Wearing a facemask for longer periods may be uncomfortable but there is no scientific evidence to prove that it can cause hypoxia.
Facemasks are a part of COVID-19 interventions to provide a breathing barrier to the mouth and nose, which helps reduce transmission of respiratory pathogens. The FDA suggests two types of masks, N-95, and surgical masks. According to the FDA, wearing an N-95 mask for long periods can inhibit breathing. Surgical masks, in comparison, are more porous and breathable. Neither World Health Organization (WHO) nor the Centre for Disease Control and Prevention (CDC) state that wearing any of these masks would result in hypoxia.
Hypoxia is a condition in which oxygen supply in tissues is low or insufficient. According to Mayo Clinic, a healthy oxygen intake level, or the amount of oxygen circulating in your blood, ranges from 95-100%, with anything below 90% considered low.
A study published by NCBI involving ten nurses found that while carbon dioxide levels were significantly elevated while wearing N-95 respirators for a 12-hour shift, those levels did not meet the clinical definition of hypercapnia (build-up of carbon-di-oxide in the bloodstream). However, while wearing respiratory protection, study participants reported perceived shortness of breath, headache, lightheadedness, and other subjective symptoms.
WHO stated that prolonged usage of medical masks might be uncomfortable, but it does not lead to oxygen deficiency or hypoxia. WHO also advises people to make sure their masks fit properly and are tight enough to allow one to breathe normally.
The CDC states that cloth face masks should not be worn by children under the age of two, anyone with breathing difficulties, the unconscious, the incapacitated, or those unable to remove the covering without assistance. However, the CDC does not state that the use of masks could result in hypoxia.
Clinical trials also state that the degree of hypoxia associated with surgical or N-95 masks is unclear as yet. There is no evidence to verify that prolonged use of face masks could cause hypoxia. It warrants further investigation, given the current widespread use of both due to the COVID-19 pandemic.
The COVID-19 pandemic has given rise to a lot of potentially dangerous misinformation. For reliable advice on COVID-19 including symptoms, prevention and available treatment, please refer to the World Health Organisation or your national healthcare authority.