The density of the air decreases as altitude increases since gravity keeps most of the air molecules close to the Earth's surface. This decrease in density means fewer gas molecules, such as oxygen, are found at higher altitudes. This corresponds to lower air pressure at high altitudes.
Temperature also decreases with altitude in the first several kilometers above the earth.
The lower temperatures, oxygen levels, and air pressures associated with high altitude can cause altitude sickness , even in physically fit people. The reduced pressure and oxygen levels cause the body to have a shortage of oxygen in the blood. To deal with this, the body produces more red blood cells in the hopes of capturing more oxygen. This condition can cause nausea, headache, weakness, hyperventilation, insomnia, and diminished alertness. Not everyone experiences all of these symptoms, and there is no way to determine if high altitudes will adversely affect someone.