At what height can altitude sickness occur?

There is no dependable personal height below which symptoms are impossible. Pay attention to how far and how quickly your sleeping altitude changes, not only to the elevation of a summit or daytime excursion.

Air pressure falls as elevation rises, so each breath delivers less oxygen. The body adapts by changing breathing and circulation, but those adjustments need time. Headache accompanied by nausea, dizziness, fatigue, or poor sleep after an ascent can fit acute mountain sickness.

Fitness does not reliably prevent the condition. A rapid journey by road, cable car, or aircraft can place an active traveller at risk because physical conditioning and acclimatisation are different processes. A previous symptom-free trip also does not set a safe ceiling for every future journey.

Build time for gradual acclimatisation, especially when the route involves a large change in sleeping altitude. Keep the first days physically gentle, avoid alcohol, maintain ordinary food and fluid intake, and travel with someone who can recognise changes in behaviour or balance.

If mild symptoms appear, stop ascending and rest at the same altitude. Do not sleep higher while symptoms persist. Descend if they worsen or fail to settle, and never let a fixed itinerary, booking, or summit goal override symptoms.

Confusion, difficulty walking in a straight line, unusual drowsiness, breathlessness at rest, chest congestion, blue or grey lips, or coughing frothy fluid require urgent descent and emergency medical help. A person with these signs should not descend alone.

Get medical guidance before travel if you have significant heart or lung disease, are pregnant, previously became severely unwell at altitude, or cannot make a gradual itinerary. Advice should account for the destination, route, sleeping stops, and access to descent.

Bring your full itinerary and ask whether its changes in sleeping altitude allow reasonable acclimatisation. Clarify what symptoms should stop further ascent, where descent is possible, and whether any current medicines affect breathing, hydration, or alertness.

A clinician can also explain whether preventive prescription medicine belongs in your plan and how it should be used alongside gradual ascent rather than as permission to ignore illness.