Carbon dioxide build up in the body can cause a lot of things and have a lot of issues. The mask restricts the amount of fresh air you get, it also concentrates the carbon dioxide we exhale in a small area, and then you end up re-breathing it. Surgeons do not wear a mask to protect themselves, they wear a mask to protect the patient. The minute they are out and away from the patient the very first thing they do is take the mask away from their face and try to breath deep.
About the longest I can wear a mask is 20 minutes tops. I have reduced lung capacity, I need the air I breath, fresh air, not the same air I just exhaled.