COVID-19 has been frustrating for exercise rats. Even before scientists knew much about this particular virus, it was pretty clear that it was an easy way to get respiratory diseases by breathing heavily in a confined space with lots of other people around, and gyms were among the first companies to closed early in the pandemic. These suspicions have since been confirmed by science: aerosols – small droplets that disperse through the air as we breathe – have been identified as a significant source of COVID-19 infection, especially when people breathe faster and deeper. During the entire pandemic, training at spin classes, fitness clubs and sports games has been identified as the source of dozens of new cases.

Now a new experiment has given us a more accurate sense of how many aerosols a single person can spit out during an intense workout – and the results are not pretty. According to research conducted by researchers in Germany published in PNAS on May 23132 times as many aerosols per minute during high-intensity training than when at rest, researchers warn, increases the risk of a person infected with COVID-19 triggering a superspreader event. At rest, people emitted an average of 580 particles per minute, but during maximal training – where researchers gradually increased the intensity until the subjects were exhausted – people emitted an average of 76,200 particles per minute.

The authors of the study acknowledge that their work has limitations. First of all, the sample size was only 16 people. In addition, none of the subjects were infected with COVID-19; in the paper, the researchers note that there was no way to make it safe, due to ethical concerns about health risks for the participants.

Nevertheless, there were some valuable results to get out of the work. “[As an exercise physiologist], and we knew before that when you train, more air comes out of a person, ”says Henning Wackerhage, co-author and professor of training biology at the Technische Universität München. “But we did not know it before, and what I honestly did not expect is that even when we train hard: there are more particles per liter of air.”

The unusual experimental design allowed the researchers to get a more accurate sense of the released particles. While training on a stationary bike, each of the 16 subjects breathed clean air through a silicone face mask and then exhaled in a plastic bag. This allowed researchers to eliminate sources of pollution and get more reliable results, says Christian Kähler, professor at the Institute of Fluid Mechanics and Aerodynamics at the Universität der Bundeswehr München, who co-authored the study.

Some of the participants also emitted much more aerosols during high-intensity training than others; especially fitters with more experience in endurance training emitted 85% more aerosols than people without such training. 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 get their muscles to do a tremendous amount of work, and they have to support that by giving their muscles huge amounts of oxygen and helping to remove waste products,” he says.

If this gives you a break from your current workout program, keep in mind that not all gyms are the same – and the right policies and setup can help keep you safe. For example, the amount of space per. person crucial; large spaces, especially those with high ceilings, give the air more space, says Thomas Allison, director of Cardiopulmonary Exercise Testing Laboratories at the Mayo Clinic. Other things to look for in a gym, Klompas says, are a vaccination requirement, a facility that has professionally measured airflow and put air filters in place, and ideally a test requirement. In Klompas’ opinion, masks are potentially useful, but they are probably not reliable during training – looser masks will not do much during vigorous training, and it is impractical to expect people to wear N95s while exerting themselves.

The researchers note that factors besides fitness status can also affect how many aerosols people emit. Wackerhage says they are also investigating how factors such as body mass index, age and lung condition play a role.

Ultimately, says Klompas, whether you go to a gym or not, your risk tolerance and weighting of the costs and benefits of going to the gym for you personally depends. But, he says, do not pretend that exercising indoors and around other people does not pose a risk. “If you’re not willing to get COVID, then do not go,” Klompas says. “At a time like this, when there is a lot of COVID around, it’s a high-risk proposal.”

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