The relationship between exercise intensity and infection risk
The relationship between exercise intensity and the emission and concentration of aerosol particles in exhaled air has not been well understood up to this point. A Munich research team has shown using a unique experimental setup that aerosol emissions rise exponentially with high levels of physical activity. This means indoor athletic events have an increased risk of infectious diseases like COVID-19.
Before the research, it was known that untrained individuals’ respiratory volumes rise during exercise from 5 to 15 liters per minute at rest to over 100 liters per minute. In fact, well-trained athletes can reach 200 l/min levels. It was also recognized that a lot of individuals had contracted the SARS-CoV-2 virus while working out indoors.
However, it was unclear how exercise intensity was related to the number of aerosols that a person actually inhaled per minute and the concentration of aerosol particles in exhaled air, and thus the potential danger of transmitting infectious diseases like SARS-CoV-2. However, this knowledge is urgently required, for instance, to build mitigation measures for school gyms and other indoor sports facilities, fitness studios, or discos to prevent a shutdown in case of major waves of infection.
The new methodology delivers individually measurable aerosol values
A team led by Henning Wackerhage, a Professor of Exercise Biology at the Technical University of Munich (TUM), and Prof. Christian J. Kähler, the Director of the Institute of Fluid Mechanics and Aerodynamics at the Universität der Bundeswehr München, has developed a new investigative method for studying these questions.
Their experimental apparatus initially filtered out the aerosols already present in the ambient air. In the subsequent ergometer stress test, the test subjects inhaled the purified air through a special mask covering the mouth and nose. The exercise intensity was gradually increased from rest to the point of physical exhaustion. The mask was connected to a two-way valve through which only the exhaled air can escape.
The number of aerosol particles emitted per minute was then measured and directly linked to the current performance of the healthy, 18-40-year-old test subjects.
Moderate aerosol emissions at medium exertion
The researchers were thus able to investigate for the first time how many aerosol particles are exhaled per minute by an individual at various levels of exercise intensity. The result: aerosol emissions during exercise initially increased only moderately up to an average workload of around 2 watts per kilogram of body weight. Above that point, however, they rose exponentially. That means that an individual who weighs 75 kilograms reaches that threshold at an ergometer reading of around 150 watts. This corresponds to moderate effort for a casual athlete, perhaps comparable to the exercise intensity of moderate jogging.
The aerosol emissions of well-trained athletes were significantly higher than those of untrained test subjects at maximum effort due to their much higher minute ventilation. The researchers did not find significant differences in particle emissions between genders.
Protective measures are important for high-intensity training
Although the aerosol experiments provide only indirect knowledge on the number of viruses in exhaled air, the study suggests useful starting points for managing indoor activities when a wave of infection combined with a poorly immunized population threatens to overwhelm the healthcare system.
“Based on our results, we distinguish between moderate endurance training with an intensity of up to 2 watts per kilogram of body weight and training at high to maximum intensity. Due to the sharp rise in aerosol emissions at high-intensity workloads above that initial benchmark, special protective measures are needed in case of a high risk of infections with serious consequences,” says study leader Prof. Wackerhage: “Ideally, that kind of training would be moved outdoors. If that is not possible, testing should be done to ensure that no infected individuals are in the room. The participants should also maintain a proper distance and a high-efficiency ventilation system should be running. In addition, infection risks are reduced by training at lower intensities and keeping sessions shorter. It might also be possible for fit, young athletes to wear masks while training.” At low workloads such as easy to moderately intense endurance training, adds Prof. Wackerhage, less protection is needed and the infection risk can be controlled through distancing and ventilation systems.
The research team is currently conducting experiments to compare aerosol emissions in strength and endurance training and to correlate them with test subjects’ ages and physical characteristics.
The study was funded by the German Federal Institute of Sports Science (BISp) and the German Research Foundation (DFG).
Reference: “Aerosol particle emission increases exponentially above moderate exercise intensity resulting in superemission during maximal exercise” by Benedikt Mutsch, Marie Heiber, Felix Grätz, Rainer Hain, Martin Schönfelder, Stephanie Kaps, Daniela Schranner, Christian J. Kähler and Henning Wackerhage, 23 May 2022, Proceedings of the National Academy of Sciences.