This is an amazing study which is well-worth reading in full. The following are excerpts.
The researchers in great detail outline the numerous potential negative effects of exercising with facemasks, including: hypercapnia, cardiac overload, renal overload, a shift to metabolic acidosis, potential falls, and much, much more. Thanks to these researchers for their valuable studies and information.
“Exercise With Facemask; Are We Handling a Devil’s Sword?” – A Physiological Hypothesis
Baskaran Chandrasekaran et al. Med Hypotheses. 2020.
Straying away from a sedentary lifestyle is essential, especially in these troubled times of a global pandemic to reverse the ill effects associated with the health risks as mentioned earlier. In the view of anticipated effects on immune system and prevention against influenza and Covid-19, globally moderate to vigorous exercises are advocated wearing protective equipment such as facemasks. Though WHO supports facemasks only for Covid-19 patients, healthy “social exercisers” too exercise strenuously with customized facemasks or N95 which hypothesized to pose more significant health risks and tax various physiological systems especially pulmonary, circulatory and immune systems. 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.
Keywords: Cardiometabolic risk; Complication; Exercise; Facemask; Immunity; Physiology.
Pathophysiological changes associated during exercise with facemasks.
Potential changes that occur in the immune, muscular, renal, brain, cardiovascular and metabolic systems contributing to anxiety and depression. GFR – Glomerular Filtration Rate; PaCO2 – Partial pressure of Carbondixide; PaO2 – Partial pressure of Oxygen; ↓- decreased; ↑- increased
Facemask and physiology alteration during exercise
Exercising with customized tight facemasks induces 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. Fig. 1 illustrates the possible physiological alterations while exercising with facemasks.
❗❗ Poor immune responses
Substantial evidence exists concerning the long term effects of exercise and the improvement in adaptive immunity . Though, moderate exercise, in the long run, is found to increase natural killer cell count and downregulate inflammatory factors such as tumour necrosis factors, acute bouts of vigorous exercise over a while may influence these changes negatively. Exercising with facemasks induces an acidic environment, and thus mobility of hypoxic natural killer cells to the target cells would be affected, aggravating the chances of infection during the pandemic. A further change in humidity and temperature in the upper airway causes immotile cilia syndrome predisposing individuals to lower respiratory tract infections by deep seeding of oropharyngeal flora 
Increased cardiorespiratory stress
The reduced availability of O2 and CO2 would increase the heart rate and blood pressure exponentially even at low workloads. This physiological alteration may increase aortic pressure and left ventricular pressures, leading to an upsurge of cardiac overload and coronary demand . Further increased respiratory load against the “valve breathing”, leads to increased respiratory muscle load and pulmonary artery pressure, in turn, adding to the cardiac overload. These changes may be subtle in healthy individuals during exercise. Still, in persons with established chronic illness, these changes may aggravate the underlying pathophysiology, leading to hospitalization or increased use of medication.
Altered renal function
Hypercapnic hypoxia reduces renal blood flow and glomerular filtration rate posing a risk of reduced renal functions. Thus, aciduria and resulting tubular damage may potentially aggravate the compromised renal functions in individuals with established chronic diseases . Further, the autonomic dysfunction and reduced immune responses, increase the inflammatory substances such as C reactive protein, interleukins (IL-6, IL-12) resulting in generalized nephritis in chronic kidney failure patients . Additionally, poor renal artery flow causes hypoxemia in nephrons perpetuating the pathophysiology of poor renal functions.
Brain metabolism and mental health
Acute hypercapnia, a double-edged sword, on the one hand, elevates intracranial pressure, lowers cerebral perfusion, and triggers cerebral ischemia and, on the other hand, it is found to be neuroprotective decreasing the excitatory amino acids and minimizing the cerebral metabolism . Studies from obstructive sleep apnea  provide irrefutable evidence of hypercapnic hypoxemia affecting the postural stability, proprioception, altered gait velocities and falls. The above findings can be extrapolated to elderly persons as well as individuals with established respiratory diseases exercising with N95 respirator masks
Can facemasks increase the risks of Coronavirus?
Though the respirator masks are perceived to be the barriers for preventing aerosol depositions to the respiratory tract, the bitter reality is that masks increase the risk of more in-depth respiratory tract infections…
You can read the full study here https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7306735/#!po=30.0000
You can download the PDF here https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7306735/pdf/main.pdf