Catching coronavirus outdoors is rare but not impossible

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Washington (AFP)

Almost all documented transmission of the coronavirus has taken place indoors, but experts say wearing a mask outdoors is justified because there is always a risk of infection.

The likelihood of catching the virus increases during events where people stand close to each other and talk for long periods of time, such as parties or election rallies.

– Relative risk –

Since the start of the pandemic, studies have described cases of infection in restaurants, homes, factories, offices, conferences, trains and airplanes.

A study published in April identified a single case of transmission outdoors, between two Chinese villagers, out of more than 7,000 studies.

In an analysis of 25,000 cases, which has not yet been independently reviewed, six percent of the cases were related to environments with an external element, such as sporting events or concerts.

These were closed areas where social distancing was not observed, or where people stayed awhile, moving around and talking loudly or singing.

“There have been virtually no cases that we have been able to identify in some kind of everyday outdoor life,” study author Mike Weed, professor and researcher at the University of Canada, told AFP. Canterbury Christ Church.

The data indicates that “outdoors is much safer than indoors, for the same activity and the same distance,” according to a group of scientists and engineers, including professors from US, UK and US universities. German.

– ‘Dilution in the atmosphere’ –

“The risk of transmission is much lower outdoors than indoors because viruses that are released into the air can quickly dilute into the atmosphere,” the group explained, comparing the “aerosols” carriers virus to cigarette smoke.

Since February, multiple studies and health authorities have singled out the airborne route of transmission, through invisible clouds of microscopic droplets (aerosols) that we release by breathing, talking and singing.

This is in addition to the relatively larger droplets that we expel when coughing or sneezing, which can land directly on someone else’s face within a radius of one or two meters (up to six feet).

The smallest droplets float in the air for minutes or hours, depending on how ventilated an area is. In a poorly ventilated room, but also outdoors between two buildings without air circulation, droplets can accumulate and be inhaled by a passerby.

The dose of viral particles needed to cause infection is unknown, but the higher the dose, “the greater the likelihood of infection,” Harvard University geneticist and expert Steve Elledge told AFP. virus.

Time spent near a contagious person will be a key factor: a second on the sidewalk doesn’t seem to be enough to catch Covid-19. It probably takes at least several minutes.

“While it is not impossible, there is no evidence that Covid-19 has been transmitted when people pass each other on the outside,” the group of scientists concluded.

– ‘Precautionary principle’ –

Linsey Marr, a well-known airborne virus transmission expert from Virginia Tech, told AFP that she recommends wearing masks outside if the area is crowded and “you’ll walk past people frequently, let’s say. , more than one per minute as a guide, but not a hard and fast rule. ”

“When we walk past people outside, we can feel a whiff of their exhaled plume of breath,” she says. “Any brief, transient exposure is low risk, but these exposures can build up over time. ”

“My advice follows the precautionary principle and the fact that wearing a mask is not harmful,” Marr added.

On restaurant patios, the group of scientists recommends keeping a safe distance between tables and wearing masks when you are not eating.

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

Ultraviolet rays deactivate the virus, but the speed at which they do so depends on the intensity of the sun (from a few minutes to an hour).

Knowledge is limited because scientists find it difficult to measure viral concentrations outdoors and to conduct experiments as they do in the laboratory.

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

“Having universal agreement on continued mask use is really the safest strategy,” said Kristal Pollitt, professor of epidemiology and environmental engineering at Yale University.

Not to mention that on a sidewalk, a passer-by can sneeze as you pass by, she told AFP.

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