Most people know about mountain or altitude sickness—but they may not know about the more serious forms of it, or why it’s so important to descend immediately when symptoms appear.

High altitude pulmonary edema (HAPE) is a rare and serious medical condition that most commonly affects mountaineers who venture above 8,200 feet. It is caused by the failure of the body to adapt quickly enough to the decrease in oxygen levels at high altitudes, which can lead to fluid accumulation in the lungs. This fluid makes it difficult for the lungs to exchange oxygen and carbon dioxide and, in severe cases, can lead to collapse of the lungs and death.

HAPE often starts with mild symptoms such as headache, nausea, vomiting, general fatigue, and dizziness. Other symptoms may include a dry cough and low-grade fever. As the condition progresses, symptoms such as shortness of breath, chest tightness, and cough become more severe. It is important to be aware of the symptoms of HAPE so that you can get help immediately—HAPE can become life-threatening in only a few hours—and prevent respiratory failure.

The best way to prevent HAPE is to ascend slowly to high altitudes, allowing your body time to adjust. If you are planning to climb a mountain or ski in a high-altitude area, be sure to bring plenty of water and snacks so that you don’t have to rush the ascent. If you experience HAPE symptoms, stop ascending and return to a lower altitude immediately.

Treatment for HAPE includes rest, hydration, and oxygen therapy as needed. In some cases, drugs such as nifedipine may be prescribed to help improve blood flow through the lungs. If caught early, HAPE can be reversed and there are usually no long-term consequences. However, if left untreated, HAPE can be fatal.

Here are a few tips on how to prevent and treat HAPE:

• Ascend slowly to give your body time to adjust.  
• Stay hydrated.  
• Eat light meals rich in simple sugars and carbohydrates. This will help your body burn oxygen more efficiently.   
• Know your limits: If you feel sick, don’t push yourself harder. Descend to a lower altitude where you feel better.  
• Listen to your body. Pay attention to how you’re feeling and don’t ignore warning signs like shortness of breath, lightheadedness, or headache.  
• Acclimatize. Altitude sickness expert Dr. Peter Hackett advises climbers to sleep at least 100 meters or 300 feet higher than they did the night before so their bodies have time to adjust while they rest.  
• If you start feeling ill, especially if you develop a headache or coughing fit, act quickly. HAPE can progress very rapidly.

Click to read more about acute mountain sickness and HACE, high altitude cerebral edema.