Air seems thinner at high altitudes because air pressure is lower, which allows individual air molecules to occupy a larger volume than air molecules do at low altitudes. This decreasing pressure with increasing altitude occurs because there are increasingly fewer air molecules exerting pressure on the next-lowest altitude.
On the Earth's surface, the air pressure is highest because it bears the combined weight of the air molecules in the atmosphere. When a human travels unprotected through higher and higher altitudes, the amount of breathable air necessary to sustain normal functioning becomes difficult for the lungs to capture as the air molecules become increasingly dispersed. Once the air pressure outside the body becomes lower than is necessary for the lungs to process the incoming air, especially if the air contains an insufficient oxygen concentration, a person can suffocate.
An insidious condition called hypoxia, which occurs when the air contains an inadequate concentration of oxygen, can begin to manifest in the human body at altitudes exceeding 10,000 feet above sea level. The lack of a sufficient concentration of oxygen above this altitude begins to cause impairments to normal functioning. This impairment can begin with dizziness, nausea and the appearance of a blue coloration to the nail beds and the lips, which if left untreated can lead to loss of consciousness and death.