I grew up in Santa Fe, NM as a kid, which is about 7000 feet altitude. My adult life has been in flatlands mainly so have always been popping from low flatlands to higher Northern New Mexico and Colorado. For what ever reason, when transitioning from the low to high altitude I have two breakpoints where a few days of adaptation benefit me. The first is at 7000 feet, which is largely regaining stamina and the second hits at 10,000-11,000 feet where functioning at a greater rate than a potato takes a few days of adaptation. 13,000+ feet the high altitude impacts hits me hard with respect to stamina and I have never been at this level long enough to adapt.
Where we (Colorado) see a lot of this is during ski season. Folks from the east coast / low lands, hop on an early morning flight, land in Colorado by noon, grab a shuttle to the ski resort and by 2 or 3 in the afternoon are settled in.
That is when they start thinking that there is enough light left to get a couple of runs in and hit the slopes. When the ski lift takes them to the top of the mountain, they have gone from sea level to 12,000 feet in the space of 8 to 12 hrs or so. And then start doing some heavy exercise (yes skiing can take it’s toll) and end up blowing out a vein in their lungs. Quick way to ruin a vacation.
I ALWAYS tell visitors to give yourself a day or two to at least adjust somewhat to the altitude and drink more water as the air is drier and you will loose fluid through your lungs.
But nobody listens to me.