A recent article from the Technical University of Munich published in PNAS reached national newspapers in several countries worldwide. The team showed that aerosol emissions increase exponentially with intense physical exertion, indicating that indoor sporting activities result in higher risk of infectious diseases as COVID. The authors suggested the use of face masks, social distancing and ventilation to prevent viral infections during (vigorous) indoor exercise.
However, the study as presented in the newspaper article does not yet prove a higher risk for viral infections by aerosols emitted by healthy persons during indoor sports. The recommendation wearing face masks during vigorous exercise has not been argued to be safe and effective …
A recent study (not peer-reviewed yet) showed the increase of CO2 while wearing a mask. The CO2 content reached levels above the set risk level of 5,000 ppm (acceptable threshold for workers) for 40.2% of the people wearing medical masks and for 99.0% of the people wearing a FFP2 mask. A special article COVID-19 and masks in sports also found an enhancing effect on pCO2 in case of intense physical exercise.
The use of masks in athletes causes hypoxic and hypercapnic breathing as evidenced by increased effort during exercise. Another study found that wearing a face mask during exercise intensity mostly affects perceptual responses, causing an increase in the rate of perceived breathlessness and overall exertion with limited influence on pulse oxygen, blood lactate, and heart rate responses.