how safe is it to go back to the gym?

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 May 23 in the Proceedings of the National Academy of Sciences provides some rather startling answers.

The study looked at the number of aerosol particles 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 increase again in much of the nation and raise questions about how to best reduce risks of exposure when we work out.

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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.

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

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).

The more particles, the more possibility of COVID-19 infection if any exercisers are infected.

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

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"In-room air filters can be really effective in reducing transmission risk by removing the virus from the air. Also, stay well away from other exercisers. Social distancing of 6 feet or more is always important. But it may not be enough during strenuous, indoor exercise classes." The new study did not track where cyclists’ aerosol particles flowed, but it is likely they stream well beyond 6 feet, he said. So keep at least 8-10 feet apart during strenuous workouts, which requires large rooms and small classes.

The classes themselves should likewise be well spaced. “If there are back-to-back exercise classes, some of the air from that first class will carry over to the second,” Cappa said. Be sure there are breaks of at least 15 and preferably 30 minutes between sessions to allow the air to clear.

Mask up as well. Respiratory face masks reduce aerosol emissions.

If you find a tight N95 mask uncomfortable during intense exercise, “I’d suggest wearing a good surgical mask,” Cappa said, which may feel slightly less constricting and steamy.

Finally, check the incidence of COVID-19 in your area. “The higher the local case rates,” Cappa said, “the more likely it is that an infectious person might be in the class with you.”

But keep moving. This study “is more incentive to ensure great ventilation and no crowding in gyms, but it is not a reason to skip workouts".