Traveling to a place at a higher elevation than you're used to can make you sick. Learn the symptoms of altitude sickness and what to do if you get it. Altitude sickness, the mildest form being acute mountain sickness (AMS), is a harmful effect of high altitude, caused by rapid exposure to low amounts of oxygen at high elevation. [1][2][3] People's bodies can respond to high altitude in different ways. When you feel symptoms of altitude sickness, your body is reacting to getting less oxygen out of the thinner air that exists at higher altitudes than your body is used to. You can try multiple methods to prevent altitude sickness. The right option for you may depend on your situation. Here are the top tips from experts. What is altitude sickness? At higher altitudes, the pressure of the air around you (barometric pressure) decreases so there is less oxygen in surrounding air. People can live comfortably at moderately high altitudes, but the body must make some adjustments, and this takes time. Fatigue, insomnia, and headaches are early symptoms of altitude sickness. What is the medical definition of altitude sickness? Altitude sickness is due to a rapid ascent to higher altitudes (4800 to 11,200 ft or more) due to the decreasing amount of oxygen (low PO2) that occurs at high altitudes. Altitude sickness is the term for medical conditions that can happen when you move to a higher altitude too quickly. The higher up you go, the thinner the atmosphere gets. That means breathing in the same amount of air gets you less oxygen than at a lower altitude. Since the primary cause of altitude sickness is gaining too much altitude without acclimatizing, the best treatment is to reduce your elevation. Backtrack to spend a day at a lower elevation and wait for your mild or moderate symptoms to subside. Learn how to prevent altitude illness among international travelers visiting high altitudes. NHS information about altitude sickness, including symptoms, treatment and how to prevent it.