The air is thinner at higher altitudes, which means there is less oxygen available to breathe. The higher you go, the thinner the air becomes. This is because the air pressure decreases as you move away from the Earth's surface. At sea level, the air pressure is about 14.7 pounds per square inch (psi). At the top of Mount Everest, the highest peak in the world, the air pressure is only about 0.33 psi. This means that there is only about one-fifth as much oxygen available to breathe at the top of Mount Everest as there is at sea level.

The human body is able to compensate for the thinner air at higher altitudes by increasing its breathing rate and heart rate. However, this can only go so far. At very high altitudes, the body may not be able to get enough oxygen, which can lead to altitude sickness. Altitude sickness can cause a variety of symptoms, including shortness of breath, headaches, nausea, and dizziness. In severe cases, altitude sickness can be fatal.