Introduction: Due to the mandatory use of a mask, and the authorization to do outdoor sports in Catalonia, we aimed to assess the physiological impact of the hypercapnia hypoxia generated by the masks during aerobic sports practice. Methods: Eight subjects (2 women, 6 men) were assessed at baseline with and without a mask, and immediately after a 21-flex test performed following the Ruffier protocol with a mask. Measures of HR (heart rate), concentration of O-2 and CO2 inside the mask and SatO(2) were assessed. The test was carried out in ambient air in squares in the city of Barcelona. Results: A decrease in O-2 was recorded, and when comparing the, baseline 20.9%, baseline mask 18.3%, post-exercise 17.8% (p < 0.001). An increase in CO2 in the three preconditions (464, 14162, 17 000 ppm; p < 0.001). Basal saturation O-2 was 97.6 +/- 1.5% and post exercise 92.1 +/- 4.12% (p 0.02). Conclusions: The use of masks in athletes causes hypoxic and hypercapnic breathing as evidenced by increased effort during exercise. The use of masks during a short exercise with an intensity around 6-8 METS, decreases O-2 by 3.7% and increases the CO2 concentration by 20%.