At height above 8000 m from sea level, the partial pressure of $\mathrm{O}_2$ in air is decreased. As a result less haemoglobin is formed and the person suffers from dizziness, breathlessness etc. This is called mountain sickness. A continuous exposure to this height increases ventilation to about 3 to 7 times normal then there is significant increase in RBC count and haemoglobin content in blood and breathing becomes normal.