A friend of mine, a fellow Cat fan who posts here from time to time, a professional in his field of work, recently related a story.
He went to his doctor for a checkup. This guy is mid 30's, very, very fit, works out each day, eats right, etc. They did a blood draw because of something they saw made them pause. Turns out his CO2 level was VERY elevated, to the point it was a danger. His doctor asked him if he wore a mask, to which he replied he had to at work, and then he wore one anytime he was outside his home.
His doctor told him he was seeing this more and more of late, and it's due to needlessly wearing a mask in places where you aren't in immediate contact with a known Covid patient. It's obvious when you think about it: when you exhale you are exhaling CO2, but the mask is preventing the majority of that CO2 from exiting, and you're breathing it right back in when you inhale.
On top of that, the mask prevents you from inhaling enough oxygen, thereby depriving your body of what it needs. Combine the two and you can see why wearing a mask, outside of when you're near a known Covid patient, is actually a detriment.