Why the air at the gym may be more likely to spread COVID

Many gyms and health clubs seem to be filling up again with people eager to return to their old routines and communities or get in shape for summer — at the same time that new omicron variants are pushing COVID infections up. So how safe is it to go back to the gym?

Put another way, how many microscopic aerosol particles are the other cyclists in your spin class breathing out into the room? How many is the runner on the nearby treadmill spewing forth? A small study about respiration and exercise published recently in the Proceedings of the National Academy of Sciences provides some rather startling answers.

The study looked at the number of aerosol particles that 16 people exhaled at rest and during workouts. These tiny bits of airborne matter — measuring barely a few hundred micrometers in diameter, or about the width of a strand of hair, and suspended in mist from our lungs — can transmit the coronavirus if someone is infected, ferrying the virus lightly through the air from one pair of lungs to another.

The study found that at rest, the men and women breathed out about 500 particles per minute. But when they exercised, that total soared 132-fold, topping out above 76,000 ppm, on average, during the most strenuous exertion.

These findings help explain why several notable COVID superspreader events since 2020 have occurred at indoor gym classes. They also could renew some people's concerns about indoor gym programs as COVID-19 cases rise again in much of the nation and raise questions about how to best reduce risks of exposure when we work out.

In general, packing hard-breathing bodies into enclosed spaces is a bad way to avoid transmission of COVID-19 or other respiratory diseases. In 2020, 54 South Koreans developed COVID after Zumba classes with infected instructors and then passed it to family members and acquaintances. Later that year, all 10 members of a spin class in Hawaii taught by an infected instructor tested positive afterward, as did another 11 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 respiration rates among the exercisers contributed to the wildfire-like spread of COVID at the affected gyms. But the scientists could only guess about the extent to which exercise had increased the levels of aerosol particles in the gym areas. Accurately measuring the rise in floating particles during exercise is difficult.

The researchers first measured people's particle production as they sat still and then as they rode at an increasingly punishing pace until they were too exhausted to continue. Particles were counted constantly.

The scientists expected the exercisers' aerosol output to grow as intensity ramped up. We all breathe more deeply and swiftly as we work out harder. But the extent of the increase "surprised us," said Henning Wackerhage, a professor of exercise biology at the Technical University of Munich and a senior author of the new study.

The rise in aerosol emissions began moderately as riders warmed up and started pedaling harder. But as they reached a threshold at which their exercise became noticeably more strenuous — about when a jog becomes a run or a spin class switches into intervals — the rise in emissions became exponential. The riders started huffing out about 10 times as much air per minute as at rest, while the numbers of particles per minute soared more than 100-fold as riders approached exhaustion (with considerable variation from person to person).

In a room filled with spin-bike riders, treadmill runners or boot campers, "the aerosol particle concentration would increase a lot," said Benedikt Mutsch, a graduate student at the Institute for Fluid Mechanics and Aerodynamics at the University of the German Armed Forces in Munich and study co-author. The more particles, the more possibility of COVID-19 infection if any exercisers are infected.

"The study provides mechanistic data to back up the assumption that exercising indoors is a higher-risk activity when it comes to transmission of COVID-19" than taking your exercise outside, said Linsey Marr, a professor of civil and environmental engineering at Virginia Tech and expert on airborne transmission of viruses.