Altitude sickness can cause a range of symptoms, which typically begin within 6 to 24 hours after ascending to high altitudes—usually above 2,500 meters (8,200 feet). Symptoms can be mild at first but may become more serious if ignored or if the person continues to ascend without proper acclimatization.

The most common early symptom is a headache, which may feel dull or throbbing and is often not relieved by usual pain medication. Many people also experience nausea, vomiting, or a general loss of appetite. Dizziness or lightheadedness is common, particularly when standing up or moving around. Fatigue is often present and may not improve with rest. People may notice shortness of breath during physical activity, even if the activity is not strenuous.

Sleep disturbances are also frequent and may include difficulty falling asleep, frequent waking, or vivid dreams. Swelling of the hands, face, or feet (known as peripheral edema) can occur due to fluid retention. Other signs include a rapid heartbeat, general weakness, and irritability or feeling mentally "foggy."

In more severe cases, altitude sickness can progress to high-altitude cerebral edema (HACE), a potentially fatal condition involving swelling of the brain. Symptoms of HACE include severe, persistent headache, loss of coordination or balance (ataxia), confusion, slurred speech, drowsiness, visual disturbances, and in some cases, hallucinations or unconsciousness. HACE requires immediate descent and emergency medical care.

Another life-threatening form is high-altitude pulmonary edema (HAPE), which involves fluid buildup in the lungs. Symptoms of HAPE include shortness of breath even while resting, a dry or productive cough that may produce pink, frothy sputum, chest tightness or congestion, and extreme fatigue. Breathing may become noisy or labored, and lips or fingertips may appear blue or gray due to lack of oxygen. A person with HAPE may be unable to walk or speak clearly and can rapidly deteriorate without treatment.

If any of the severe symptoms of HACE or HAPE occur—such as confusion, difficulty walking, severe breathlessness at rest, or loss of consciousness—it is a medical emergency. Immediate descent and oxygen therapy are essential for survival.

Altitude sickness occurs when the body cannot adjust quickly enough to the **lower oxygen levels and reduced air pressure** found at **high altitudes**, usually above **2,500 meters (8,200 feet)**. As you ascend, the air becomes thinner, meaning each breath contains fewer oxygen molecules. This results in less oxygen reaching the brain, muscles, and other organs.

The primary cause of altitude sickness is **ascending too quickly** without allowing the body time to **acclimatize**. Rapid ascent doesn't give your body enough time to adapt to the changing environment, leading to symptoms such as headache, nausea, and fatigue.

Other contributing factors include **overexertion shortly after arrival**, **dehydration**, **alcohol consumption**, and **underlying medical conditions** such as lung or heart disease. Sleeping at a high altitude, especially on the first night, also increases the risk of symptoms. While physical fitness can help with endurance, it does **not protect against altitude sickness**—even healthy, athletic individuals can be affected.

Frequently Asked Questions