Many gyms appear to be packed with people and groups eager to resume old activities or get in shape for the summer as new variants of the Omicron virus increase Covid infections. So is it safe to go to the gym?
Put another way, how many microscopic aerosol particles are the cyclists in your spinning class exhaling through the room? How many spit out the runner on the next treadmill? A small study on breathing and movement, published in May in the Proceedings of the National Academy of Sciences, offers some answers.
The study looked at the number of aerosol particles exhaled by 16 people at rest and during exercise. These tiny airborne particles — which measure just a few hundred microns in diameter, or about the width of a hair, and hover in the mist that leaves our lungs — can transmit the coronavirus if someone is infected, and carry the virus through the air from one pair of lungs to the other.
The study found that men and women exhale about 500 particles per minute at rest. But when they exercise, that total increases 132fold, averaging more than 76,000 ppm during the most strenuous effort.
These results help explain why there have been several notable Covid superspreading events in indoor fitness classes since 2020. They could also renew some people’s concerns about indoor fitness programs as cases of the disease are rising again in many parts of the world, and raise questions about how best to reduce the risk of exposure from exercise.
In general, huddling heavily breathing bodies indoors is bad for preventing the transmission of Covid19 or other respiratory diseases. In 2020, 54 South Koreans contracted Covid after attending 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 a kickboxing instructor.
Scientists studying this and similar outbreaks speculated that poor ventilation and high breathing rates among practitioners contributed to the spread of Covid in affected gyms. However, scientists could only guess to what extent exercise increases the concentration of aerosol particles in gyms. Accurately measuring the addition of airborne particles during exercise is difficult.
In the study, a group of exercise scientists and fluid dynamics researchers in Germany developed a new method to measure aerosol emission using a single exercise bike and a cyclist in an airtight tent. The cyclists wore silicone masks that caught their exhaled breath and sent the air through tubes to a machine that counted every particle as it passed.
The researchers measured the individuals’ particle production first while sitting and then as they walked at increasing speeds until they were too tired to continue. The particles were constantly counted.
Scientists expected that practitioners’ aerosol production would increase as the intensity increased. We all breathe deeper and faster the more we move. But the extent of the increase “surprised us,” says Henning Wackerhage, professor of movement biology at the Technical University of Munich and first author of the study.
The increase in aerosol emissions started moderately as cyclists warmed up and began pedaling harder. But when they reached a threshold where exercise became noticeably more strenuous — when a brisk walk became a jog or a spinning class switched to intervals — the increase in emissions became exponential.
Cyclists began blowing about 10 times more air per minute than when they were resting, while the number of particles per minute increased more than 100 times as cyclists neared exhaustion (with significant persontoperson variability).
In a room full of spinning cyclists, treadmill runners or on a military course, “the concentration of aerosol particles would increase sharply,” said Benedikt Mutsch, a doctoral student at the Institute of Fluid Mechanics and Aerodynamics at the University of German Armed Forces in Munich and coauthor of the study. The more particles, the greater the possibility of Covid infection if a practitioner is contaminated.
“The study provides mechanistic data supporting the notion that indoor exercise poses a higher risk when it comes to transmission of Covid19,” said Linsey Marr, a professor of civil engineering at Virginia Tech and an expert on the 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.
Stay away from other practitioners as well. “Social distancing of two meters or more is always important,” said Mutsch. But for strenuous indoor fitness classes, it may not be enough.
The study didn’t track how far the cyclists’ aerosol particles traveled, but it’s likely they traveled well over 6 feet, he said. So, during strenuous workouts that require large spaces and small class sizes, keep at least 8 to 10 feet away from others.
The classes themselves should also be spatially separated. “When there are backtoback training courses, some of the air from the first class is carried over to the second,” Cappa said. Make sure there are breaks of at least 15 and preferably 30 minutes between sessions to allow air to renew.
And also wear a mask. “Respirators reduce aerosol emissions,” said Wackerhage.
Translated by Luiz Roberto M. Gonçalves