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. People can respond to high altitude in different ways. Symptoms may include headaches, vomiting, tiredness, confusion, trouble sleeping, and dizziness. Acute mountain sickness can progress to high-altitude pulmonary edema (HAPE) with associated shortness of breath or high-altitude cerebral edema (HACE) with associated confusion. Chronic mountain sickness may occur after long-term exposure to high altitude.
Altitude sickness typically occurs only above , though some are affected at lower altitudes. Risk factors include a prior episode of altitude sickness, a high degree of activity, and a rapid increase in elevation. Diagnosis is based on symptoms and is supported for those who have more than a minor reduction in activities. It is recommended that at high altitude any symptoms of headache, nausea, shortness of breath, or vomiting be assumed to be altitude sickness.
Sickness is prevented by gradually increasing elevation by no more than per day. Being physically fit does not decrease the risk. Generally, descent and sufficient fluid intake can treat symptoms. Mild cases may be helped by ibuprofen, acetazolamide, or dexamethasone. Severe cases may benefit from oxygen therapy and a portable hyperbaric bag may be used if descent is not possible. Treatment efforts, however, have not been well studied.
AMS occurs in about 20% of people after rapidly going to and in 40% of people after going to . While AMS and HACE occurs equally frequently in males and females, HAPE occurs more often in males. The earliest description of altitude sickness is attributed to a Chinese text from around 30 BCE that describes "Big Headache Mountains", possibly referring to the Karakoram Mountains around Kilik Pass.
People have different susceptibilities to altitude sickness; for some otherwise healthy people, acute altitude sickness can begin to appear at around above sea level, such as at many mountain ski resorts, equivalent to a pressure of .