I don't know if it actually applies to a facemask with unidirectional flow (on at the top, out at the bottom). But the CO2 buildup mechanism is sometimes descri

I don't know if it actually applies to a facemask with unidirectional flow (on at the top, out at the bottom). But the CO2 buildup mechanism is sometimes described as a reason for medical facemasks to be close-fitting. When you exhale into a mask with a big space, and then inhale again, you (very roughly) first inhale everything that was in the mask before inhaling new air. The extreme example is breathing from a long skinny tube. If the volume of the tube is bigger than that of your lungs, you never inhale new air.