What is altitude sickness?

If you’ve ever hiked up a mountain and felt yourself getting nauseous or lightheaded, you may have experienced altitude sickness, also called mountain sickness. This condition happens when you travel to a high altitude (elevation) too quickly.

It doesn’t happen only to hikers. Just visiting a high-altitude location can cause problems for some. Symptoms happen when your body tries to adjust to the lower air pressure and lower oxygen levels at high altitudes.

Who is at risk for altitude sickness?

Anyone can get altitude sickness. Your age, sex and general health don’t seem to affect your risk. You may be at higher risk if you:

  • Have a lung or heart condition: Your healthcare provider may recommend avoiding high altitudes if possible.
  • Are pregnant: Talk to you provider before traveling to a high-altitude location.
  • Live at low elevation: Since your body isn’t used to higher altitudes, you have a greater risk for symptoms. If you’re planning a trip to a high-altitude location, be aware of the symptoms of altitude sickness and how to treat it.
  • Previously had altitude sickness: Talk to your provider about prevention and treatment before your next trip.

What is considered a “high elevation” in terms of getting altitude sickness?

Climbing to these elevations can bring on symptoms of altitude sickness:

  • High altitude: 8,000 to 12,000 feet above sea level.
  • Very high altitude: 12,000 to 18,000 feet.
  • Extremely high altitude: 18,000+ feet.

For context, New York City is at an elevation of 33 feet above sea level. Denver (known as the “Mile High City”) sits at 5,000 feet, and many Rocky Mountain ski slopes are at 11,000 feet or higher. The Grand Canyon is 6,600 feet above sea level. The summit of Mount Everest is over 29,000 feet.

How common is altitude sickness?

Altitude sickness may occur in up to half of people who climb to elevations above 8,000 feet.

What are the different forms of altitude sickness?

Most people who get altitude sickness get AMS, acute mountain sickness. Higher than 10,000 feet, 75% of people will get mild symptoms . There are three categories of AMS:

  • Mild AMS: Symptoms, such as mild headache and fatigue, don’t interfere with your normal activity. Symptoms improve after a few days as your body acclimates. You can likely stay at your current elevation as your body adjusts.
  • Moderate AMS: Symptoms start to interfere with your activities. You may experience severe headache, nausea and difficulty with coordination. You’ll need to descend to start to feel better.
  • Severe AMS: You may feel short of breath, even at rest. It can be difficult to walk. You need to descend immediately to a lower altitude and seek medical care.

Two severe forms of altitude illness occur less frequently but are more serious. Both can be life-threatening. You need to descend immediately and receive medical treatment for:

  • HAPE (High-altitude pulmonary edema): HAPE produces excess fluid on the lungs, causing breathlessness, even when resting. You feel very fatigued and weak and may feel like you’re suffocating.
  • HACE (High-altitude cerebral edema): HACE involves excess fluid on the brain, causing brain swelling. You may experience confusion, lack of coordination and possibly violent behavior.

What causes altitude sickness?

Altitude sickness results from a rapid change in air pressure and air oxygen levels at higher elevations. You may have symptoms if you travel to a high elevation without giving your body time to adjust to less oxygen. Even if you’re physically fit, you can still experience altitude sickness.

In addition, high altitude and lower air pressure can lead to fluid leaking from blood vessels. Researchers don’t understand exactly why this happens. This leakage causes fluid to build up in your lungs and brain. Ignoring moderate or severe symptoms can lead to a life-threatening situation.

What are the symptoms of altitude sickness?

You’ll likely feel nauseous and lightheaded. You may vomit and have a headache. Different levels of altitude sickness have different symptoms:

Symptoms of mild, short-term altitude sickness usually begin 12 to 24 hours after arriving at high altitude. They lessen in a day or two as your body adjusts. These symptoms include:

  • Dizziness.
  • Fatigue and loss of energy.
  • Shortness of breath.
  • Loss of appetite.
  • Sleep problems.

Symptoms of moderate altitude sickness are more intense and worsen instead of improve over time:

  • Worsening fatigue, weakness and shortness of breath.
  • Coordination problems and difficulty walking.
  • Severe headache, nausea and vomiting.
  • Chest tightness or congestion.
  • Difficulty doing regular activities, though you may still be able to walk independently.

Severe altitude sickness is an emergency. The symptoms are similar to moderate AMS, but more severe and intense. If you start experiencing these symptoms, you must be taken to a lower altitude immediately for medical care:

  • Shortness of breath, even when resting.
  • Inability to walk.
  • Confusion.
  • Fluid buildup in the lungs or brain.

HAPE, when fluid builds up in the lungs, prevents oxygen from moving around your body. You need medical treatment for HAPE. Symptoms include:

  • Cyanosis, when your skin, nails or whites of your eyes start to turn blue.
  • Confusion and irrational behavior.
  • Shortness of breath even when resting.
  • Tightness in the chest.
  • Extreme fatigue and weakness.
  • Feeling like you’re suffocating at night.
  • Persistent cough, bringing up white, watery fluid.

HACE happens when the brain tissue starts to swell from the leaking fluid. You need medical treatment for HACE. Symptoms include:

  • Headache
  • Loss of coordination.
  • Weakness.
  • Disorientation, memory loss, hallucinations.
  • Psychotic behavior.
  • Coma.

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy