Travelling at altitude without ruining your trip

This article is general information written for travellers, not medical advice. Altitude illness can be fatal. Speak to a doctor before a high-altitude trip, particularly if you have any heart or lung condition, and follow the advice of local guides over anything you read online.

Altitude catches people out because the trigger is invisible and the timing is delayed. You land in Cusco or La Paz or Leh feeling completely normal, walk up one flight of stairs, and discover that your body has opinions. Six hours later you have a headache that paracetamol does not touch.

Almost all of this is preventable, and almost none of it is about fitness.

What is actually happening

The percentage of oxygen in the air does not change with altitude - it is about 21% at sea level and about 21% on Everest. What changes is pressure. At 3,500 metres, air pressure is around two thirds of sea level, so each breath delivers roughly a third less oxygen. At 5,500 metres it is about half.

Your body compensates: you breathe faster, your heart rate rises, and over days you produce more red blood cells. That adaptation is what acclimatisation means, and it takes time that cannot be shortened by being fit. Marathon runners get altitude sickness. So do people who have been to altitude before without problems. Susceptibility is largely genetic and it is not a measure of anything.

The numbers that matter

• Below 2,500 m: most people feel nothing.
• 2,500–3,500 m: mild symptoms are common. Cusco is 3,400 m, Quito 2,850 m, Bogotá 2,640 m, Addis Ababa 2,355 m.
• 3,500–5,500 m: acute mountain sickness is likely without proper ascent. La Paz sits around 3,600 m, Leh 3,500 m, the Pamir Highway crosses 4,655 m, Everest Base Camp is 5,364 m.
• Above 5,500 m: no one acclimatises indefinitely. The body degrades over time at that height.

The rules that prevent most problems

1. Climb high, sleep low. Sleeping altitude is what counts. Walking to 4,500 m and sleeping at 3,900 m is fine; sleeping at 4,500 m the first night is not.
2. Above 3,000 m, raise your sleeping altitude by no more than 300–500 m per night, with a rest day every 1,000 m or so.
3. Take the first day off. If you fly into a high city, do almost nothing for 24 hours. No hiking, no heavy lunch, no drinking.
4. Drink far more water than feels necessary. Dry air and fast breathing dehydrate you quickly, and dehydration mimics and worsens altitude symptoms.
5. No alcohol for the first 48 hours, and be careful with sleeping tablets, which suppress breathing.
6. Eat carbohydrates. Appetite disappears at altitude; eat anyway.
7. Walk slower than feels natural. Pole pole, as every Kilimanjaro guide says roughly four hundred times in five days. On summit night you will pass the fast group sitting down in the dark.

Recognising acute mountain sickness

AMS is common, usually mild, and entirely manageable if you respect it. The classic signs are headache plus at least one of: nausea or loss of appetite, fatigue out of proportion to effort, dizziness, and difficulty sleeping. It typically appears 6 to 12 hours after arriving at a new altitude.

The response is simple: stop ascending. Rest, drink, eat, and give it a day. If it improves, carry on. If it does not, go down. Ascending with symptoms is the mistake that turns a bad night into an emergency.

The two conditions that are emergencies

Both are uncommon and both are life-threatening. Descent is the treatment for both, and it should not be delayed until morning.

HACE (high altitude cerebral oedema): confusion, unusual behaviour, severe headache, and - the classic test - an inability to walk heel-to-toe in a straight line. Someone with HACE may not recognise their own condition, which is why companions matter more than self-assessment.

HAPE (high altitude pulmonary oedema): breathlessness at rest, a persistent cough, tightness in the chest, and extreme fatigue. Breathlessness that does not settle after several minutes of rest is the warning sign.

For either: descend immediately, by at least 500–1,000 m, at night if necessary. Supplementary oxygen and medication help but do not replace descent.

Medication, honestly

Acetazolamide speeds acclimatisation and is widely prescribed for high-altitude trips. It is a genuine tool, not a licence to ascend faster, and it has side effects including tingling in the fingers and a strange metallic taste in fizzy drinks. It requires a prescription and a conversation with a doctor, ideally weeks before you travel.

Coca tea and coca leaves are traditional across the Andes and locals will offer them constantly. They help mildly with symptoms. They do not acclimatise you, and they can show up on a drug test at home.

Planning a trip so altitude is not the problem

Sequence matters more than anything else. In Peru, spending a few days in the Sacred Valley - which is several hundred metres lower than Cusco - before returning to the city is far better than the reverse, and it is what experienced operators do. In Nepal, the standard Everest itineraries build in acclimatisation days at Namche and Dingboche precisely because the arithmetic demands it, and the trekkers who get into trouble are usually the ones who skipped them to save time.

Build in slack. Two spare days in a two-week high-altitude trip is not lost time - it is the thing that lets you stop for a day without abandoning the trip.

The small stuff nobody warns you about

• UV increases roughly 10% per 1,000 m, and snow reflects it. Sunburn at altitude happens in an hour, including under your chin.
• Sealed packets inflate and sometimes burst. So do shampoo bottles.
• Water boils below 100°C, so pasta and eggs take noticeably longer.
• Sleep is genuinely worse. Periodic breathing at night is normal and unnerving.
• Alcohol hits considerably harder. One drink behaves like two or three.
• Everything takes longer, including getting dressed. Plan shorter days.

Handled properly, altitude is a scheduling problem rather than a health problem. Give it the days it asks for and the mountains are yours.