Washington (AFP)

Almost all of the documented contaminations with the new coronavirus have occurred in indoor spaces, but experts believe that wearing a mask outside is justified because the risk is not zero.

And all the more so in crowded places where people talk to each other nearby and for a long time, such as parties or electoral meetings.

- Relative risk -

Since the start of the pandemic, studies have described contagions in restaurants, homes, factories, offices, conferences, coaches, airplanes ... An April study identified a single case of external transmission , between two villagers in China, out of more than 7,000 studied.

In an analysis of 25,000 cases, and not independently assessed, 6% of cases were linked to environments with an outdoor component, such as sports, parties or concerts: crowded places where physical distancing was not respected, where people often stayed for a long time, moved around and spoke loudly or sang.

"We hardly found any cases in circumstances outside of everyday life," says author Mike Weed of Canterbury Christ Church University, who cites open markets in China and Latin America, or a construction site in Singapore. .

The available data indicates that "the exterior is much safer than the interior, for similar activity and distance," writes a group of scientists and engineers specializing in the spread of respiratory viruses and the quality of the air, in frequently asked questions summarizing the state of knowledge (https://tinyurl.com/FAQ-aerosols).

- Dilution in air -

"The risk of transmission is much lower outside than inside because the viruses released into the air can quickly dilute in the atmosphere," continue these experts, who compare it with cigarette smoke.

Since February, multiple studies have accredited the so-called aerial contamination pathway, by invisible clouds of microscopic droplets that we release just by breathing (aerosols) - not just by the relatively larger droplets that we expel when coughing and in sneezing, and which can land directly on a person's face within a radius of one or two meters.

The smallest droplets float in the air for minutes or hours, depending on the ventilation.

In a poorly ventilated room, but also outside between two buildings without drafts, they can accumulate and be inhaled by someone else.

We do not know the dose of viral particles needed to create an infection, but the larger it is, "the more the probability of infection increases," Steve Elledge, virus geneticist at Harvard, told AFP.

The time spent near an infectious person will be a key factor: a second on a sidewalk does not seem to be enough, it probably takes several minutes at least.

"Although it is not impossible, there is no evidence that the Covid-19 has been transmitted when people pass each other outside," concludes the group of researchers, which recommends the mask in crowded places .

- Precautionary principle -

Linsey Marr, a highly cited expert at Virginia Tech University, tells AFP she recommends the mask outside if the area is very busy, and that "you see people frequently, for example more than one person per minute. , more or less, it is not an absolute rule ".

“When you pass people outside, you can inhale some of the air they breathe out. Any single, brief exposure carries a very low risk, but those exposures add up over time. My advice is based on principle. precaution and on the fact that wearing a mask does not hurt ", adds the scientist.

On restaurant terraces, the group advises to keep a good distance between the tables and to hide when you are not eating.

There are too many variables to quantify the exact risk on a sidewalk or in such and such a park, it depends on the wind, the number of people but also the sun.

UV rays deactivate the virus, but more or less quickly depending on the intensity (from a few minutes to an hour).

Knowledge is limited because scientists find it difficult to measure virus concentrations outdoors, and to perform experiments as in laboratories.

In terms of public health, experts ultimately believe that it is more effective to have simple and clear instructions.

"A universal recommendation is the safest strategy," Kristal Pollitt, professor of epidemiology and environmental engineering at Yale University, told AFP.

Not to mention that on a sidewalk, a passer-by can sneeze the instant you pass him, she recalls.

© 2020 AFP