Altitude Sickness is a pathological condition caused by exposure to low partial pressure of oxygen at high altitudes. Some people are more susceptible than others to the condition. It generally occurs above 2,400 meters, though some may experience it at altitude as low as 1800 meters, and some may not experience it until over 4000 meters.
The percentage of oxygen in the atmosphere at sea level is about 21%. As altitude increases, the percentage of oxygen remains the same. What changes is the density of the air – hence the saying that the air gets thinner at higher altitudes. The number of molecules per given volume, or in our case, per breath, drops as altitude increases. At over 3,000 meters, the barometric pressure is just over 60% of that of the sea level. In other words, the human body gets roughly 40% less oxygen per breath compare to when at sea level.