[adhesive_wombat]

Does that stay true when exercising? At rest, with a tidal volume of 500ml ish, lungs are least 5/6 full of stale air, round down to 3% ish CO2.

When exercising, the tidal volumes are far higher (2-3L), and you breathe faster, so the lung air is at least twice as fresh as it was at rest, probably more, since fresh air is functionally 0% CO2, and you're moving maybe 6 times the gas volume in each direction.

Does that massive change in volume and diffusion gradient (at least at the start of a breath) means that you might be able to tolerate a higher CO2 level in the inhaled air?