I don't know why it's so hard for Mask Karens to understand, that the human respiratory system was not designed to be limited by a cloth barrier like this. The bacteria trap, but also the limited airflow and the trapping of CO2 you breathe back in that takes the place of oxygen you actually need. CO2 is a poison for humans. Sure, you're inhaling it in small quantities but that adds up over time. The longer you go with a stunted blood-oxygen level, the more damage that can be done. You're robbing your cells of the oxygen they need to do what they do.