Where are you making the connection that wearing a mask increases CO2 concentrations by any measurable amount? Like, sure, breathing in more CO2 is bad, but are masks doing that, or are you just guessing that they might?
And, the obvious follow-up: are said side effects, if present, worse than getting/spreading Covid? For example, the meds I take for epilepsy have quite a few downsides, including the price, but it's still a lot fucking better than having seizures.