researchers, this confirms that normal speech can also transmit SARS-CoV-2 if you get too close. "The study underlines the need for social distancing and illustrates the potential of the virus to spread in narrow spaces such as offices or factories," said Lawrence Young, researcher from the University of Warwick, who was not involved in the study. "It also explains the transmission of SARS-CoV-2 by indicated people without symptoms."

However, both Young and Stadnytskyi and his team emphasize that their results do not reveal how infectious the viruses contained in the speech droplets still are. "We need more research to understand the viability of SARS-CoV-2 in aerosol droplets and how it depends on viral load and droplet size," says Young. (Proceedings of the National Academy of Sciences (PNAS), 2020; doi: 10.1073 / pnas.2006874117)

Source: PNAS, Science Media Center UK

May 15, 2020

- Nadja Podbregar