Mountain sickness on Mont Blanc: symptoms, prevention and what to do
Published May 30, 2026 · Updated July 5, 2026
Acute mountain sickness (AMS) is the most common cause of turning back and of accidents on high-altitude ascents. On Mont Blanc (4,805 m), the drop in oxygen is real and progressive. Understanding it means knowing how to prepare for it.
Understanding
Why altitude affects the body
At the summit of Mont Blanc (4,805 m), atmospheric pressure is around 575 hPa versus 1,013 hPa at sea level. The oxygen content of the air stays the same (21%), but each breath delivers fewer oxygen molecules — around 53% of what you absorb at sea level.
The body responds by increasing breathing and heart rate. For most people, this adaptation is partial and gradual: it takes between 12 and 48 hours. This is why acclimatization — spending time at intermediate altitude before aiming for the summit — is the main prevention tool.
Recognize
Severity levels and what to do
| Level | Signs to recognize | What to do |
|---|---|---|
| Mild AMS | Headache, unusual fatigue, mild nausea, loss of appetite, mild dizziness. | Take a break, hydrate, do not go higher. Wait for improvement before continuing. |
| Moderate AMS | Intense headache not relieved by ibuprofen, vomiting, extreme fatigue, unsteady gait. | Descend immediately. Do not wait. A 500 m descent is often enough to improve condition. |
| HACE — Cerebral edema | Confusion, hallucinations, loss of balance, inability to walk in a straight line, deep drowsiness. | Absolute emergency. Immediate descent and call for rescue (PGHM: +33 4 50 53 16 89). |
| HAPE — Pulmonary edema | Breathlessness at rest, cough with pink sputum, cyanosis of the lips, fast and labored breathing. | Life-threatening emergency. Call PGHM immediately. Descend even at night if possible. |
PGHM Chamonix (mountain rescue): +33 4 50 53 16 89. In an emergency: 15 (SAMU), 18 (fire brigade) or 112.
Prevention
How to reduce the risk of mountain sickness
Gradual acclimatization — the 300 m rule
Above 3,000 m, do not gain more than 300 m of sleeping altitude per night. If you sleep at the Goûter refuge (3,835 m), a prior night at Tête Rousse (3,167 m) helps significantly.
Stay well hydrated
Dehydration amplifies the effects of mountain sickness. Drink at least 3 liters of water a day at altitude, even without feeling thirsty. Avoid alcohol the night before and during the ascent.
Respect the pace — the slow-foot rule
Climbing slowly and steadily beats alternating sprints and breaks. A high-mountain guide knows the optimal pace for their clients — one of the major benefits of guiding.
Don't ignore the warning signs
The most dangerous reflex in the mountains is minimizing symptoms out of ego or social pressure. A persistent headache above 3,500 m is a signal to take seriously.
Preventive medication — medical advice required
Acetazolamide (Diamox) is sometimes prescribed as prevention. It reduces certain AMS effects but is not free of side effects. Discuss it with a doctor before the ascent — never self-medicate.
Key takeaway
The absolute rule: never keep climbing if symptoms worsen.
In the high mountains, the wrong choice isn't turning back. The wrong choice is continuing despite clear warning signs. A high-mountain guide is trained to recognize these signals and make the decision objectively.
Frequently asked questions
Mountain sickness FAQ
At what altitude does mountain sickness start?
The first symptoms can appear as early as 2,500 m in some sensitive people, but the risk rises significantly above 3,000 m. At the Goûter refuge (3,835 m) and at the summit of Mont Blanc (4,805 m), almost all mountaineers feel an amplified level of effort due to oxygen scarcity.
Is mountain sickness predictable?
No. Neither age, fitness nor mountaineering experience guarantees immunity. Elite athletes can develop severe AMS, while untrained seniors may have no issues at all. The only real prevention: gradual acclimatization.
Can you keep climbing with a headache?
A mild headache that goes away with rest and a painkiller (ibuprofen) is manageable. A persistent headache or one that intensifies while climbing is a signal to stop. The absolute rule: never keep climbing with worsening symptoms.
Do you need to descend if you have mountain sickness?
For moderate AMS or any symptom that doesn't improve with rest, yes — descent is the only reliable solution. Mountain sickness doesn't 'pass' by staying at the same altitude. Descending 300 to 500 m is usually enough to quickly improve condition.