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's bodies can respond to high altitude in different ways. Symptoms of altitude sickness 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 2,500 metres (8,000 ft), though some people 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. Being physically fit does not decrease the risk. 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 300 metres (1,000 ft) per day. 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. The only definite and reliable treatment for severe AMS, HACE, and HAPE is to descend immediately until symptoms resolve. Other treatment efforts have not been well studied.

AMS occurs in about 20% of people after rapidly going to 2,500 metres (8,000 ft) and in 40% of people after going to 3,000 metres (10,000 ft). While AMdS 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 2,000 metres (6,600 ft) above sea level, such as at many mountain ski resorts, equivalent to a pressure of 80 kilopascals (0.79 atm). This is the most frequent type of altitude sickness encountered. Symptoms often manifest within ten hours of ascent and generally subside within two days, though they occasionally develop into the more serious conditions. Symptoms include headache, confusion, fatigue, stomach illness, dizziness, and sleep disturbance. Exertion may aggravate the symptoms.^{[citation needed]}

Those individuals with the lowest initial partial pressure of end-tidal pCO₂ (the lowest concentration of carbon dioxide at the end of the respiratory cycle, a measure of a higher alveolar ventilation) and corresponding high oxygen saturation levels tend to have a lower incidence of acute mountain sickness than those with high end-tidal pCO₂ and low oxygen saturation levels.

Headaches are the primary symptom used to diagnose altitude sickness, although a headache is also a symptom of dehydration.^{[citation needed]} A headache occurring at an altitude above 2,400 metres (7,900 ft) – a pressure of 76 kilopascals (0.75 atm) – combined with any one or more of the following symptoms, may indicate altitude sickness:

Symptoms that may indicate life-threatening altitude sickness include: