There was a paper trying to measure that, but it has been withdrawn.So if it slows your exhaling breath (this leaving more Carbon Dioxide close to your face) it must also slow your inhale. Therefore drawing less oxygen and more carbon dioxide (since its somewhat “trapped” between your mouth and the mask) with every breath.
I’m sure that oxygen deprivation (however slight it may be) is great for kids in a learning environment much less an athletic one.
You can do a swag. A normal resting breath is about 0.5 liters. If your mask traps 100ml of air, then you’re only getting 80% new air.
The rebreathing effect is why I went with lightweight cloth instead of N95. The cloth mask fits tight across my face and doesn’t feel stuffy because it traps a lot less air.
I mostly just avoid any situation where an N95 would be appropriate. If I want to talk to someone, we meet outside.