Acute Mountain Sickness


Inadequate time to acclimate may lead to Acute Mountain Sickness (AMS) in any traveler accending to 8,000 ft (2,500 meters) or higher. Susceptibility and resistance to altitude illness are genetic traits, and no simple screening tests are available to predict who will experinece the symptoms. Risk is not affected by training or physical fitness. How a traveler has responded to high altitudes in the past is the most reliable guide for pridicting symptoms of acute Mountain Sickness in future trips. Risk is largely influenced by rate of ascent and exertion and can be catogorized by low, moderate and high risk. See full table 2-07 at the CDC web site

Low risk

  • People with no prior history of altitude illness and ascending to <9,000 ft (2,750 meters)
  • People taking >2 days to arrive at 8,000-9,000 ft (2,500-3000 meters), with subsequent increases in sleeping elevation <1,600 ft (500 m) per day, and an extra day for acclimatization every 3,200 ft (1,000 meters)

Moderate risk

  • People with prior history of AMS and ascending to 8,000-9,000 ft (2,500-2,750 meters) in 1 day
  • No history of AMS and ascending to >9,000 ft (2,750 m) in 1 day
  • All people ascending >1,600 ft (500 meters) per day (increase in sleeping elevation) at altitudes above 9,000 ft (2,750 meters), but with an extra day for acclimatization every 3,200 ft (1,000 m)

High risk

  • History of acute mountain sickness and ascending to >9,000 ft (2,750 meters) in 1 day
  • All people with a prior history of high-altitude cerebral edema (HACE), and high-altitude pulmonary edema (HAPE).
  • All people ascending to >11,400 ft (3,500 meters) in 1 day
  • All people ascending >1,600 ft (500 meters) per day (increase in sleeping elevation) above 9,000 ft (2,750 meters), without extra days for acclimatization
  • Very rapid ascents (such as <7-day ascents of Mount Kilimanjaro)

Symptoms of Acute Mountain Sickness

To have an idea of the extent of the problem, approximately 25% of all travelers sleeping above 8,000 ft (2,500 meters) in Colorado experience acute mountain sickness. Symptoms are similar to those of an alcohol hangover. Headache is the primary symptom, sometimes accompanied by fatigue, loss of appetite, nausea, and occasionally vomiting. The onset of headache is usually 2-12 hours after arrival at a higher altitude and often is experienced during or after the first night. Symptoms of acute mountain sickness generally resolves with 24-72 hours of acclimatization.

Tips for acclimatization

  • Ascend gradually, if possible. Try not to go directly from low altitude to >9,000 ft (2,750 meters) sleeping altitude in 1 day. Once at >9,000 ft (2,750 meters), move sleeping altitude no higher than 1,600 ft (500 meters) per day, and plan an extra day for acclimatization every 3,300 ft (1,000 meters).
  • Discuss with your health care providers medication such as acetazolamide to speed acclimatization, if abrupt ascent is unavoidable.
  • Avoid alcohol for the first 48 hours.
  • Participate in only mild exercise for the first 48 hours.
  • Having a high-altitude exposure at >9,000 ft (2,750 meters) for 2 nights or more, within 30 days before the trip, is useful.


* One severe and rare consequence of altitude illness is swelling of the brain (high-altitude cerebral edema (HACE). Symptoms include extreme fatigue, drowsiness, confusion, and loss of coordination. HACE is rare, but it can be fatal. If it develops, the person must immediately descend to a lower altitude.

Swelling of the lungs, (high-altitude pulmonary edema (HAPE) is also a rare consequence of acute moutain illness. Symptoms include being out of breath, weakness, and cough. A person with HAPE should also descend and may need oxygen.