Hypoxia is caused by the lack of pressure of oxygen in the atmosphere at altitude. Hypoxia occurs because there is a smaller and smaller pressure differntial between the pressure of oxygen in the blood. The body, and specifically the blood, becomes less efficient in carrying oxygen to the tissues as altitude increases.

While the effects vary from person to person, these effects are more pronounced in women, who size for size, carry less hemoglobin (the oxygen transport cells in the blood) than a man of comparable size. In a normal, healthy individual, sea level air pressure is sufficient to cause the blood leaving the lungs to be almost 97% saturated with oxygen - at 10,000 ft. the saturation has dropped to almost 90% - still sufficient for nearly all usual life functions.