What is altitude sickness?

Altitude sickness (also called Acute Mountain Sickness or AMS) is a group of symptoms that can occur when you ascend to high altitudes too quickly and your body doesn’t have enough time to adjust to lower oxygen levels. It is the most common health issue on Mount Kilimanjaro and the main reason people fail to summit.

Altitude sickness (AMS) is brought about by climbing to high altitudes where there is little air pressure than required. There is a great calculator here https://baillielab.net/critical_care/air_pressure/ that calculates atmospheric pressure in reference to altitude.

When you reach the peak of Mount Kilimanjaro air pressure decreases to a mere 49%. This means that every air that you inhale, contains about half amount of oxygen. Due to the respiratory strain that this brings to your respiratory system, it is advised to take it slow on the mountain. As you do not want fluid to build up in your lung cavities causing pneumonia and the gap between the brain and the skull causing severe headaches and migraines. The result can be fatal at times.

With great emergency precautions and great acclimatization practices, we will help minimize or avoid altitude sickness (AMS). Read more here about altitude sickness and acclimatization rules.

Why It Happens

At higher altitudes (especially above 2,500–3,000 m / 8,200–9,800 ft), air pressure drops and there is less oxygen per breath. Your body needs time to adapt by breathing faster, producing more red blood cells, and making other changes. If you ascend too fast, these adaptations lag, leading to symptoms.

Common Symptoms (Mild AMS)

Headache (often the first sign)
Nausea or vomiting
Fatigue or weakness
Dizziness or lightheadedness
Shortness of breath (especially with exertion)
Loss of appetite
Trouble sleeping

Symptoms usually start 6–24 hours after reaching higher altitude and can feel like a bad hangover.

Serious Forms (Rare but Dangerous)

HAPE (High Altitude Pulmonary Edema) — Fluid in the lungs. Symptoms: severe shortness of breath (even at rest), cough (sometimes with frothy or bloody sputum), chest tightness.
HACE (High Altitude Cerebral Edema) — Brain swelling. Symptoms: severe headache, confusion, loss of coordination (ataxia), hallucinations, or unconsciousness.

These can be life-threatening and require immediate descent.

Acclimatization for Mount Kilimanjaro

On Kilimanjaro Specifically

Most climbers experience some mild symptoms.
Risk increases rapidly above 4,000–5,000 m.
Shorter routes (5–6 days) have much higher rates of severe sickness than longer ones (7–9 days).

Prevention (Most Important)

Slow ascent — “Climb high, sleep low.” Longer routes give your body time to acclimatize.
Hydration — Drink 3–5+ liters of water daily.
Pace — Go slow (“pole pole”) and don’t overexert.
Medications — Acetazolamide (Diamox) can help prevent symptoms (consult a doctor).
Listen to your body — Never push through worsening symptoms—descend if needed.

Most cases are mild and resolve with rest, hydration, or descent. With good preparation and a reputable operator (who monitors health daily), serious altitude sickness is very avoidable. It’s the main reason longer itineraries have much higher summit success rates.