Why gyms have a higher risk of COVID

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A major topic of research during the ongoing coronavirus disease 2019 (COVID-19) pandemic has been the emission patterns of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). SARS-CoV-2 is primarily transmitted through aerosols that are produced during breathing, singing, shouting, and other common activities.

Taken together, these factors play an important role in the transmission of SARS-CoV-2 indoors during group physical activities. A new PNAS study discusses the marked change in aerosol emission over a range of physical exercises.

Study: Aerosol Particle Emission Increases Exponentially Above Moderate Exercise Intensity Resulting in Superemission During Maximal Exercise. Image Credit: Kamil Macniak / Shutterstock.com

Study: Aerosol Particle Emission Increases Exponentially Above Moderate Exercise Intensity Resulting in Superemission During Maximal Exercise. Image Credit: Kamil Macniak / Shutterstock.com


COVID-19 is primarily an infectious viral respiratory condition; however, it can lead to multi-organ damage and death as a result of acute respiratory distress syndrome (ARDS). As of May 26, 2022, over 6.3 million deaths have been reported worldwide as a result of COVID-19.

In an effort to reduce viral spread, non-pharmaceutical interventions (NPIs) like mask use for interactions with others, social distancing, restrictions on group activities including leisure and travel groups, as well as the closure of schools and businesses have been implemented. Furthermore, many countries declared a complete lockdown for varying periods of time. However, this kind of restriction also had counter-productive effects, such as a loss of physical fitness, emotional well-being, and overall resilience.

Group exercise indoors without very high ventilation promotes the spread of SARS-CoV-2 through the expulsion of respiratory droplets and aerosols. While droplets tend to drop to the ground within 1.5 meters of the source, aerosols can float in the air for much longer.

The concentration of aerosol particles in exhaled air varies significantly between individuals. However, one in five people is considered a superemitter, as they exhale air containing over 156 particles per liter of air. Talking, coughing, sneezing, singing, and physical activity are all known to increase the number of emitted aerosol particles.

Dehydration of the airways also leads to increased aerosol emission. This can occur during exercise or with high rates of airflow.

High airflow rates are typically within the range of five to 15 liters per minute at rest. Comparatively, during exercise, these airflow rates can increase to between 100-200 liters per minute, depending on the level of training.

Earlier research suggests that mild infection with SARS-CoV-2 is associated with greater aerosol emission than its absence.

This suggests that exercising SARS-CoV-2-infected individuals will “blow out” more SARS-CoV-2 into a room and that exercising, noninfected individuals will inhale more SARS-CoV-2-contaminated aerosol particles, when compared to rest.”

In the current study, researchers develop a method to assess aerosol concentration and emission in individuals from rest to maximal exercise.

Study findings

The researchers found that the concentration of aerosol particles in both sexes increased by ten-fold, from 56 to 630 particles per liter at rest and maximal exercise, respectively, using cycle ergometry. However, untrained individuals emitted significantly lower numbers of particles than those who had endurance training, at 500 and 877 particles per liter at maximal exercise, respectively.

The highest concentration of aerosol particles was over 1,000 particles per liter, which was observed in one man and two women. The particle size remained similar in women and men, as well as over the range of exercise at less than 0.5 µm.

The airflow rate increased from nine liters per minute at rest to over 100 liters per minute at maximal exercise in women. In men, the resting rate was 13, whereas the rate increased to 160 liters per minute at maximal exercise. Ventilation was higher in men at peak rates. Training did not significantly affect peak ventilation.

The emission of aerosol particles increased by more than 130-fold, with 580 particles per minute emitted at rest and 76,200 particles per minute emitted at peak exercise. The difference between trained and untrained individuals was about 85% and was in favor of trained individuals. The point at which aerosol particle emission exceeded 10,000 per minute was at an exercise intensity of 2 W/kg.  

Moreover, the two individuals with the highest emission rates at rest also had higher emissions at peak exercise. Importantly, no reliable pattern was observed in the relationship of these parameters.


The striking rise in aerosol particle emission with exercise, by over 100-fold with peak exercise, shows that peak exercise performance is a primary factor that contributes to the superemission of aerosols in young and healthy people of both sexes.

The emission of aerosol particles is a function of exercise intensity; however, an exponential relationship is observed at and above an exercise intensity of about 2 W/kg. Notably, there was no observable relationship between aerosol particle emission at rest and during exercise.

The study findings indicate the need for mitigation strategies to avoid transmission in indoor exercise groups. Moreover, the researchers also provide evidence supporting the value of this experimental method in measuring the concentrations of particles in a partial flow of exhaled air.

The researchers were also able to arrive at a more accurate estimate of the risk of viral spread from aerosol particle emission by individuals, particularly when compared to those obtained using the more indirect method of estimating the concentration in exhaled or room air.

The extremely high increase in aerosol particle emission and concentration in exhaled air during maximal exercise, as well as the difference associated with endurance training, will have to be accounted for and presents a question for further studies. Some possible explanations for these observations include the increased ventilation due to deeper breaths and exhalations, dehydration of the airway, changes in the velocity of airflow, and changes in the fluids lining the airway mucosa.


The researchers of the current study recommend maintaining a distance of over 1.5 meters between people during exercise, increasing the ventilation of the room, and keeping sessions between 45-90 minutes to limit exposure. At maximal exercise, which is when superemission is likely, the room should be aired freely for 15 minutes between classes. It would also be beneficial for participants to be tested for COVID-19 before each class.

Safety shields between individuals, masks, and mobile air filters are other potential ways in which viral transmission can be reduced in high-intensity exercise settings. However, the practicability of these suggestions remains to be explored.

Aerosol particle emission increases moderately up to an exercise intensity of 2 W/kg and exponentially at higher exercise intensities. This information should be used to develop more data-based mitigation measures for indoor group exercise.”

Journal reference:
  • Mutsch, B., Heiber, M., Gratz, F., et al. (2022). Aerosol Particle Emission Increases Exponentially Above Moderate Exercise Intensity Resulting in Superemission During Maximal Exercise. PNAS. doi:10.1073/pnas.2202521119.
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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