CHICAGO (NEXSTAR) — A new report from the Centers for Disease Control and Prevention suggests that some indoor exercise facilities may have trouble preventing the spread of COVID-19 among exercisers.

The report studied a single Chicago gym in August 2020 and found that out of 81 attendees of indoor high-intensity classes, 55 people fell sick with COVID-19 — or 68 percent.

Twenty-two of those infected attended classes on or after the day symptoms began, and most attendees (76%) wore masks infrequently, including persons with (84%) and without COVID-19 (60%).

The gym appeared to be following most public health protocols. All classes were held at 25% capacity or less, and mask use, temperature checks and symptom screenings were required on entry. Exercisers were stationed six feet apart and brought their own equipment.

Here’s the catch: Patrons were allowed to remove masks during exercise.

“After receiving notification of a COVID-19 case in one of its patrons, the exercise facility closed and informed all attendees of possible COVID-19 exposure,” the CDC said.

The CDC said it attributes the outbreak to “the high proportion of attendees with COVID-19 who participated in class while symptomatic, or asymptomatic and infectious.”

The report underlines the need for “combined COVID-19 prevention strategies,” including mask use in public settings, testing of symptomatic persons, self-isolation after symptom onset or a positive test result and quarantining of persons who have been exposed to COVID-19.

While data on transmission of COVID-19 in exercise facilities is limited, past outbreak reports indicate that increased respiratory exertion may facilitate transmission.