A key research topic during the ongoing coronavirus disease 2019 (COVID-19) pandemic has been the emission patterns of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). SARS-CoV-2 is primarily transmitted through aerosols generated from breathing, singing, shouting and other common activities.
Taken together, these factors play an important role in indoor transmission of SARS-CoV-2 during group physical activities. A new PNAS Study discusses the marked change in aerosol emission over a range of physical exercise.
To learn: The emission of aerosol particles increases exponentially above moderate exercise intensity, resulting in superemission during maximal exercise. Photo credit: Kamil Macniak / Shutterstock.com
COVID-19 is primarily an infectious viral respiratory disease; however, it can cause multiple 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.
To reduce the spread of the virus, non-pharmaceutical measures (NPIs) such as the use of masks for interactions with others, social distancing, restrictions on group activities including recreational and travel groups, and the closure of schools and businesses have been implemented. In addition, many countries have declared a full lockdown for varying periods of time. However, this type of restriction also had counterproductive effects, such as a loss in physical fitness, emotional well-being and general resilience.
Indoor group exercises without very high ventilation promote the spread of SARS-CoV-2 through the expulsion of respiratory droplets and aerosols. While droplets tend to fall to the ground within 1.5 meters of the source, aerosols can remain airborne for much longer.
The concentration of aerosol particles in exhaled air varies significantly between individuals. However, one in five people is considered a super emitter, as they exhale air that contains over 156 particles per liter of air. Speaking, coughing, sneezing, singing and physical activity are known to increase the number of aerosol particles emitted.
Dehydration of the respiratory tract also leads to increased aerosol emissions. This can occur during exercise or at high airflow rates.
High air flow rates are typically in the range of five to 15 liters per minute at rest. In comparison, during exercise, these airflow rates can increase to between 100 and 200 liters per minute, depending on your exercise level.
Previous research suggests that mild infection with SARS-CoV-2 is associated with greater aerosol emission than its absence.
This suggests that exercisers infected with SARS-CoV-2 “blow out” more SARS-CoV-2 into a room and that exercisers who are not infected inhale more SARS-CoV-2-contaminated aerosol particles than when at rest.”
In the current study, researchers are developing a method for assessing aerosol concentration and emission in individuals from rest to maximum exposure.
The researchers found that the concentration of aerosol particles increased tenfold in both sexes, from 56 to 630 particles per liter at rest and at maximum exercise, respectively, using bicycle ergometry. However, with 500 or 877 particles per liter at maximum exertion, untrained people emitted significantly fewer particles than those who did endurance training.
The highest concentration of aerosol particles was over 1,000 particles per liter, observed in one man and two women. Particle size remained similar in males and females, and was less than 0.5 µm across the exposure range.
The air flow increased from nine liters per minute at rest to over 100 liters per minute at maximum exertion in women. In men, the resting rate was 13, while the rate at maximum exertion increased to 160 liters per minute. Ventilation was higher in men at peak frequencies. Training had no significant impact on peak ventilation.
Aerosol particle emission increased more than 130-fold, emitting 580 particles per minute at rest and 76,200 particles per minute at maximum exposure. The difference between trained and untrained people was about 85% in favor of the trained people. The point at which the aerosol particle emission exceeded 10,000 per minute was at an exposure intensity of 2 W/kg.
In addition, the two subjects with the highest emission rates at rest also had higher emissions at maximum exertion. Importantly, no reliable pattern was observed in the relationship of these parameters.
The striking increase in aerosol particle emission during exercise by more than 100-fold at peak exercise shows that peak exercise is a major factor contributing to aerosol superemission in young and healthy individuals of both sexes.
Aerosol particle emission is a function of exercise intensity; however, an exponential relationship is observed from an exercise intensity of about 2 W/kg. Remarkably, there was no observable relationship between aerosol particle emission at rest and during exercise.
The study results indicate the need for mitigation strategies to avoid transmission in indoor training groups. In addition, the researchers also provide evidence of the value of this experimental method in measuring the concentration of particles in a partial flow of exhaled air.
Researchers were also able to provide a more accurate estimate of the risk of virus spread through the emission of aerosol particles by individuals, particularly when compared to those obtained using the more indirect method of estimating concentrations in exhaled or indoor air.
The extremely large increase in aerosol particle emission and concentration in exhaled air at maximum exercise, as well as the difference associated with endurance training, must be taken into account and raises a question for further study. Some possible explanations for these observations include increased ventilation due to deeper breaths and exhalations, Dehydration of the airways, changes in the speed of airflow, and changes in the fluids lining the lining of the airways.
Researchers in the current study recommend maintaining a distance of over 1.5 meters between people during exercise, increasing ventilation in the room, and keeping sessions between 45 and 90 minutes to limit exposure. At maximum load, when superemission is likely, the room should be freely ventilated for 15 minutes between classes. It would also be beneficial if participants were tested for COVID-19 before each class.
Safety screens between people, masks and mobile air filters are other ways that virus transmission can be reduced in high-intensity training environments. However, the practicability of these proposals has yet to be explored.
The aerosol particle emission increases moderately up to an exposure intensity of 2 W/kg and exponentially at higher exposure intensities. This information should be used to develop more data-based mitigation measures for indoor group exercises.”
- Mutsch, B., Heiber, M., Gratz, F., et al. (2022). The emission of aerosol particles increases exponentially above moderate exercise intensity, resulting in superemission during maximal exercise. PNAS. doi:10.1073/pnas.2202521119.