Question: Does wearing face mask/surgical masks for hours cause ‘hypercapnia’ or carbon dioxide toxicity?

It doesn’t really, not unless there is something very wrong with the mask, or the wearer has an underlying condition. Technically, “hypercapnia” means any detectable increase in blood CO2 concentration. Realistically, we can measure very, very small increases—at levels far too small to affect the body.

Why does it happen?

Every mask traps a small amount of air between it and the face.