Climb a 4,000 m pass after driving straight from sea level and you may feel fine in the morning — until the headache starts at 3 a.m. Altitude illness does not care about fitness: marathon runners get it, and seasoned mountaineers respect it. It happens because the partial pressure of oxygen drops as you climb, so every breath carries less fuel to your brain and muscles. Your body can adapt, but only if you give it time and follow a few well-proven rules.
1. What altitude does to your body
At 3,000 m, the air holds roughly 30% less oxygen than at sea level; at 4,500 m it is nearly 40% less. The immediate effects are faster, deeper breathing, a faster pulse, and — on a too-fast ascent — headache, nausea, poor sleep and loss of appetite. You also lose more moisture through faster breathing and dry air, so dehydration compounds altitude stress. Physical effort simply costs more: the same hike that felt easy at 1,500 m becomes a grind at 4,000 m, and you will bonk faster if you under-fuel.
2. The acclimatization rules that work
Acclimatization is the process by which your body builds more red blood cells and learns to breathe harder at rest. It takes days, and it cannot be rushed. Two rules dominate every high-altitude itinerary:
- Climb high, sleep low: you may hike up to a high viewpoint during the day, but always descend to sleep lower. Gaining daytime altitude stimulates adaptation without exposing you to the worst symptoms overnight.
- The ~300 m per night rule: once above ~3,000 m, do not gain more than about 300–500 m of sleeping elevation per night. After every 2–3 nights of gain, add a rest (acclimatization) day at the same elevation.
Fly directly to a high city (3,000 m+) and spend the first day resting, hydrating and doing almost nothing. Walk up gradually, and the trip goes far better than driving or flying straight to altitude.
| Elevation band | Typical effect | Planning rule |
|---|---|---|
| 1,500–2,500 m | Mild shortness of breath on exertion | No special acclimatization needed |
| 2,500–3,500 m | Noticeable effort, some breathlessness | Ascend gradually; rest if symptomatic |
| 3,500–4,500 m | AMS risk rises; sleep disturbed | +300–500 m sleeping gain/night, acclimatization days |
| 4,500–5,500 m | Significant risk; slow going | Strict climb-high/sleep-low; monitor symptoms |
| Above 5,500 m | Death zone effects; continuous risk | Only with guided, fully acclimatized teams |
3. Recognizing altitude illness: AMS, HAPE, HACE
Mild acute mountain sickness (AMS) is common and manageable; severe forms — fluid in the lungs (HAPE) and swelling in the brain (HACE) — are medical emergencies that can kill within hours. Learn the symptoms before you go, and check on each other every evening.
| Condition | Key symptoms | Action |
|---|---|---|
| Mild AMS | Headache, tiredness, mild nausea, poor sleep, dizziness on exertion | Stop ascending; rest, hydrate; do not go higher tonight |
| Moderate AMS | Worsening headache, nausea/vomiting, no appetite, breathless at rest, lousy sleep | Do not ascend further; consider early descent if not improving |
| HAPE (lungs) | Persistent cough, gurgling breath, breathless at rest, blue lips, feverish feel | Descend immediately; oxygen/portable chamber; emergency |
| HACE (brain) | Confusion, poor coordination (stumbling), altered behavior, severe headache, vomiting | Descend immediately — life-threatening |
A quick test for HACE is the "straight line" walk: ask the person to walk heel-to-toe in a straight line. If they stumble and cannot, descend immediately. A tell-tale sign that symptoms are serious is that they worsen when you go uphill to sleep, or do not improve after a rest day.
4. Prevention checklist
Most altitude illness is preventable with discipline. Before and during the trip:
- Ascend slowly above 3,000 m — this is the single most effective prevention.
- Climb high, sleep low on every acclimatization day.
- Hydrate well — urine should stay pale; altitude dehydrates you fast.
- Eat enough carbs; appetite drops but your energy needs stay high.
- Avoid alcohol and sleeping pills the first nights at altitude — they suppress breathing.
- Go slow on arrival day; no strenuous activity the first 24 h at a new elevation.
- Monitor each other; the affected person often does not notice themselves getting worse.
5. When to turn around
The hardest decision in high-altitude travel is turning back, and it is also the most important. Turn around — and descend — if anyone has a headache plus nausea that does not ease with rest, a persistent cough, trouble walking a straight line, or symptoms that get worse instead of better overnight. Summits are always there for another day; your health is not. Experienced teams carry a plan B for every pass, a turnaround time set before the climb, and the discipline to use both.
Pace your ascent and fuel your body
The Pace & Time calculator builds slow, realistic stage times that respect acclimatization days; the Calorie Calculator accounts for the extra burn at altitude so you pack enough food.