Coronavirus particles escape the human body in microscopic droplets generated from the fluid lining of the respiratory tract.
Droplets are expelled into the air when infected people sneeze or cough, shout or sing, talk or breathe. While most of those actions can be controlled to some extent, you can’t really stop a person from breathing, especially during exercise.
Regular physical activity has numerous beneficial effects on health, helping to treat chronic diseases and improve mental wellbeing, so it’s vital to keep exercising during the pandemic. But at the same time, it’s also important to consider how breathing during exercise might aid the spread of Covid-19.
Aerobic exercise causes you to breathe in and out more often, and more deeply when you get your breath back during recovery periods. If you’re infected with the SARS-CoV-2 coronavirus — and aren’t aware because you’re an ‘asymptomatic carrier’ with no symptoms of disease — then exercising near other people could increase their risk of infection, particularly in an enclosed space. Conversely, you may be more likely to catch the virus from someone else.
The possibility that breathing during exercise produces more virus particles is based on the assumption that exhalations carry more droplets. But is that true?
One study on how physical activity affects the amount of droplets was carried out by air quality researchers Graham Johnson and Lidia Morawska at Queensland University of Technology in Australia. In the pair’s 2009 paper, entitled ‘The mechanism of breath aerosol formation’, they studied 17 participants and found that rapid inhalation raised aerosol concentration 2-3 times more than normal, whereas deep exhalation caused a 4- to 6-fold increase.
A 2010 study led by Ann-Charlotte Almstrand from the Institute of Medicine at the University of Gothenburg, showed that the level of respiratory particles is influenced by how much air is already in your lungs when breathing out. The Swedish team measured particles exhaled by 10 adults following varying degrees of airway closure, from ’empty’ to ‘full’ lung volumes. Compared to when the airways weren’t closed, the concentration of exhaled particles increased 2-18 times. This result indicates that deep breaths would produce more aerosol.
The impact of lung volume on spreading Coronavirus makes sense when you consider children, according to Michael Riediker of the Swiss Centre for Occupational and Environmental Health in Winterthur, and Lidia Morawska. In a 2020 letter, they point out that kids have a smaller lung capacity (they’re still developing) so their lungs form fewer respiratory droplets. The two researchers propose that this might explain why children are relatively poor transmitters of SARS-CoV-2.
The above studies suggest that physical activity could produce more droplets — and potentially more virus particles — because people breathe faster and deeper while active or recovering. The experiments didn’t involve participants who were actually exercising, however, so more research is needed before scientists can say for sure that exercise releases more viruses.
There’s some indirect evidence that large exhalations help spread Covid-19. In one well-known ‘super-spreader’ event, a symptomatic patient infected 87% of the 61 people who attended a choir practice in Skagit County, Washington (two people later died of the disease). As the report concluded, “Transmission was likely facilitated by close proximity (within 6 feet) during practice and augmented by the act of singing.” This implies that exhaling forcefully — as you might do while exercising — raises the likelihood of spreading the virus.
More specific to physical activity, medical researchers at Dankook University Hospital in Cheonan, South Korea, recently reported that 112 people were infected by SARS-CoV-2 after attending fitness dance classes at a dozen sports centres, which implies that intense activity in a busy indoor facility increases the risk of infection. As the researchers conclude, “Vigorous exercise in confined spaces should be minimized during outbreaks.”
While there’s no smoking gun to prove that the rapid and deep breathing which occurs during and after exercise helps spread Coronavirus, the circumstantial evidence points to it being a contributing factor. As a consequence, you should avoid exercising close to other people while inside in order to protect them from infection.