Acute mountain sickness is an illness that can affect mountain climbers, hikers, skiers, or travelers who climb too fast. It usually occurs when people rapidly reach a high altitude (typically above 8,000 feet or 2,400 meters). 

Acute mountain sickness occurs from the combination of reduced air pressure and a lower concentration of oxygen at high altitude. Symptoms can range from mild to life threatening, and can affects the nervous system, lungs, muscles, and heart.

In most cases the symptoms are mild. In severe cases fluid collects in the lungs (pulmonary edema) causing extreme shortness of breath, which further reduces how much oxygen a person gets. Brain swelling may also occur (cerebral edema). This can cause confusion, coma, and, if untreated, death.

The chance of getting acute mountain sickness increases the faster a person climbs into a high altitude. The severity of the symptoms also depend on this factor, as well as how much the person pushed (exerted) him or herself. Persons who normally live at or near sea level are more prone to acute mountain sickness.

Approximately 20% of people will develop mild symptoms at altitudes between 6,300 to 9,700 feet, but pulmonary and cerebral edema are extremely rare at these heights. However, above 14,000 feet, a majority of people will experience at least mild symptoms. Some people who stay at this height can develop pulmonary or cerebral edema.

Symptoms generally associated with mild to moderate altitude illness include:

• Headache
• Difficulty sleeping
• Loss of appetite
• Nausea or vomiting
• Fatigue
• Dizziness or light-headedness
• Rapid pulse (heart rate)
• Shortness of breath with exertion

Symptoms generally associated with more severe altitude illness include:

• Cough
• Shortness of breath at rest
• Chest tightness or congestion
• Bluish discoloration of the skin
• Coughing up blood
• Inability to walk in a straight line, or to walk at all
• Decreased consciousness or withdrawal from social interaction
• Confusion

• Gray or pale complexion (cerebral edema)

Listening to the chest with a stethoscope (auscultation) reveals sounds called crackles in the lung, which can mean pulmonary edema.

A chest x-ray may be performed.