As gyms start to reopen around the country, a new study suggests that you might want to consider skipping your fitness classes for the near future.

South Korean researchers found a cluster of coronavirus cases associated with high-intensity fitness classes there. In 24 days, 112 people were infected from fitness dance classes at 12 sports facilities in one city. Just over half of the cases were the result of transmission from the instructors to participants. None of the instructors or participants had symptoms during the classes.

Large class size, small spaces, and intense workouts appear to be the key factors. Like many fitness classes here in the U.S., these classes were 50 minutes long, meeting twice a week. All of the people infected exercised in classes with 5 to 22 participants. No cases of COVID-19 were found in classes with less than 5 participants. Interestingly, an instructor taught yoga and pilates in the same facility as some of the more intense workouts. There were no coronavirus cases from these yoga and pilates sessions.

If you are adamant about getting out of your living room and returning to your group fitness classes, here are some steps you might consider taking.

Avoid classes with 5 or more people. Stick to group exercise classes held outside. Only participate in low-intensity exercise classes, like yoga or pilates, and do high-intensity training on your own. And think about wearing a mask.