evidence: Numerous studies showed that wearing masks during physical exercise causes non-health relevant decreases in blood oxygen as well as slight increases in blood carbon dioxide2,4,5,6,7,8,9,10. However, the majority of these studies focused on relevant clinical parameters, sporting leisure activities and investigated short-term loads of up to 300 watts and more on a bicycle ergometer which does not represent typical conditions in everyday life or at workplaces11,12,13,14,15. Some authors also reported physiological effects after continuous or moderate loads10,15,16,17,18,19. In most studies, during cardiopulmonary exercise test (CPET), the face mask was worn underneath the silicone mask required for CPET, which has been discussed as influencing factor4,5,6,7,13. Additionally, measurements were predominantly performed on young, well-trained people10,14,20,21,22 and none of the preceding studies was blinded with respect to the type of mask to be worn by the subjects or focused on breathing physiology or breathing mechanics.