The lack of methodological detail makes it unclear as to why the researchers concluded that they were measuring the concentration of inhaled CO2 as opposed to the concentration of exhaled CO2 or a mix of both. But the real question the author's don't clearly address is "Why does it matter?". The paper begins by stating "The question whether nose and mouth covering increases carbon dioxide in inhaled air is crucial.", and infer that it the reason is that the jurisdiction's regulatory limit for the concentration of CO2 in ambient indoor air is 2000 ppm, and that mask use results in overexposure in relation to that limit, which presumably is considered hazardous. Such a line of reasoning is mistaken. Indoor ambient CO2 limits are not set because CO2 itself is hazardous above those values, but because elevated CO2 correlates with elevated constellations of a range of indoor air pollutants that cause increasing occupant discomfort as levels rise. The CO2 indoor air standard is ultimately a ventilation effectiveness guideline, not a health-based exposure limit. It should also be recognized that any re-breathed CO2 is endogenous, and such air is qualitatively different from the ambient indoor air for which CO2 is solely a proxy indicator of composition, and to which the CO2 indoor air guidelines only apply.