Since the start of the pandemic, an unprecedented and astonishing amount of research has been conducted on issues related to COVID-19. And yet so much related to this virus remains a mystery. A new study, however, offers some clarity on how exercise intensity affects the emission and concentration of aerosol particles in exhaled air, which is one of the primary ways in which novel coronaviruses like COVID are spread. With a new experimental setup, a Munich research team has shown that aerosol emissions increase exponentially with intense physical exertion, which has implications for indoor exercise and one’s risk of contracting COVID.

Prior to the study, it was known that the respiratory volume for untrained people increases from around 5 to 15 liters per minute (L/min) at rest to more than 100 L/min when exercising. Highly trained athletes actually reach levels of 200 L/min. It was also known that people have become infected with COVID while exercising indoors.

However, it was unclear how exercise intensity was linked to the concentration of aerosol particles in exhaled air and the actual quantity of aerosols exhaled by an individual per minute and thus to the potential risk of spreading infectious diseases such as COVID. This information is urgently needed, however, for example to design mitigation measures for health clubs, fitness studios, training facilities, school gyms and other indoor sports facilities to avoid shutdowns during future waves of infection.

An Innovative Study Design

A team lead 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, recruited eight male and eight female healthy, 18- to 40-year-olds to participate in the study. The team developed a new investigative method for studying these questions. Their experimental apparatus initially filtered out the aerosols already present in the ambient air. When participants then performed an ergometer stress test, they inhaled the purified air through a 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 quantity of aerosol particles emitted per minute was then measured and directly linked to the current performance of each participant.

Moderate Aerosol Emissions at Moderate-level Exertion

Using this methodology, the researchers were able to identify how many aerosol particles are exhaled per minute by an individual at various levels of exercise intensity. They found that aerosol emissions during exercise initially increased only moderately up to an average workload of around 2 watts per kilogram of body weight. (Note: Respiration generally increases with body weight.) Above that point, however, they rose exponentially with increasing exercise intensity. An individual weighing 165 pounds (75 kilograms), for example, reaches that threshold at an ergometer reading of around 150 watts. This corresponds to moderate effort for a casual athlete, comparable to the exercise intensity of moderate jogging. Beyond this intensity, that same individual exhales significantly more aerosol particles.

Due to the much higher minute ventilation of well-trained athletes, their aerosol emissions were significantly higher than those of untrained test subjects at maximum effort. The researchers did not find significant differences in particle emissions between sexes.

What the Research Means to Health and Exercise Professionals

As a health and exercise professional, it is important that you have the knowledge you need to protect yourself and your clients, especially when performing high-intensity training in an indoor environment. Although the aerosol experiments provide only indirect knowledge on the quantity of viruses in exhaled air, the study suggests useful starting points for managing indoor activities, This information will be particularly helpful when the next wave of infection arises or if COVID reaches an endemic stage in which we learn to live with it, much like influenza viruses.

“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-maximal intensity, explains Dr. Wackerhage. “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.” 

Ideally, Dr. Wackerhage says, this kind of training should be performed outdoors. When outdoor exercise is not an option, he recommends having participants maintain a proper distance from each other and ensuring that the club or facility has a high-efficiency ventilation system. Training intensity should also be considered.

“Infection risks are reduced by training at lower intensities and keeping sessions shorter,” explains Dr. Wackerhage. “It might also be possible for fit, young athletes to wear masks while training.” At low workloads, such as light-to-moderate intensity cardiorespiratory training, less protection is needed and the infection risk can be controlled through distancing and ventilation systems.

Additionally, the researchers noted that “dehydration of the airways can be caused by exercise and increased ventilation and both could lead to a higher number of emitted aerosol particles.” For this reason, remind your clients and participants about the importance of maintaining proper hydration before, during and after the exercise session. 

As the world moves into a new phase of the pandemic, it remains each individual’s responsibility to manage their risk and that of the individuals in their care. For those who have a more challenging risk profile, these recommendations may serve to help to lower their risk so they can continue to enjoy the health benefits of physical activity.