Preventing and managing altitude sickness: the complete guide to high-mountain hiking
The high mountains attract thousands of hikers and trekkers every year, seeking magnificent landscapes and new challenges. Whether you dream of reaching Kilimanjaro's summit, walking Nepal's trails, discovering the Andes or simply hiking in the Alps, one thing is certain: altitude requires preparation.
Above 2,500 metres, lack of oxygen can quickly turn a hike into a real ordeal. Headaches, unusual tiredness, nausea… These early signs of altitude sickness, also called acute mountain sickness (AMS), are often mild when addressed promptly. Ignoring them, however, can lead to more serious complications.
I experienced this while climbing Kilimanjaro. This legendary Tanzanian summit reaches 5,895 metres and presents no particular technical difficulty. Yet many hikers turn back before the summit, not because of the terrain, but because they have not acclimatised sufficiently.
In this guide, discover how to recognise the first symptoms, understand why they occur and, above all, prevent altitude sickness so you can fully enjoy your high-mountain hikes.
What is altitude sickness?
Altitude sickness, or acute mountain sickness (AMS), is the body's reaction to ascending too quickly.
Contrary to popular belief, air still contains around 21% oxygen, even at 4,000 or 5,000 metres. What changes is atmospheric pressure. The higher you climb, the lower it becomes. Air becomes less dense, so each breath supplies less oxygen to the body.
The heart and lungs must then work harder to supply the brain, muscles and other organs. When this adaptation does not happen quickly enough, the first symptoms appear.
Fortunately, our bodies can adapt. Over the days, breathing rate increases and more red blood cells are produced to improve oxygen transport. This process is called acclimatisation.
This is why a marathon runner may suffer from altitude sickness while an occasional but properly acclimatised hiker continues their trek without difficulty.
At what altitude does mountain sickness occur?
There is no precise altitude at which everyone develops altitude sickness.
For most people, the first symptoms appear between 2,500 and 3,000 metres. Some may be sensitive from 2,300 metres, while others feel no discomfort before 4,000 metres.
This sensitivity depends on many factors:
- speed of ascent;
- level of acclimatisation;
- tiredness;
- lack of sleep;
- state of health;
- dehydration.
Sensitivity to altitude sickness varies enormously between individuals, and between treks. Never having suffered from it does not guarantee you will avoid it on your next ascent. Conversely, someone who has experienced acute mountain sickness can perfectly well complete another trek successfully with better-planned acclimatisation.
What are the symptoms of altitude sickness?
The most common symptom is a headache. It generally appears a few hours after arriving at altitude or after a significant ascent.
It may be accompanied by several other signs:
- unusual tiredness;
- more breathlessness than usual;
- loss of appetite;
- nausea or vomiting;
- feeling weak;
- dizziness;
- difficulty sleeping;
- irritability.
The severity of symptoms varies between individuals. Some hikers only experience a mild headache, while others quickly become unable to continue.
Even when mild, these symptoms should never be dismissed. The biggest mistake is to keep ascending in the hope that “it will pass”. In the mountains, this choice can turn a simple headache into a genuine medical emergency.
When does altitude sickness become dangerous?
In most cases, symptoms disappear with rest, good hydration and no further ascent.
However, if you continue gaining altitude despite the first signs, acute mountain sickness can develop into more serious complications.
Oedema
In the vast majority of cases, altitude sickness is limited to headaches, tiredness or nausea. However, if symptoms are ignored and the ascent continues regardless, they may occasionally develop into more serious complications, such as high-altitude pulmonary or cerebral oedema. These situations remain rare among properly acclimatised hikers, but they underline an essential rule: if symptoms worsen, descend without delay.
These complications require descent and medical care.
How can you prevent altitude sickness?
The best way to avoid altitude sickness is to give your body time to adapt. No medication or exceptional level of fitness can replace proper acclimatisation.
Plan proper acclimatisation
If you remember just one piece of advice, make it this: do not rush.
My experience on Kilimanjaro convinced me that good acclimatisation often makes all the difference. Before our ascent, we spent a week hiking on Mount Kenya. This first part of the trip allowed us to adapt gradually to altitude. The result: 11 of 12 people reached Kilimanjaro's summit, while many trekkers who chose a shorter programme had to turn back before Uhuru Peak.
Of course, every body reacts differently.
In my personal experience, which is obviously not a universal rule, you need at least four days' acclimatisation before considering several days of hiking above 3,500 metres. Some people need more time, while others acclimatise faster.
Ascend gradually
The principle is simple: the more gradually you ascend, the more time your body has to adapt.
From around 3,000 metres, try to limit increases in sleeping altitude as much as possible. A rise of around 500 metres between two nights is a useful guideline where the route allows. If you must ascend further, plan an extra day at the same altitude where possible to aid acclimatisation.
Climb high, sleep lower
In the mountains, a commonly recommended principle is: “climb high, sleep low”.
In practice, this means gaining altitude during the day before descending a few hundred metres to spend the night. This method promotes acclimatisation while limiting the risk of symptoms appearing during sleep.
Headaches associated with altitude sickness often tend to be worse when lying down. If the route allows, sleeping slightly below the day's highest point generally helps you recover better.
Keep a steady pace
At altitude, performance takes second place. The aim is to maintain a steady effort without excessive breathlessness.
Walk at your own pace, take regular breaks and do not wait until you are completely exhausted to recover. If you feel your breathing accelerate or your heart race, slow down. A steady pace is often more effective than a series of intense efforts.
Eat and drink well
Altitude promotes dehydration. The air is drier, breathing speeds up and water loss increases. It is therefore important to drink regularly, even if you are not thirsty.
Food also plays an important role. Loss of appetite is common at altitude, but your body uses more energy to adapt to the lack of oxygen. Keep eating enough to maintain your physical abilities throughout the trek.
Adapt the group's pace
One of the most common mistakes is to walk at the pace of the fastest members.
In the mountains, the opposite is needed. The group should move at the pace of the person most sensitive to altitude. Forcing a hiker to keep up with an unsuitable pace can quickly turn a simple headache into a much more serious problem.
Better to slow the group down than have to stop the trek entirely.
Acclimatisation does not last forever
You sometimes hear that acclimatising a few weeks before a trek is enough to avoid problems.
In reality, physiological adaptations gradually disappear when you return to low altitude. Acclimatisation a month before departure will therefore have practically no effect left.
To be truly useful, it needs to take place immediately before the trek or at its start.
What should you do if the first symptoms appear?
Despite every precaution, you may still experience the first signs of altitude sickness.
The first thing to do is simple: stop ascending.
Take time to rest, hydrate properly and observe how symptoms develop. In many cases, stopping for a few hours or spending an extra night at the same altitude allows your body to adapt.
However, if symptoms persist, worsen or return as soon as you start moving up again, you must descend.
A descent of a few hundred metres is often enough to bring rapid relief.
The hardest part is sometimes accepting that you must turn back. Yet continuing to ascend despite persistent symptoms is the main cause of worsening altitude sickness.
On some very-high-altitude expeditions, guides also carry oxygen cylinders or a portable hyperbaric chamber. These are for emergencies and help stabilise someone while awaiting evacuation. They are never a means of continuing the ascent despite altitude sickness.
The most common mistakes
Some mistakes crop up regularly among hikers:
- wanting to reach the summit at any cost;
- ascending too quickly during the first few days;
- ignoring the first symptoms;
- sleeping much higher each night than the night before;
- not drinking enough;
- keeping up with the fastest members of the group.
Yet these mistakes are easy to avoid with good preparation.
Conclusion
Altitude sickness can affect any hiker, regardless of ability or fitness. The best prevention remains gradual ascent and appropriate acclimatisation.
Before departure, take time to study your route and, if possible, allow several days for acclimatisation. Once in the mountains, walk at a steady pace, stay hydrated, listen to your body and never try to ignore the first symptoms.
Finally, remember two essential principles.
First: in a group, always walk at the pace of the person most sensitive to altitude.
Second: if in doubt, stop ascending. Descend. A mountain will still be there tomorrow. Your health, however, deserves to remain the priority.
FAQ
At what altitude does altitude sickness occur?
For most people, the first symptoms appear between 2,500 and 3,000 metres. However, some may be sensitive from 2,300 metres, while others feel no symptoms before 4,000 metres.
Does being athletic protect against altitude sickness?
No. Excellent fitness does not prevent acute mountain sickness. Acclimatisation remains the most important factor.
How long does acclimatisation take?
It depends on the individual. In my experience, four days is a good minimum before considering several days above 3,500 metres, but some people need more time.
What should you do if symptoms appear during a trek?
Stop ascending, rest, hydrate and monitor symptoms. If they persist or worsen, descend immediately.
Can you take Diamox to prevent altitude sickness?
Diamox may be prescribed in certain situations, but only following medical advice. It never replaces good acclimatisation and must not be used to mask symptoms so you can continue ascending.
Is acclimatisation several weeks before departure effective?
No. The body's adaptations gradually disappear when you return to low altitude. To be effective, acclimatisation must take place immediately before or during the trek.