It is plain ole air served at a different pressure. The vast majority do not feel the difference - that is because they are sitting down and when they do walk it is at very slow pace. So the reduced PPO2 is not an issue. However, those who have a compromised system (pneumonia, emphysema, etc) though sitting could well have issues and for them on-board O2 at a low flow rate will be needed.

Around here in Oootah this time a year we have what is called the Texas heart attack. A skier comes from sea level, hits the ski slopes at 7k-10k feet, does a few runs, feels weak, shortness of breath, etc. Thinks they are having a heart attack - nope just did not acclimate very well. They get sent down to the valley.

Similarly many sports teams like basketball teams will come to Oootah and stay up in Park City the night before a game to acclimate - even 12 hours is helpful for the body to respond to a change altitude.