The scientists expected the exercisers’ aerosol output to grow, as intensity ramped up. We all breathe deeper and faster 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.
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.
“Open windows, especially with fans, can often be as effective as active ventilation systems,” he said. If windows at your gym are closed, ask the manager to fling them wide and crank up the fans. If the weather is stifling and air conditioning necessary, make sure your gym’s units draw air from outside, so fresh supplies replace the air filled with aerosol emissions from you and your classmates.
You might also suggest the gym install in-room air filters in each workout area, Dr. Cappa said. “These can be really effective in reducing transmission risk by removing the virus from the air.” They can be purchased commercially or even made at home, he said.