Causes of Altitude sickness (acute mountain sickness - AMS)

At high altitude, atmospheric pressure drops: less oxygen is available to the body. The amount of oxygen available at 3,000 meters is two-thirds that available at sea level. At 5,000 meters, it’s half that. The body reacts to this lack of oxygen (or hypoxia) in two ways:

  • Immediate reaction

Acceleration of ventilation and heart rate, to capture more oxygen from the air and transport it more rapidly to the organs.

This reaction is costly for the body in terms of energy, as it makes the respiratory muscles and heart work harder.

  • Progressive reaction

The immediate reaction is replaced or supplemented by a more economical mechanism: increasing the number of oxygen carriers, i.e. red blood cells.

Red blood cells take a long time to produce, and you need to spend at least a week at altitude to see their numbers increase.

Ventilation rate and heart rate can then slow down, without returning to baseline values.

Who is affected by altitude sickness?

Altitude sickness (or soroche in the Andes) is thought to affect around:

  • 15% of people above 2,500 metres
  • 60% of people above 4,000 metres
  • everyone at high altitudes (over 5,000 metres)

Like seasickness, altitude sickness can affect anyone. Age and gender are not risk factors.

What are the symptoms of altitude sickness?

Each individual’s reaction to altitude is unique. Most of the time, altitude sickness is benign, and manifests itself in a variety of disorders that occur separately or in association with others, from the very first hours of a stay at altitude.

The main symptoms are:

  • headaches
  • feeling tired and drowsy
  • shortness of breath
  • sleep disorders
  • dizziness and balance problems
  • loss of appetite and digestive disorders
  • nausea
  • irritability
  • decreased urine output

These symptoms, which are common and normal, generally disappear on their own (or at least lessen considerably) after a few hours at altitude and with the help of painkillers (aspirin or paracetamol).

In some cases, serious complications can occur: pulmonary edema (coughing, spitting, blue lips, respiratory failure) or cerebral edema (severe headaches not relieved by aspirin, vomiting, visual disturbances, sometimes psychiatric disorders).

This is an extreme form of altitude sickness, and you must react immediately by :

  • immediately lowering the sufferer to the lowest altitude without delay
  • consult a specialist without delay

We are aware of one case of a severe attack every two years or so (usually occurring within the first few days at altitude). Our teams of field experts, guides and escorts are trained to diagnose edema when it occurs, and to react in the most appropriate way.

How do you prepare your body for altitude?

Physical training before a stay at altitude does not protect against altitude sickness, but it is obviously advisable: cycling, swimming or hiking (at altitude if possible) are good ways to prepare. You can also consult your GP to detect any deficiencies or ailments.

You may also wish to consult a center specializing in mountain medicine for reassurance (download the list of such centers in France below).

If your stay includes sporting activities (trekking, mountaineering) above 3,000 meters, we’ll ask you to fill in a health questionnaire (download below): depending on the answers you provide, we’ll ask you to present a certificate, issued by a doctor, declaring you fit for sporting activities at altitude.

Plan for gentle acclimatization on site

To mitigate the effects of MAM, and limit the risk of aggravation, there are two golden rules to follow.

  • Start slowly

One week at altitude is generally sufficient for proper acclimatization.

We take particular care to ensure that our programs get off to a gentle start, with activities or excursions that are not too strenuous in the first few days, close to the health centers if necessary, while allowing for periods of rest.

  • Build up gradually

Above 3,500 metres, we advise you not to climb more than 700 metres from one night to the next. For example, if you start at 3,500 metres, you can easily climb to a pass at 4,500 metres (+1,000 m), provided you sleep lower down, at 4,000 metres for example, as the difference in altitude between this bivouac and the previous one is only 500 metres. All our trekking and Andinism programs are designed to respect this progressive approach.

A few tips for your stay

A few tips for the first few days after your arrival:

  • Particularly in the first 24 hours, try to conserve energy, walk slowly and ventilate well.
  • Don’t make unnecessary or ill-considered physical efforts “to test yourself”, even when you feel in top form (this can encourage the onset of MAM).

A few tips for trekking or climbing:

  • Avoid violent exertion (giant steps, jumping, running, etc.)
  • Don’t overextend yourself (for example, by playing the “last to arrive buys you a drink” game)
  • For a good night’s sleep, once you’ve arrived at the bivouac, climb 200 to 300 metres without a pack, stay there for an hour and come back down to camp.
  • In case of pain (with the exception of headaches relieved by taking aspirin), stop climbing and rest
  • If signs persist, descend 300 to 400 metres, and do not resume your ascent until the following day if the signs have disappeared; otherwise, do not continue your trek or climb


  • Always stay well hydrated:

To avoid dehydration (favoured by the combination of altitude + sun + effort), it’s essential to drink plenty of fluids (3 to 4 liters every day).

This is a good opportunity to enjoy “maté de coca” (an infusion of coca leaves, a typical Andean drink), which is energetic and reputed to help alleviate the effects of altitude.

When trekking or climbing, be sure to drink mineral water or purify the water in streams, otherwise you’ll run the risk of other problems, particularly gastric.


  • Always eat well:

During your stay, and especially when trekking or climbing, always remember to :

  • eat a balanced diet (vegetarians may be deficient in iron, which slows down the production of red blood cells)
  • eat a generous and complete breakfast
  • regularly replenish your supplies: for each day in the mountains, race supplies (cereal bars, dried fruit, sugar) are distributed by our support teams.

For your first meal at altitude, we advise you to eat lightly and avoid alcohol.


  • What medication to take for altitude sickness?

If you are taking any medication, ask your doctor for advice. He or she will be able to tell you if there are any contraindications to taking it at altitude, what precautions to take, what symptoms to watch out for…

For headaches, take aspirin (which thins the blood) rather than paracetamol.

Do not take sleeping pills, which encourage pauses in breathing during sleep, aggravating hypoxia (lack of oxygen).

  • Diamox® for altitude sickness

Diamox® (acetazolamide) acts by increasing the ventilatory response to oxygen deprivation.

We advise against taking it as a preventive measure, preferring natural acclimatization through a gradual climb in altitude.

It should also be noted that:

  • Diamox® can cause undesirable effects (metabolic disturbances)
  • Diamox® is a diuretic, which may lead to dehydration
  • administered preventively, it makes it difficult to detect altitude-related problems

However, it can be useful to take it with you, and we recommend that you do so:

  • in the event of poor acclimatization to altitude, particularly if headaches persist despite painkillers
  • when ascending too quickly
  • for the treatment of peripheral oedema (swelling of the face, hands and ankles, significant on awakening and subsiding during the day) with no other signs of MAM.


  • Don’t refuse diagnosis

If you suffer at altitude, it’s MAM until proven otherwise!

Don’t tell yourself that you’ve got a headache because you’ve been walking in the sun without a hat, or that you’re throwing up because the eggs you ate the day before weren’t fresh.

The diagnosis can be vexing for a well-trained sportsman, or frustrating because it means putting an end to a trek or climb.

But refusing it can lead to disaster.

It’s essential to be sincere with yourself and with the guides who are supervising the trek/climb.

They have a great deal of experience of the terrain, and will be able to give sound advice and provide first aid if necessary.

It is up to them to decide whether or not to allow a participant to continue with a trek or climb.

Contraindications at altitude

Staying at altitude, and especially engaging in sporting activities (trekking, mountaineering) at altitude, is strongly contraindicated for people suffering from:

  • vascular insufficiency
  • chronic respiratory insufficiency
  • epilepsy
  • diseases requiring repeated injections (e.g. insulin-dependent diabetes)
  • unstabilized heart disease
  • certain blood disorders

It is also contraindicated for people who have undergone neurosurgery, or for those who have suffered cerebral or pulmonary edema during a previous stay at altitude.

Altitude stays are also not recommended for pregnant women and infants (under 18 months).

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