Gym air is more likely to generate Covid infections, study suggests

Many gyms appear to be full of people and groups eager to resume their old activities again or get in shape for the summer, while new variants of the omicron virus are increasing Covid infections. So is it safe to go to the gym?

Put another way, how many microscopic aerosol particles are the cyclists in your spin class exhaling across the room? How many are spitting out the runner on the next treadmill? A small study on breathing and exercise published in May in the Proceedings of the National Academy of Sciences offers some answers.

The study looked at the number of aerosol particles that 16 people exhaled at rest and during workouts. These tiny pieces of airborne matter — which measure just a few hundred micrometers in diameter, or roughly the width of a hair, and are suspended in the mist that leaves our lungs — can transmit the coronavirus if someone is infected, carrying the virus through the air from one pair of lungs to another.

The study found that, at rest, men and women exhale about 500 particles per minute. But when they exercise, that total increases 132-fold, reaching more than 76,000 ppm on average during the most strenuous effort.

These findings help explain why several notable Covid superspreading events since 2020 have taken place in indoor fitness classes. They may also renew some people’s concerns about indoor fitness programs as cases of the disease are on the rise again in many parts of the world and raise questions about how best to reduce the risks of exposure when we exercise.

In general, huddling hard-breathing bodies in closed spaces is a bad way to prevent the transmission of Covid-19 or other respiratory diseases. In 2020, 54 South Koreans developed Covid after Zumba classes with infected instructors, and then passed the disease on to family members and acquaintances.

Later that year, 10 members of a spinning class in Hawaii taught by an infected instructor tested positive, as did 11 others who came into close contact with one of the class members: a personal trainer and kickboxing instructor.

Scientists investigating these and similar outbreaks speculated that inadequate ventilation and high breathing rates among practitioners contributed to the spread of Covid in affected gyms. But scientists could only guess to what extent exercise increases levels of aerosol particles in gym areas. Accurately measuring the addition of floating particles during training is difficult.

So in the study, a group of exercise scientists and fluid dynamics researchers in Germany developed a new way to measure aerosol emission, using a single exercise bike and a cyclist inside an airtight tent. The cyclists wore silicone masks that captured their exhaled breath, sending the air through tubes to a machine that counted each particle as it passed.

The researchers first measured the people’s particle production while they were sitting and then as they walked at an increasingly fast pace until they were too tired to continue. The particles were constantly counted.

The scientists expected the practitioners’ aerosol production to increase as the intensity increased. We all breathe deeper and faster as we exercise more. But the extent of the increase “surprised us,” said Henning Wackerhage, a professor of exercise biology at the Technical University of Munich and the study’s lead author.

The increase in aerosol emissions started moderately, as cyclists warmed up and began pedaling harder. But when they hit a threshold where exercise became noticeably more strenuous — when a brisk walk becomes a jog or a spin class switches to intervals — the rise in emissions became exponential.

Cyclists began to blow approximately ten times more air per minute than at rest, while the number of particles per minute increased more than one hundred times as the cyclists approached exhaustion (with considerable variation from person to person).

In a room full of spinning cyclists, treadmill runners or on a military circuit, “the concentration of aerosol particles would go up a lot,” said Benedikt Mutsch, a graduate student at the Institute of Fluid Mechanics and Aerodynamics at the University of the Armed Forces. Germans in Munich and co-author of the study. The more particles, the greater the possibility of Covid infection if any practitioner is contaminated.

“The study provides mechanistic data to support the assumption that exercising indoors is a higher risk activity when it comes to Covid-19 transmission” than exercising outdoors, said Linsey Marr, professor of civil engineering and at Virginia Tech and an expert on airborne virus transmission.

But these risks can be mitigated. “Good ventilation and air exchange is a great way to reduce the risk of transmission,” said Chris Cappa, a professor of civil and environmental engineering at the University of California at Davis and an expert in airflow dynamics.

Also, stay well away from other practitioners. “Social distancing of six feet or more is always important,” Mutsch said. But it might not be enough during those strenuous indoor fitness classes.

The study didn’t track how far the cyclists’ aerosol particles flowed, but it’s likely they go well beyond 6 feet, he said. So keep at least 8 to 10 feet away from others in strenuous workouts that require large rooms and small class sizes.

The classes themselves should also be spaced out. “If there are back-to-back exercise classes, some of the air from the first class will transfer to the second,” Cappa said. Make sure there are breaks of at least 15 and preferably 30 minutes between sessions to allow the air to renew itself.

And also wear a mask. “Respiratory masks reduce aerosol emissions,” Wackerhage said.

Translated by Luiz Roberto M. Gonçalves