At what altitude can altitude sickness start?

The body responds to lower oxygen availability rather than to a signposted height. One traveller may develop headache, nausea, unusual tiredness, dizziness or poor sleep on an itinerary that causes no symptoms in a companion. Prior uneventful trips do not ensure that a later ascent will feel the same.

Where you sleep matters because the body spends prolonged time adapting there. Rapid gains, driving or flying directly to a high destination, hard exertion soon after arrival and a personal history of altitude illness can reduce the opportunity to acclimatise.

Build an itinerary with gradual increases in sleeping elevation and easier activity after a major gain. Learn the early symptom pattern before departure, travel with someone who will speak up about changes, and know where descent and medical help are available along the route.

If a headache with nausea, fatigue, dizziness or disturbed sleep appears after ascent, stop climbing. Rest at the current elevation and reassess. Descend if symptoms become worse or do not improve; medication or oxygen should never be treated as permission to push upward while ill.

The seriousness of symptoms, not the number on an altimeter, determines urgency. Confusion, loss of coordination, inability to walk straight, severe drowsiness, breathlessness at rest, chest congestion, blue-grey lips or collapse calls for immediate assisted descent and emergency medical help.

Seek prompt assessment for repeated vomiting, an escalating headache or any rapid deterioration. Stay with the affected person, limit exertion and do not wait for a particular altitude to be reached before acting.

Bring the actual itinerary to a travel-health appointment. Ask how quickly the sleeping points rise, whether rest stops are suitable, how previous altitude symptoms affect planning and whether your heart, lung, blood or neurological health requires extra precautions or a different route.