People living at sea level have around 5 million RBC per cubic millimeter of their blood whereas those living at altitude to 5400 metres have around 8 million. This is because at high altitude
people eat more nutritive food, therefore more RBCs are formed
people get pollution-free air to breath and more oxygen is available
atmospheric O2 level is less and hence more RBCs are needed to absorb the required amount of O2 to survive
there is more UV radiation which enhances RBC production.
