Two friends fly into a high mountain town for a trek. One spends a couple of days wandering the streets and drinking water; the other, fit and eager, sets off uphill the morning they land. By that evening it's the fit one in bed with a pounding head, nausea and dizziness, while the "lazy" one feels fine.
That's the thing almost everyone gets wrong. Marathon runners get altitude sickness; sedentary travellers sometimes sail through. What matters is giving your body time to adjust to thinner air, and noticing the early signs in time to act.
What actually happens up high
As you climb, the air thins and each breath delivers less oxygen. Your body adapts remarkably well, but it needs time. Push too high too quickly and you outrun that adaptation — which is when altitude sickness sets in. It typically becomes a real consideration above roughly 2,500 metres (about 8,000 feet), and the higher and faster you go, the greater the risk.
The mildest and most common form is acute mountain sickness, which feels a lot like a hangover: headache, nausea, fatigue, dizziness and poor sleep. Most cases are mild and pass with rest. Ignored, a small number progress to serious conditions, which is why the early signs deserve respect rather than gritted teeth.
The golden rule: climb high, sleep low
The single most effective prevention is a slow ascent. Mountaineers live by "climb high, sleep low" — go up during the day, but sleep at a more modest elevation so your body recovers overnight. Once you're high, a common guideline is to avoid raising your sleeping altitude by more than a few hundred metres per night, with a rest day for every roughly thousand metres gained.
The basics help too: drink plenty of water, go easy on alcohol the first days, eat well, and don't overexert early on. Some travellers also discuss preventative medication with a doctor before a high-altitude trip — a conversation worth having in advance rather than improvising on the mountain.
Estimate your risk
This is a rough gauge based on the altitude you're heading to and how quickly you'll get there — not a medical assessment.
Altitude risk → destination height + ascent speed
A guide only — individual susceptibility varies a lot, so treat this as a prompt to plan and acclimatise, not a verdict.
When to stop, and when to go down
The most important rule is also the simplest: if you have symptoms, don't go higher. Rest where you are and let your body catch up. If symptoms get worse, or you develop anything beyond a mild headache and tiredness — severe breathlessness at rest, confusion, trouble walking straight, a persistent cough — descend, without delay. Going down even a few hundred metres often brings rapid relief, and descent is the one treatment that reliably works. No view, summit or schedule is worth pushing through serious symptoms.
Go deeper
Questions
Does being fit protect me?
No — fitness makes no real difference. Very fit people get altitude sickness too, sometimes worse because they push harder and climb faster. What protects you is a slow ascent, not strong legs.
At what altitude does it usually start?
It becomes a real consideration above roughly 2,500 metres (about 8,000 feet), and risk rises the higher and faster you go. Susceptibility varies, so some feel it lower and others higher.
What are the warning signs?
Mild altitude sickness feels like a hangover: headache, nausea, fatigue, dizziness and poor sleep. Signs that need action include worsening symptoms, severe breathlessness at rest, confusion, trouble walking straight, or a persistent cough — descend if these appear.
How do I prevent it?
Ascend slowly — limit how much you raise your sleeping altitude each night, build in rest days, and follow "climb high, sleep low." Stay hydrated, go easy on alcohol early, eat well, and don't overexert in the first days. Some travellers discuss preventative medication with a doctor beforehand.
What if I get symptoms?
Don't go any higher — rest where you are. If symptoms worsen or become severe, descend without delay; going down even a few hundred metres usually brings rapid relief and is the one reliable treatment. Seek medical help for serious symptoms.
Can I fly straight to a high-altitude city?
You can, but it's exactly the scenario that causes trouble, because you skip the gradual ascent. If you fly into somewhere high, plan a couple of easy days to acclimatise before any strenuous activity or going higher, and watch for symptoms.
This is general information, not medical advice, and the estimator is a rough guide only. Altitude affects everyone differently. Consult a doctor before high-altitude travel, especially if you have any heart, lung or other condition, and seek medical help promptly if serious symptoms develop.