Although the percentage of oxygen in the atmosphere is constant, its partial pressure decreases proportionately as atmospheric pressure decreases. Above 10 000 ft, the partial pressure of oxygen gets lower and the lungs cannot effectively transfer oxygen from the ambient air to the blood to be carried to all tissues in the body. Consequently, for instance, when flying above 10 000 ft on an unpressurized aircraft or with a pressurization malfunction, oxygen deficiency in the body will most likely result in hypoxia. For best protection, you are encouraged to use supplemental oxygen above 10 000 feet MSL. At night, because vision is particularly sensitive to diminished oxygen, a prudent rule is to use supplemental oxygen when flying above 6 000 feet MSL. So, when you fly at high altitudes, supplemental oxygen is the only solution.