What Happened: A recent study published in the Journal Medical Hypothesis titled "Exercise with facemask; Are we handling a devil's sword? – A physiological hypothesis" claims the following:
Exercising with facemasks may reduce available Oxygen and increase air trapping preventing substantial carbon dioxide exchange. The hypercapnic hypoxia may potentially increase acidic environment, cardiac overload, anaerobic metabolism and renal overload, which may substantially aggravate the underlying pathology of established chronic diseases. Further contrary to the earlier thought, no evidence exists to claim the facemasks during exercise offer additional protection from the droplet transfer of the virus. Hence, we recommend social distancing is better than facemasks during exercise and optimal utilization rather than exploitation of facemasks during exercise.
According to the authors, exercising with facemasks induced as "a hypercapnic hypoxia environment [inadequate Oxygen (O2) and Carbon dioxide (CO2) exchange] . This acidic environment, both at the alveolar and blood vessels level, induces numerous physiological alterations when exercising with facemasks: 1) Metabolic shift; 2) cardiorespiratory stress; 3) excretory system altercations; 4) Immune mechanism; 5) Brain and nervous system.'
Further, poor saturation of haemoglobin would be anticipated due to increased partial pressure of CO2 at higher exercise intensity . Fig. 2 demonstrates the extreme right shift of the oxyhemoglobin dissociation curve, which would be higher than that expected during exercise. This acidic environment would unload O2 faster at the muscle level, but due to higher heart rate and reduced affinity at the alveolar junction, the partial pressure of O2 would substantially fall, creating a hypoxic environment for all vital organs.
In the figure below, the authors present a dissociation curve that "is showing the extreme right side shift with the carbon dioxide rebreathing (PaCO2) and inadequate available Oxygen (PAO2). Red dotted lines show the right shift of the curve due to exercise without masks (↑PaCO2, PH and temperature). Violet dotted lines show the extreme curve shift during exercise with masks (↑↑↑↑PaCO2, PH and temperature). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)"