The Invisible Risk Above 3,000 Metres
Altitude sickness doesn't discriminate by fitness level. Elite athletes have been brought to their knees at altitude while unfit first-timers sailed through. Understanding why this happens — and how to manage it — is one of the most important skills any high-altitude trekker can develop.
This guide focuses on practical, evidence-based strategies for acclimatization, helping you trek higher with confidence.
What Is Altitude Sickness?
As elevation increases, air pressure drops, meaning each breath delivers fewer oxygen molecules to your lungs. Your body must adapt — a process called acclimatization. When you ascend faster than your body can adapt, you develop Acute Mountain Sickness (AMS).
Symptoms of AMS include:
- Headache (the primary indicator)
- Nausea or vomiting
- Fatigue and weakness out of proportion to exertion
- Dizziness and poor coordination
- Poor sleep quality, especially Cheyne-Stokes breathing (periodic breathing at night)
In more serious cases, AMS can progress to High Altitude Pulmonary Edema (HAPE) or High Altitude Cerebral Edema (HACE) — both life-threatening conditions requiring immediate descent.
The Golden Rule: Ascend Slowly
Above 3,000 metres (approximately 10,000 feet), most wilderness medicine guidelines recommend gaining no more than 300–500 metres of sleeping altitude per day, with a rest day for every 600–900 metres gained.
The key phrase is sleeping altitude. You can hike higher during the day as long as you descend to sleep lower — this is the "climb high, sleep low" principle widely used on Himalayan expeditions.
Practical Acclimatization Strategies
1. Pre-Acclimatize Where Possible
If you're flying directly to altitude (e.g., La Paz at 3,600m or Lhasa at 3,650m), spending even two or three nights at a moderate elevation beforehand makes a measurable difference. Many trekkers flying to Nepal spend a night in Kathmandu (1,400m) before heading to higher trailheads.
2. Hydrate Consistently
Dehydration worsens AMS symptoms significantly. Aim to drink at least 3–4 litres of water per day at altitude. Reduce alcohol and caffeine intake, particularly in the first few days.
3. Eat Carbohydrate-Rich Meals
Carbohydrates require less oxygen to metabolise than fats or proteins. Shifting toward a carbohydrate-heavy diet (dal bhat, rice, bread) while at altitude supports your body's adaptation process.
4. Consider Acetazolamide (Diamox)
Acetazolamide (sold as Diamox) is a prescription medication that speeds acclimatization by stimulating faster breathing. It's widely used by trekkers on routes like Kilimanjaro and the Annapurna Circuit. Consult a travel medicine physician before your trip — it's not appropriate for everyone and has side effects including increased urination and tingling in the extremities.
Recognising When to Descend
The most dangerous mistake trekkers make is pushing through worsening symptoms. The rule is simple: if symptoms are getting worse, descend. Even dropping 300–500 metres can produce rapid, dramatic improvement. Never leave a symptomatic trekker alone, and never ascend with any AMS symptoms present.
Altitude Sickness Risk by Elevation
| Elevation | Risk Level | Examples |
|---|---|---|
| 2,400–3,000m | Low | Mont Blanc base, many Alpine huts |
| 3,000–5,500m | Moderate to High | Kilimanjaro, Everest Base Camp, Annapurna Circuit |
| 5,500m+ | Very High | High Himalayan passes, summit attempts |
The Bottom Line
Acclimatization cannot be rushed. The mountains will still be there if you take an extra rest day. The trekkers who summit successfully are almost always the ones who were patient in the valleys below.