Symptoms that are caused by decreased oxygen in the environment as a result of being significantly higher above sea level; usually above 2,400 meters or approximately 8,000 feet: "A person may get altitude sickness when an airplane suddenly goes up to a high altitude, or slowly, as when mountain climbing."

"A lack of oxygen causes headache, shortness of breath, malaise, decreased ability to concentrate, lack of judgment, lightheadedness, fainting, and when it is severe, death."

"The beginning indicator of altitude sicknesses may be euphoria, meaning individuals are unaware of the cause of the problem; so, it is important that people live in high altitudes for a period of weeks, or even months, in order to safely adapt to the high altitude conditions."