evidence: Breathing in excessive carbon dioxide is dangerous for the body. Some people with preexisting respiratory illnesses may face health issues only with prolonged use of tight-fitting masks, such as respirators. [...] During the COVID-19 coronavirus disease pandemic in the spring of 2020, questions arose about protective measures. Social media users shared related images, and in one case, a newspaper column claimed that continual or prolonged use of face masks resulted in breathing too much of one's own exhaled carbon dioxide, which can lead to health concerns. [...] The CO2 will slowly build up in the mask over time. However, the level of CO2 likely to build up in the mask is mostly tolerable to people exposed to it. You might get a headache but you most likely [would] not suffer the symptoms observed at much higher levels of CO2. The mask can become uncomfortable for a variety of reasons including a sensitivity to CO2 and the person will be motivated to remove the mask. It is unlikely that wearing a mask will cause hypercapnia.