Altitude Sickness Guide:

Whether you are planning a trek to Everest Base Camp, climbing Kilimanjaro, or visiting high-altitude cities like La Paz or Lhasa — understanding altitude sickness is not optional. It is essential.

Altitude sickness affects millions of travelers every year, regardless of age, fitness level, or prior mountain experience. Even elite mountaineers are not immune. The good news? With the right knowledge, proper planning, and early recognition of symptoms, altitude sickness is largely preventable — and always manageable.

This complete altitude sickness guide covers everything you need: what causes it, how to recognize it at every stage, evidence-based prevention strategies, acclimatization schedules, treatment protocols, and the critical warning signs that demand immediate descent.

What Is Altitude Sickness?

Altitude sickness — medically known as Acute Mountain Sickness (AMS) — is a pathological condition caused by exposure to low oxygen levels (hypoxia) at high elevations. The condition occurs when you ascend too rapidly for your body to adapt to the reduced air pressure and decreased oxygen availability.

At sea level, the air you breathe contains approximately 21% oxygen at a pressure of 760 mmHg. As you ascend, the percentage of oxygen remains the same, but the atmospheric pressure drops, meaning each breath delivers fewer oxygen molecules to your lungs.

At 3,500 metres (11,500 feet), you receive roughly 65% of the oxygen available at sea level. At 5,500 metres (18,000 feet), that drops to just 50%. Your body — especially your brain — is acutely sensitive to this reduction.

Causes and Risk Factors

The primary cause of altitude sickness is ascending too fast. Your body needs time to produce more red blood cells, increase breathing rate, and make other physiological adjustments to compensate for reduced oxygen.

However, several risk factors increase your susceptibility to altitude sickness:

Interestingly, physical fitness does not protect against altitude sickness. In fact, very fit athletes sometimes ascend too rapidly, increasing their risk. Genetics play a role, but there is no reliable way to predict your susceptibility without prior altitude exposure.

Types of Altitude Sickness: AMS, HACE, and HAPE

Altitude sickness exists on a spectrum. Understanding the three distinct types is critical because HACE and HAPE are life-threatening emergencies that require immediate action.

1. Acute Mountain Sickness (AMS)

AMS is the mildest and most common form of altitude sickness. It typically begins 6–12 hours after reaching a high altitude and resolves with rest, hydration, and acclimatization — provided you do not ascend further.

2. High Altitude Cerebral Edema (HACE)

HACE is a severe, life-threatening condition where the brain swells due to fluid accumulation. It is considered the end-stage of AMS. Without prompt treatment and descent, HACE can cause coma and death within hours.

3. High Altitude Pulmonary Edema (HAPE)

HAPE occurs when fluid builds up in the lungs, severely impairing the ability to breathe. It is the most common cause of death from altitude sickness. HAPE can develop without prior AMS symptoms and may worsen rapidly during sleep.

Altitude Sickness Symptoms by Severity

Recognizing altitude sickness symptoms early is the most effective way to prevent the condition from escalating into a dangerous emergency. Here is a comprehensive breakdown by severity level:

The “walk the line” test is a simple field check for HACE: ask the person to walk heel-to-toe in a straight line. Stumbling or inability to do so is a serious red flag requiring immediate descent.

Diagnosis and the Lake Louise Score

In clinical and field settings, altitude sickness is often assessed using the Lake Louise Scoring System — a standardized questionnaire developed at a consensus conference in Lake Louise, Canada.

The score evaluates five symptoms: headache, gastrointestinal symptoms, fatigue/weakness, dizziness/lightheadedness, and difficulty sleeping. Each is scored 0–3. A total score of 3 or more in the presence of headache indicates AMS.

Prevention: The Golden Rules of Altitude Safety

The most effective treatment for altitude sickness is prevention. Follow these evidence-based golden rules to dramatically reduce your risk:

Acclimatization Schedule & Timeline

Acclimatization is the process by which your body physiologically adapts to reduced oxygen at altitude. Key adaptations include increased breathing rate, elevated heart rate, production of more red blood cells (via erythropoiesis), and increased 2,3-DPG levels in blood cells to improve oxygen release to tissues.

Here is a sample safe acclimatization timeline for trekking to Everest Base Camp (5,364m) — one of the world’s most popular high-altitude treks:

Treatment Options for Altitude Sickness

Treatment of altitude sickness depends entirely on the severity of the condition. Here is a structured approach:

For Mild AMS

Stop ascending immediately. Rest at your current altitude for 24–48 hours. Stay well-hydrated. Take Ibuprofen (600mg) or Paracetamol for headache. Avoid alcohol and sedatives. Most mild altitude sickness resolves on its own with adequate rest.

For Moderate AMS

Consider descent of 300–500 metres. Begin Acetazolamide (Diamox) if not already taking it. Use supplemental oxygen if available. Monitor symptoms every 2–4 hours. Do not ascend until symptom-free for 24 hours.

For Severe HACE

Descend immediately — minimum 1,000 metres or to below 2,400m. Administer Dexamethasone 8mg immediately, then 4mg every 6 hours. Use a Gamow bag (portable hyperbaric chamber) if descent is delayed. Supplemental oxygen at 2–4 litres per minute. Emergency evacuation by helicopter if available.

For HAPE

Descend immediately. Administer Nifedipine 30mg extended-release or 10mg immediate-release followed by 30mg extended-release. Supplemental oxygen is critical. Avoid exertion during descent — use a stretcher or helicopter if possible. Seek hospital care urgently.

Medications for Altitude Sickness

Several medications are used both to prevent and treat altitude sickness. Always consult a doctor before taking any medication for altitude purposes, as individual responses vary significantly.

When to Descend Immediately

The decision to descend is the most critical decision you may make on a mountain. The following symptoms demand immediate descent — no waiting, no sleeping it off, no hoping for improvement:

Remember: descent is the definitive treatment. Even descending 300 metres can produce a dramatic, life-saving improvement in symptoms. A Gamow bag or supplemental oxygen can buy time, but they are never substitutes for descent.

Do’s and Don’ts at High Altitude

Altitude Sickness in Popular Trekking Destinations

Altitude sickness is particularly relevant for trekkers in Nepal’s Himalayas, Peru’s Andes, and East Africa’s volcanic peaks. Understanding destination-specific risks can help you plan more safely.

Nepal (Everest Base Camp, Annapurna Circuit)

Nepal’s high-altitude treks are among the world’s most iconic — and most dangerous for altitude sickness. Everest Base Camp reaches 5,364m, while the Thorong La Pass on the Annapurna Circuit crests at 5,416m. Both require 2+ weeks of gradual acclimatization. For comprehensive information, visit Nepal Tourism Board. You can also read our internal guide on Everest Base Camp Trekking: Complete Preparation Guide for detailed advice.

Peru (Machu Picchu, Cusco, Rainbow Mountain)

Cusco sits at 3,400m — and many tourists fly directly from sea level, arriving with immediate altitude sickness symptoms. Spend 1–2 days in Cusco before visiting Machu Picchu (2,430m), which is lower and safer for acclimatization. Rainbow Mountain (Vinicunca) at 5,200m should only be visited after several days of acclimatization in the region.

Tanzania (Mount Kilimanjaro)

Kilimanjaro rises to 5,895m and is frequently climbed by non-mountaineers who underestimate altitude risks. Studies show that up to 75% of climbers on rapid routes experience AMS. The 7-day Lemosho route significantly improves acclimatization compared to the 5-day Marangu route. Consult The Altitude Medicine Initiative for up-to-date research on Kilimanjaro-specific altitude risk.

Frequently Asked Questions About Altitude Sickness