As states ease coronavirus-related restrictions, many Americans are wondering whether it's safe to head back to the gym. Two new CDC reports are raising doubts, however—with one study finding the novel coronavirus infected 68% of gym-goers at one location.

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An alarming case study shows rapid coronavirus spread in a Chicago gym

For one study, published Thursday as a CDC Morbidity and Mortality Weekly Report, researchers collaborated with the Chicago Department of Public Health (CDPH) to review a coronavirus outbreak that occurred at an exercise facility in the city from late August to early September 2020.

CDPH during routine coronavirus case investigations on Sept. 8 identified a cluster of coronavirus cases linked to the exercise facility. CDPH contacted the facility on Sept. 10 and learned that a patron had notified the facility on Sept. 1 that they'd tested positive for the virus. The facility then notified all other patrons and employees who may have been exposed, and during those notifications, the facility learned that 41 other patrons had tested positive or had developed symptoms of Covid-19. The facility then gave CDPH contact and attendance information for all people who had attended classes there from Aug. 24 through Sept. 1.

The facility had offered four to eight high-intensity, indoor exercise classes each day. The facility had constrained attendance at the classes to a maximum of 25% capacity and required all patrons to receive temperature checks and symptom screenings before entering. Although the facility required patrons to wear masks upon arrival, masks were not required during exercise. All patrons brought their own mats and weights and were spaced at least six feet apart.

When CDHP conducted its investigation, researchers found that at least 49 of the 81 individuals who had attended classes tested positive for the coronavirus. Six additional attendees developed probable infections, the researchers said, as they had experienced symptoms of Covid-19 but were not tested for the virus. In all, 68% of attendees developed confirmed or probable infections.

CDHP found that 22 of the patrons who developed Covid-19 had attended a class on the day they developed their first Covid-19 symptom or the day after—including three who attended a class on the day or the day after they received a positive coronavirus test result. In all, the researchers found that 43 patrons with Covid-19 attended a class at the exercise facility "during their estimated infectious periods."

According to the researchers, patrons of the facility who later developed Covid-19 reported using masks during the exercise classes less frequently than patrons who did not develop Covid-19.

A second study details a similar incident in Hawaii

For the second study, also published Wednesday as a CDC Morbidity and Mortality Weekly Report, investigators at the Hawaii Department of Health (HDH) looked into a cluster of coronavirus cases after a fitness instructor on July 2, 2020, notified the department that they had tested positive for the virus.

During the two days before the instructor first developed symptoms of Covid-19, the instructor had taught classes at two exercise facilities—including one that occurred just four hours before symptoms emerged.

During that class, which lasted an hour, 10 participants joined the instructor in a stationary cycling room that had all doors and windows closed, the researchers said. The room also featured three large fans that were pointed at the class participants, according to the researchers. The instructor was facing the participants from a distance of more than six feet, but he was "shouting instructions and encouragement" at the participants throughout the class, the researchers noted. Neither the instructor nor participants wore masks during the class.

Ultimately, all of the participants tested positive for the coronavirus between July 2 to July 6, and all of them developed symptoms of Covid-19, the researchers found.

One of those participants also was a fitness instructor, and during the days leading up to his first symptoms, he led five personal training and small-group kickboxing sessions at a third facility. Mask use was mixed during the classes, and 11 of the participants tested positive for the coronavirus in the following days.

Overall, HDH traced a total of 21 coronavirus cases to the original fitness instructor.

What do the findings mean?

Researchers in the reports said that heavy breathing during workouts in confined, indoor spaces increases the chance of coronavirus transmission, as do fitness instructors shouting instructions or encouragement.

In the instance in Chicago, the researchers said the outbreak emphasizes the importance of strictly following safety precautions when working out in a gym. "To reduce [coronavirus] transmission in exercise facilities, employees and patrons should wear a mask, even during high-intensity activities while less than six feet apart," the researchers said.

Alex Larcom, senior manager of health promotion and health policy for the International Health Racquet and Sportsclub Association, said it appears that the outbreaks in both reports stemmed from inconsistent mask use and other failures to adhere to coronavirus-related precautions.

For instance, Larcom said it's possible the gyms weren't ventilated properly. In addition, she noted, "In Chicago, you had members who went to class when they were symptomatic or Covid-positive."

"Society-wide, we are relying on people who are sick or think they are sick to remove themselves from society," she said (Vaziri, San Francisco Chronicle, 2/25; Rabin, New York Times, 2/24; Lendacki et al., CDC Morbidity and Mortality Weekly Report; 2/24; Groves et al., CDC Morbidity and Mortality Weekly Report, 2/24).