IIRC planes are pressurized to 6000' equivalent, and a quick Googling show that ...

		phkahler on March 17, 2022 \| parent \| context \| favorite \| on: In-flight surgery with a coat-hanger and silverwar... IIRC planes are pressurized to 6000' equivalent, and a quick Googling show that to be 11.7psi or about a 3 psi drop vs ground level. Also, changes of much less than that can cause discomfort in your ears if not equalized. I have no idea what 3psi does to the lungs, but with all the discussion about ventilators from 2020 I'd say that's a LOT of pressure on them.

So, the gas volume would change by a ratio of 15/12, or increase about 25%.