Could fresh air help ward off coronavirus infections?

Epidemiologists say the risk of transmission is much lower in open spaces, and warn of a new outbreak in autumn, when people are likely to spend more time indoors

Residents in Madrid, the epicenter of the coronavirus outbreak in Spain.
Residents in Madrid, the epicenter of the coronavirus outbreak in Spain.VICTOR SAINZ (EL PAÍS)
Pablo Linde

Many epidemiologists have warned of the risk of a new coronavirus outbreak in the autumn. This is not because cold weather is the source of infections – bacteria and viruses are – but rather because people are likely to spend more time indoors. Poorly ventilated areas are the perfect breeding ground for the flu, colds – and for Covid-19. Indeed, from what has been learned about the SARS-CoV-2 virus, fresh air is one of its main enemies.

While it is true that a coronavirus outbreak can happen at any moment, it is less likely to occur in the summer, just as it is less likely that a person will catch a cold. There has been a lot of speculation about whether warm weather affects the transmission of the coronavirus, but there are no final conclusions, even though outbreaks in many tropical countries have shown that it is not a decisive factor. According to health experts, fresh air – not warm weather – may play a more important role in slowing infections this summer.

If we maintain our distance in the fresh air, the possibility of contagion is very, very low
Antoni Trilla, epidemiologist

“The old saying of opening windows to let the air in is the best [way to fight] the virus,” says Antoni Trilla, an epidemiologist and a member of the expert committee advising the Spanish government’s coronavirus response. In closed spaces, he explains, it is more likely that suspended particles will concentrate and be inhaled. “If we maintain our distance in the fresh air, the possibility of contagion is very, very low,” he says.

It is a question of odds. As soon as a person goes outside, there is a risk of contagion. That individual may have the bad luck of crossing paths with someone who is infected, breathing in a droplet with the virus and catching the disease. But it is a remote possibility. The risk of transmission increases the longer a person is in contact with an infected case and the closer they are to them, given that the likelihood of infection depends on how much of the virus is inhaled.

People gather in El Retiro park in Madrid on Thursday.
People gather in El Retiro park in Madrid on Thursday.Marta Fernández Jara (Europa Press)

Studies on sites of contagion indicate that most have been closed spaces. Gwen Knight, a researcher from the Center for Mathematical Modelling of Infectious Diseases (CMMID) in the United Kingdom, has put together several scientific studies as well as different types of publications on the subject. The science communicator Luis Jiménez, who analyzed this work, explains that of the 188 outbreaks studied by Knight, only seven (3.7%) originated from a solely outdoor activity. “From the point of view of the number of confirmed infections, 150 are related to outdoor cases, while more than 9,000 are related to cases that took place indoors,” says Jiménez after reviewing the data from CMMID.

Ildefonso Hernández, spokesperson for the Spanish Public Health and Health Administration Association (SESPAS), says that all the studies published on the topic have found that outbreaks have begun in closed spaces: homes, offices, restaurants, stores, places of worship, hospitals, hotels, funerals and conferences. “In open areas there can also be [outbreaks], but the risk is much lower,” says Hernández.

Although the chances are low, they are “not non-existent,” warns María del Mar Tomás, a microbiologist at the Spanish Society of Infectious Diseases and Clinical Microbiology (SEIMC). “We must not let our guard down in the fresh air, either,” she says. “The epidemic is going much better, but we have to be careful with the feeling that it is over.”

Possibility of a new outbreak

A good example of this is what happened in the Catalan province of Lleida, when a birthday party with 20 guests led to a spike in new coronavirus cases. the event was in violation of the coronavirus lockdown rules, since at the time, the province was in Phase 1 of the national deescalation plan, which only allows social gatherings of up to 10 people. If all had gone according to plan, Lleida would have entered Phase 2 on Monday, June 1. Instead, the province has been held back due to the outbreak.

María del Mar Tomás recommends respecting safe distances even in open spaces and believes it is better for family and social gatherings to take place outdoors with a reduced number of people. “The home gives us a sensation of security that can be misleading. If we have a party at home, we may relax, forget the prevention measures, and be surprised [by the virus],” she explains.

Trilla calls on the public to take advantage of the fact that Spain is a country “that has a lot of outdoor life.” Because once autumn arrives, people will once again stay indoors. “The dry cold probably doesn’t help, but the main risk is that we will be in closed spaces, with little ventilation, where the possibility of contagion is much higher,” he explains.

Only 43 of nearly 10,000 infections took place outdoors

In the data collected by Gwen Knight on sites of coronavirus infections, only two are definitely connected to open spaces, according to Luis Jiménez: “One is the market in Wuhan [in China], the possible origin of the virus (with 41 infections), and the other is two jogger friends, in which additional circumstances increased the risk: proximity over a long time period, pulse conversation and / or intense breathing.” In total, 43 out of nearly 10,000 confirmed cases can be linked with a certain degree of certainty to outdoor situations.

In a Chinese study into coronavirus outbreaks, more than 7,000 infections were traced in search of their origin. According to Jiménez, “most were in the home or on public transportation. Only one infection took place outdoors, when two people met on the street and were talking for some time.”

English version by Melissa Kitson.

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