The first time I ever took a non-pressurized aircraft to 10,000 feet was an interesting experience. I noticed myself breathing…not heavier, that feels like the wrong word, because I had the opposite problem to “heavy.” I needed to breathe noticeably deeper and faster just sitting still at the controls of the plane doing maybe slightly more work than typing this sentence. Somebody from a lower area going up to Denver (about half the altitude I flew to that day) to play a sport has an elevated chance of Not A Good Day.