Drinking glasses again on the terraces is the activity that the French will be able to find from May 19. In Greece, this has already been the case since May 3, but with an original measure: music is prohibited there.

This idea may make you smile at first glance, but it is scientifically justified. Music forces people to speak louder, which increases the emission of postilions and aerosols, the main transmission channels of the coronavirus.
Because even if it is infinitely lower than indoors, the risk of covid-19 contamination outdoors, on the terrace is not zero.

Aerosol transmission

“The essential message remains to insist that they are much less at risk than poorly ventilated indoor spaces.“, acknowledges the epidemiologist Antoine Flahault, director of the Institute for Global Health at the University of Geneva, to AFP.

And for good reason: almost a year and a half after the start of the pandemic, specialists agree that covid is largely transmitted via aerosols. Clouds of droplets “produced by the breathing, speaking, screaming and singing of infected people“and which therefore contain the SARS-CoV-2 coronavirus, recalls Professor Flahault.

Read also: Opening the windows, an essential gesture to fight against the epidemic

18 to 20 times lower risk outdoors

“In a poorly ventilated space, this cloud can hover for several minutes or even hours before dissipating. But on the terrace, it quickly dissipates into the atmosphere“Says Professor Flahault, making the analogy with cigarette smoke. It is therefore estimated that the risk is”18 to 20 times less“outside and inside,” he continues.

“There is, a priori, no risk of massive transmission“outdoors, unlike closed environments, already explained to Allodocteurs.fr Pascal Crépey, teacher-researcher in epidemiology and biostatistics at the School of Advanced Studies in Public Health (EHESP) in Rennes.

Physical distance

But beware: the risk is not zero for all that because people “stay nearby without wearing a mask because they eat or drink together“Pascal Crépey warned again.

Under these conditions, aerosols can remain a danger if you are close to an infected person: up close, you can always inhale them before they are dispersed in the air.

“Dispersion cone”

First rule to follow, therefore: the physical distance between each person. A study in pre-publication, posted on April 30 by French researchers from the Physics Laboratory of the Ecole Normale Supérieure (LPENS) in Paris, point elsewhere “a risk of short-range aerosol transmission” outside.

The risk zone then takes the form of a “aerosol dispersion cone“, whose tip is the mouth of the infected person. According to them, this risk depends on the direction of the wind and decreases as one moves away from the infected person.

Also watch out for postilions

Professor Flahault mentions another “theoretical risk“, that of the”direct route of contamination“, or “ballistic path“. In other words: the postilions that emanate from a person when he speaks, coughs or sneezes”directly on the eyes, nostrils or mouth of the person in front of them, at a short distance, without a mask or glasses“, he develops, judging her however”more anecdotal than frequent“.

It is also for this reason that it is preferable, for Pascal Crépey, to settle down “side by side rather than face to face“, not to share a dish … And even less the bowl of peanuts in which everyone used to draw at aperitif time.

Fans on the tables?

To limit all these risks, the terraces are reopened under certain conditions, for example with a maximum of six people per table. Other solutions, blown by the researchers at LPENS: large fans facing upwards to expel aerosols, or small devices on each table that would suck in and then filter the exhaled air.