There are high altitude places in my country with 8000+ feet; I understand the oxygen levels at such altitudes are only around 75% of what they would be at low-altitude. For people who are not adapted that's a huge drop in oxygen; adapting implies in part increasing erythropoietin (the hormone that increases hemoglobin production), and in healthy people this is not a problem. Therefore, the high altitude hypoxia is partly corrected by the higher hemoglobin.I didn't mean very high altitudes like K2 and Everest. More like places such as Mexico City (height 7,000+ feet) or Baguio, Philippines (height 5,000+ ft), where oxygen isn't so scarce but the CO2 concentration in the air would be higher in the lowlands.
The problem lies, for example, when a person can't take up enough oxygen due to lung disease, or when they can't generate enough erythropoietin (this is a kidney hormone), and/or if they have a very hypoxic to begin with. So they will get even more hypoxic at higher altitudes. There are several conditions that may affect the production of that hormone, and in such cases a person may not be able to correctly adapt to the high altitude hypoxia.
I totally agree.And the point I was making was that maybe most people are not really healthy, and that is why they do not benefit from the high altitude as much as a healthy person would.