Relationship between exercise intensity and risk of infection

Before the study, it was known that tidal volume in untrained people increases from approximately 5-15 liters per minute at rest to more than 100 l/min during exercise. Highly trained athletes even reach levels of 200 l/min. It was also known that many people have become infected with the SARS-CoV-2 virus when they exercise indoors.

However, it was unclear how exercise intensity was related to the concentration of aerosol particles in exhaled air and the actual amount of aerosols an individual exhaled per minute, and thus to the potential risk of spreading infectious diseases such as SARS-CoV-2. However, this information is urgently needed, for example to design mitigation measures for gyms and other indoor sports facilities, fitness studios or discos to avoid closures in the event of serious infection waves.

New methodology produces individually measurable aerosol values

A team led by Henning Wackerhage, professor of exercise biology at the Technical University of Munich (TUM), and Prof. Christian J. Kähler, director of the Institute for Fluid Mechanics and Aerodynamics at the Universität der Bundeswehr Munich, has developed a new research method to study questions. Their experimental equipment initially filtered the aerosols already present in the ambient air. During the subsequent ergometer stress test, the subjects inhaled the purified air through a special mask that covered the mouth and nose. Exercise intensity was gradually increased from rest to the point of physical exhaustion. The mask was connected to a two-way valve that allowed only exhaled air to escape. The amount of aerosol particles emitted per minute was then measured and directly linked to the current performance of the healthy, 18-40 year old subjects.

Moderate aerosol emissions during moderate exertion

This allowed the researchers to investigate for the first time how many aerosol particles are exhaled per minute by an individual at different levels of exercise intensity. The result: aerosol emissions during exercise initially increased only moderately, reaching an average load of about 2 watts per kilogram of body weight. However, above that point they increased exponentially. This means that someone weighing 75 kilograms reaches that threshold at an ergometer setting of approximately 150 watts. This equates to moderate exertion for a casual athlete, perhaps comparable to the exercise intensity of moderate jogging.

The aerosol output of well-trained athletes was significantly higher than that of untrained subjects at maximum exertion, due to their much higher minute ventilation. The researchers found no significant differences in particle emissions between genders.

Protective measures are important for high-intensity training

Although the aerosol experiments provide only indirect knowledge about the amount of viruses in exhaled air, the research suggests useful principles for managing indoor activities when a wave of infections 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 increase in aerosol emissions at high-intensity loads above that initial benchmark, special protective measures are necessary if there is a high risk of infections with serious consequences,” says research leader Prof. Wackerhage: “Ideally, that type of training would be moved outside. If that is not possible, testing should be done to ensure that there are no infected persons in the room. Participants must also keep sufficient distance and a highly efficient ventilation system must be running. In addition, infection risks are reduced by training at lower intensity and keeping sessions shorter. It could also be possible for fitness that young athletes should wear masks during training.” At low workloads, such as light to moderate intensity endurance training, less protection is needed, says Prof. Wackerhage, and the risk of infection can be controlled through distancing and ventilation systems.

The research team is currently conducting experiments to compare aerosol emissions during strength and endurance training and to correlate this with the age and physical characteristics of subjects.