evidence: FACT: The prolonged use of medical masks* when properly worn, DOES NOT cause CO2 intoxication nor oxygen deficiency The prolonged use of medical masks can be uncomfortable. However, it does not lead to CO2 intoxication nor oxygen deficiency. While wearing a medical mask, make sure it fits properly and that it is tight enough to allow you to breathe normally. Do not re-use a disposable mask and always change it as soon as it gets damp.