Altitude
The human body can deal with high altitude by breathing faster, having a higher heart rate, and changing the blood itself to have more red blood cells that can carry oxygen.  This can take days or weeks to happen. This works for people who live in the high mountains, and mountain climbers.  For climbers, this is called acclimatization. Pilots and other people who go quickly to high altitudes need to be in a space suit, pressure suit, or inside a pressurized airplane, so the air is still the same as at the ground.  Above , the human body cannot make the needed changes and will eventually die.  Several of the world's tallest mountains are this high, the climbers usually take oxygen tanks with them, and only stay for a few hours at the highest levels.