High altitude is defined as heights between 1,550m and 3,500m above sea level, very high altitude ranges from 3,500m to 5,500 and extreme altitude is 5,500m and above.

As we gain altitude air pressure reduces and oxygen content reduces, this makes it harder for us to take oxygen into our body. In addition to this barometric pressure is higher at the equator than the poles so ascending Kilimanjaro near the equator will seem easier than Mt Vinson in the Antarctic.