On Wednesday, public health officials urged athletes to wear masks and stay three feet apart while exercising as new research describes the rapid spread of coronavirus infections during intense exercise classes at gyms in Honolulu and Chicago.

Researchers at the Centers for Disease Control and Prevention advised fitness centers to take various measures to prevent outbreaks, including enforcing proper mask use and reminding the gym and staff to stay home if they have symptoms of illness or are tested positive for the virus.

Heavy breathing during vigorous physical activity in tight indoor spaces increases the chances of transmission, and fitness instructors giving exercise instructions to members may also have contributed to the spread, according to the CDC study. Exercising outdoors or taking virtual fitness classes could reduce the risk of infection, the authors said.

“It is very important that people who want to go to a gym and exercise are aware of the symptoms of Covid and are aware that if they feel something that looks and feels like a Covid-19 symptom, they will stay home as a precaution, ”said Richard A. Teran, a CDC epidemiologist in Chicago who was one of the authors of the Chicago case study released Wednesday.

At a Chicago gym, Dr. Teran and his colleagues 55 coronavirus infections among 81 people who participated in high-intensity personal fitness classes between August 24 and September 1.

Among them were 22 people who had attended class on the day they first had symptoms of the disease or the day after. Three went to an exercise class the day they tested positive or the day after. In total, 43 members of the gym who tested positive took classes when they were potentially contagious, researchers said.

The outbreak occurred even though the gym classes were limited to 25 percent of their usual size and there were only 10 to 15 people in attendance.

Members were required to wear masks when entering the gym. At this point, temperatures were measured and examined for symptoms. But they were allowed to remove their masks during training; Those who were infected were less likely to wear masks during class, the researchers found.

In Hawaii, public health investigators linked 21 infections in late June to a 37-year-old male fitness trainer in Honolulu who taught at various facilities and developed symptoms of Covid-19 – body aches, chills, headaches and coughs, according to a CDC released on Wednesday -Report.

On June 29th, a few hours before his first symptoms, he was teaching a one-hour stationary bike class with 10 participants in which nobody wore a mask. All of these participants tested positive in early July, including a 46-year-old man who worked as a fitness trainer at another facility. He became acutely ill and was hospitalized in an intensive care unit.

Updated

Apr. 25, 2021, 9:43 p.m. ET

Twelve hours before this instructor started experiencing symptoms, he held several small kickboxing sessions and a personal training session. Of 11 exposed people, 10 were infected at the beginning of July and tested positive. All 10 developed Covid symptoms and one was hospitalized in the intensive care unit.

On July 22nd, the City of Honolulu issued emergency warrants requiring facial covers in fitness centers, including while exercising. Previously, members of the gym could remove their face coverings while exercising.

Fitness centers and gyms have been hard hit since the pandemic broke out in March last year. They were among the first to close last spring, and now they look forward to welcoming customers back.

The International Health Racquet and Sportsclub Association, an industry group, recently reported that 15 percent of the country’s clubs and studios closed last fall and further closings and bankruptcies were expected. The association has encouraged people to return to the gym and adopted the slogan “Exercise is important!”.

Many clubs have implemented new safety protocols and taken steps to improve ventilation, improve air filtration systems, and maximize outdoor air circulation, said Alex Larcom, senior manager for health promotion and health policy for the association.

The outbreaks described in the CDC studies were caused by inconsistent mask use and other behavioral disorders, as well as possibly poorly ventilated buildings, she said.

“In Chicago you had members who went to class when they were symptomatic or Covid-positive,” Ms. Larcom said. This facility wasn’t originally designed as a fitness center, she added, and symptom screening appeared to have been poorly done.

“Across society, we rely on people who are sick or who think they are sick to get out of society,” she said. Those who went to the gym when they had symptoms “probably went to the grocery store, dined, and roamed the community too.”

The CDC researchers emphasized that a diverse approach to safety is required in fitness facilities, including good ventilation, consistent and correct use of the mask, persistent reminders to stay at home for employees and customers when sick, and additional hand washing stations.

“Nothing is 100 percent certain,” said Ms. Larcom. “There is never a zero risk. But clubs are not the main driver behind the spread of Covid. “