Altitude sickness occurs when people go to a higher altitude too quickly and do not get enough oxygen in the air they breathe. Common symptoms include having a headache and not feeling like eating.
Altitude sickness most often occurs at altitudes above 8000 ft (2438 m). In mild cases, the symptoms can be treated with over-the-counter medicine, but the best way to treat altitude sickness is to go to a lower altitude quickly.
Altitude sickness can be deadly. People with symptoms should not go to a higher altitude until the symptoms are gone. Symptoms of confusion, feeling faint, or not being able to walk in a straight line mean the condition is life-threatening.
WebMD Medical Reference from Healthwise