In the scientific literature there are not many studies that relate the use of face masks to changes in the concentration of oxygen and/or carbon dioxide (CO2), but it seems a plausible hypothesis due to the barrier element that is interposed in the physiological ventilation mechanism.17 In a Taiwanese cohort of 39 patients with end-stage renal disease who wore N95 masks during the 2002 SARS outbreak, the study found a significant reduction in arterial oxygen pressure from baseline and an increase in other respiratory adverse effects.18 Another study conducted in a cohort of 130 astronauts subjected to high CO2 pressures during controlled training showed a significantly higher incidence of headache in the exposed group, in addition to respiratory symptoms and difficulty in concentrating.19 At the University of Wollongong, a study on the effects of CO2 inhalation on workers wearing respiratory protection devices showed that high levels of CO2 were associated with feelings of discomfort and significantly reduced tolerance and time of device use.20 The effect on respiratory physiology and muscle performance of wearing training masks designed to simulate a variable altitude situation has been studied. The results are mixed in terms of objective performance parameters, however, it seems that mask use reduces working speed and negatively influences levels of alertness and task focus.21 In 2014, a pilot study evaluated the consequences on respiratory physiology of surgical mask and N95 face mask use and the extent to which nasal inspiratory and expiratory resistance and discomfort were altered in the individuals. Physiological changes such as increased respiratory resistances were observed after 3 hours of use.22 Headache associated with filter mask use could be included according to ICHD-311 in the section on headaches due to homeostatic disorders where those related to alteration of oxygen and CO2 partial pressure parameters are included.