from the last thread:

CO2 inhalation from a mask is a valid concern - if you are doing intense cardio like running, biking, lots of heavy lifting, etc. Anything that gets your breathing rate up. If you’re just walking and working a light workload, a mask isn’t going to cause any harm.

we’re probably splitting hairs here

there is so little air volume between your face and a mask that i dont think any real accumulation of significant exhaled co2 is possible. (yes im guessing)

you’re not breathing, or rebreathing only from that tiny void.

the total volume of your inhale is going to way exceed the volume of that airspace and that’s going to come from outside and through the mask

and whatever is exhaled into that small space between face and mask while containing elevated co2 is still around 16% o2

again guessing - if there’s an issue with cardio and masks it the resistance of the material and whether that means reduced airflow and/or having to divert more effort to breathing