The prolonged use of medical masks does not cause carbon dioxide intoxication nor oxygen deficiency especially when it is properly worn.
The use of medical masks for a prolonged time can be uncomfortable. However, world health organization(WHO) reports that 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.
Medical masks (also known as surgical masks) are flat or pleated; they are affixed to the head with straps or have ear loops.
SOURCE – WHO