Why Altitude Sickness Matters, Explained Simply
Altitude Sickness: what it is, why it matters, and what published guidance actually asks of you.
It affects fit and unfit people alike, and the severe forms are rapidly fatal if you keep ascending.
Key takeaways
- Usually starts above: 2,500 metres
- Early symptoms: headache, nausea, fatigue, poor sleep
- Most useful single change: Ascend gradually and build in acclimatisation days.
- Commonest mistake: Ascending further with any symptoms.
- See a doctor if any of the signs under "When to speak to a doctor" below apply to you.
The short answer
Illness caused by ascending faster than the body can adapt to reduced oxygen.
Where it fits
This is part of Travel Health. The rest of this page works through the figures and then the practical side.
The numbers, in one place
| Measure | Figure |
|---|---|
| Usually starts above | 2,500 metres |
| Early symptoms | headache, nausea, fatigue, poor sleep |
| Golden rule | do not ascend further with symptoms |
| HACE and HAPE | high altitude cerebral and pulmonary oedema; emergencies |
| Only definitive treatment | descent |
What that means in practice
What that looks like in practice:
- Ascend gradually and build in acclimatisation days
- Stop ascending if you have symptoms
- Stay well hydrated and avoid alcohol early on
- Discuss acetazolamide with a travel clinic for rapid ascents
What gets people into trouble
The pitfalls worth knowing about:
- Ascending further with any symptoms
- Alcohol and sedatives at altitude
- Assuming fitness protects you
Where to go next
- Altitude Sickness: The Quick Reference
- Altitude Sickness: What the Evidence Actually Says
- Altitude Sickness — the reference entry
- Returning Traveller Illness: What Published Guidance Supports
- Travel Vaccinations: What the Evidence Actually Says
When to speak to a doctor
Descend immediately and seek emergency help for confusion, unsteadiness, inability to walk in a straight line, severe headache not relieved by painkillers, breathlessness at rest, or coughing frothy or pink sputum — these indicate cerebral or pulmonary oedema and are rapidly fatal. Descent is the treatment; do not wait.
Where this comes from
How this page was put together
Every figure, recommendation and warning above is taken from the guidance named in Where this comes from. Nothing on this page is inferred, summarised or generalised beyond what those bodies publish: where they give a number, it is their number, and where they are silent, so are we. Pages are rebuilt whenever the underlying record changes. Sign-off by a named clinician, where it has happened, is shown at the top of the page along with the date.
Medical disclaimer
This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to a GP, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 999.
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