Elevation above sea level affects a person's oxygen levels, according to WebMD. This is because air at higher altitudes is less dense and therefore contains lower concentrations of oxygen.
Lower levels of oxygen at higher altitudes can lead to altitude sickness, particularly when altitude changes occur rapidly, says WebMD. Symptoms of altitude sickness include headache, lack of appetite, nausea, dizziness and weakness. In severe cases, lack of oxygen may affect the lungs and the brain, resulting in confusion, inability to walk a straight line or faintness. Changes in lip and fingernail color may also be due to altitude sickness. Particularly severe cases may result in death.