How long can a person speak

How great is the risk of infection when speaking?

With autumn, our activities also increasingly shift indoors. It is all the more important to know the risk of infection with the coronavirus, for example during a conversation. A new study shows: While most speech sounds hardly generate more potentially virus-containing droplets and aerosols than breathing, this is different with stop consonants such as “p”. They produce fast, conical streams of air which, when accumulated, can spray droplets more than two meters away. The scientists therefore recommend not standing directly in front of each other when speaking, but rather clearly offset to one side.

The corona pandemic is still ongoing, and the number of infections is increasing in many regions and countries. One of the reasons for the difficult containment of SARS-CoV-2 is the fact that the virus is not only transmitted by coughing or sneezing, but also by the droplets and aerosols released when speaking, singing or laughing. Studies have shown that even brief speaking creates a cloud of droplets that can remain in suspension for more than eight minutes. Aerosols - tiny airborne droplets generated when breathing and speaking can stay in the air of offices, living rooms or restaurants for hours if there is not enough ventilation. In order to at least minimize the risk of infection through droplet infection, health authorities recommend a safety distance of 1.50 to two meters. But how safe is this distance in a conversation?

Stream of droplets after explosive consonants

Manouk Abkarian from the University of Montpellier and his colleagues have now investigated this in more detail. They wanted to know how different speech sounds influence the flow of air and droplets emanating from the mouth. For their experiment, they created a scatter field from green laser beams in a special chamber. At one end of the chamber sat a test person who uttered various English sentences with different speech sounds - from “We will beat the coronavirus” to “Peter Piper picked a peck”. It was no coincidence that the second movement contains a particularly large number of “P”. Because consonants like “P” in particular produce an abrupt expulsion of air, which also influences the cloud of droplets when speaking. “For stop consonants like P, B, K, it was recently demonstrated that they eject a particularly large number of droplets,” the researchers explain. What this means in concrete terms for the possible risk of infection when speaking, they have now investigated in the laser chamber with the help of high-speed recordings, among other things.

It was shown that even a single "P" sound generates a small puff of air that initially leaves the mouth at up to 1.4 meters per second. The speed of this puff of air decreases rapidly, but the impulse is sufficient to cover a distance of around one meter. The droplets ejected with the air form a vortex that widens up and down at an angle of around 40 degrees. However, over the course of a lengthy sentence or conversation, the blasts of air add up to form a conical flow of air - a directed flow of a series of consecutive puffs of air. This air flow has a considerably greater range than the individual sounds. "If you speak in a loud voice for 30 seconds, the aerosols have overcome more than two meters in this time," explain Abkarian and his colleagues. Normal social distancing would then no longer help much.

Ventilation and wearing a mask help

Further analyzes showed that the aerosol cloud was diluted to three percent of its original amount on its way over two meters. "But if longer conversations or meetings take place in closed rooms, the aerosols will accumulate in the air over a longer period of time and reach correspondingly higher concentrations," say the researchers. In fact, a recent study found that a “super spreader” in an office with 50 cubic meters of room volume can enrich the entire room air with thousands of virus copies per cubic meter after just one hour of normal breathing. If the infected person coughs, the load increases to more than seven million viruses per cubic meter. "That underscores the need for good ventilation - especially when you're having a long conversation," says Princeton University co-author Howard Stone. At the same time, their result illustrates that a safety distance of two meters is good and sensible, but not a protective wall.

The scientists recommend never standing directly in front of the person you are talking to during conversations, but always 40 to 50 degrees to the side in order to avoid the main stream of droplets. It is also advisable to wear masks indoors, because they inhibit the blasts of air associated with speaking and thus reduce the range of the aerosols. “Masks really inhibit this current tremendously,” says Stone.

Source: Manouk Abkarian (University of Montpellier) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2012156117


September 30, 2020

© - Nadja Podbregar