By Deidre McPhillips, CNN

(CNN) -- Wearing masks and other safety precautions are key to stop the spread of Covid-19 during indoor group exercise, according to two new reports published by the US Centers for Disease and Prevention.

The two studies, published Wednesday, linked Covid-19 outbreaks over the summer to exercise facilities in Chicago and Honolulu.

In the Chicago report, 60% of people who attended in-person fitness classes at one facility between August 24 and September 1 tested positive for Covid-19. Another 7% of attendees reported symptoms consistent with the disease.

While some mitigation measures were in place at the facility -- including required temperature checks and symptom screenings upon entry -- removal of masks was permitted during exercise, according to the report.

In Honolulu, 21 cases were linked to a fitness instructor who tested positive for Covid-19 on July 1.

About two days before experiencing symptoms, the fitness instructor led a yoga class for 27 people while wearing a mask. There were no reported cases among these participants.

A few hours before symptom onset, the same instructor led a stationary cycling class for 10 people, none of whom wore a mask. All participants later tested positive for Covid-19, including a second fitness instructor linked to additional cases.

According to the first report, "the increased respiratory exertion that occurs in the enclosed spaces of indoor exercise facilities facilitates transmission" of the virus.

Gyms should decrease class sizes and require physical distancing, and even when spaced 6 feet apart, the CDC recommends the use of a mask to reduce coronavirus transmission.

The agency also says that facilities should improve ventilation and encourage patrons and staff to follow proper quarantine and isolation protocol after potential exposure to Covid-19 or the onset of symptoms.