COVID-19 was a frustration for gym rats. Before scientists had any information about the virus, it was clear that heavy breathing in an enclosed space, with many people doing the same, was a way for you to contract a disease. The pandemic began early on, so gyms were one of the first to shut down. These suspicions have since been borne out by science: aerosols—tiny droplets that spread through the air when we breathe—have been identified as a major source of COVID-19 transmission, especially when people are breathing faster and more deeply. Many new cases were traced to exercise in spin classes, gyms and games during the pandemic.
Now a new experiment has given us a more exact sense of just how many aerosols a single person can spew during an intense workout—and the results aren’t pretty. In Germany, scientists published their research in PNAS on May 23, people emit about 132 times as many aerosols per minute during high intensity exercise than when they’re at rest, which the researchers warn raises the risk of a person infected with COVID-19 setting off a superspreader event. At rest, people emitted an average of 580 particles each minute, but during maximal exercise—in which researchers gradually increased intensity until the subjects were exhausted—people emitted an average of 76,200 particles a minute.
They acknowledge the limitations of their research. The sample size of 16 was the first. Aside from the fact that none of the subjects contracted COVID-19, researchers also note in their paper that there were ethical concerns over the potential health consequences for the participants.
However, the research yielded some important findings. “[As an exercise physiologist], and we knew before that when you exercise, there’s more air coming out of a person,” says Henning Wackerhage, a co-author and professor of exercise biology at Technische Universität München. “But we didn’t know before, and which, quite frankly, I didn’t expect, is that also when we exercise hard: there are more particles per liter of air.”
Researchers were able to obtain a better understanding of particles by using an unusual experiment. Each of 16 participants in the study breathed fresh air into their mouths while exercising on stationary bikes. Then they exhaled clean air into a bag. This enabled the researchers to eliminate sources of contamination and get more reliable results, says Christian Kähler, a professor at the Institute of Fluid Mechanics and Aerodynamics at Universität der Bundeswehr München who co-authored the study.
High-intensity exercise also produced more aerosols in some participants than in others. For example, those with endurance training had a higher percentage of aerosols than those who didn’t. Dr. Michael Klompas, a hospital epidemiologist and infectious disease physician at Brigham and Women’s Hospital who did not participate in the study, explains that this may be a function of the way individuals’ bodies become more efficient at moving large amounts of air. “They make their muscles do an enormous amount of work, and they need to support that by giving their muscles enormous amounts of oxygen and helping to clear waste products,” he says.
If this gives you pause about your current exercise regimen, keep in mind that not all gyms are alike—and the right policies and set-up can help to keep you safe. Thomas Allison, Director of the Mayo Clinic’s Cardiopulmonary Exercise Testing Laboratories, said that space is important. Large spaces with high ceilings and large people give more air. Klompas says other things you should look out for in a gym are a mandatory vaccination requirement, professional air flow measurements and the installation of air filters. And, perhaps, testing requirements. In Klompas’ opinion, masks are potentially helpful, but aren’t likely to be reliable during workouts—looser masks won’t do much during vigorous exercise, and it’s impractical to expect people to wear N95s while exerting themselves.
Research shows that fitness and other variables can affect the number of aerosols that people release. Wackerhage said they also investigate how body mass, age and lung conditions play into aerosol production.
Klompas says that ultimately, it comes down to risk tolerance and personal decision making about whether you want to go to a gym. However, he says, you shouldn’t pretend that working out indoors, and around other people, doesn’t pose risks. “If you’re not willing to get COVID don’t go,” says Klompas. “At a time like now, when there’s a lot of COVID around, it is a high risk proposition.”
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