Altitude Sickness Guide
Prevention, symptoms, and acclimatization for high-altitude hiking
Check Your Symptoms
AMS - Acute Mountain Sickness
Most common form. Occurs above 8,000 ft (2,500 m). Symptoms: headache plus nausea, fatigue, dizziness, or insomnia.
- Onset: 6-12 hours after arrival at altitude
- Treatment: Rest, hydrate, ibuprofen for headache. Do not ascend until symptoms resolve.
- Most people acclimatize in 1-3 days
HACE - High Altitude Cerebral Edema
Brain swelling. Can develop from untreated AMS. Occurs above 12,000 ft (3,600 m).
- Key signs: Confusion, ataxia (can't walk straight), altered consciousness
- Treatment: IMMEDIATE descent. Dexamethasone. Supplemental oxygen.
- Can be fatal within 24 hours if untreated
HAPE - High Altitude Pulmonary Edema
Fluid in lungs. Most dangerous common form. Above 8,000 ft (2,500 m).
- Key signs: Breathlessness at rest, persistent cough, gurgling breath, blue lips
- Treatment: IMMEDIATE descent. Nifedipine. Supplemental oxygen. Portable hyperbaric chamber.
- Leading cause of altitude-related deaths
Prevention Guidelines
- Ascend gradually: Above 10,000 ft, gain no more than 1,000 ft of sleeping altitude per day
- Rest days: Take a rest day every 3,000 ft of elevation gain
- Climb high, sleep low: Day hike to higher elevations but sleep at a lower camp
- Hydrate: Drink 3-4 liters of water per day at altitude
- Avoid alcohol for the first 48 hours at a new altitude
- Eat carbohydrates: High-carb diet is easier to digest at altitude
- Medication: Acetazolamide (Diamox) 125mg twice daily, starting 24h before ascent
- Know your limits: Previous altitude sickness increases future risk
Acclimatization Schedule
| Altitude | Zone | Oxygen | Recommendation |
|---|---|---|---|
| 0-5,000 ft | Low | ~100% | No concerns |
| 5,000-8,000 ft | Moderate | ~80% | Minor effects possible; hydrate well |
| 8,000-12,000 ft | High | ~70% | AMS risk begins. 2-3 days to acclimatize. |
| 12,000-18,000 ft | Very High | ~60% | Gradual ascent required. Rest days every 3,000 ft. |
| 18,000+ ft | Extreme | ~50% | Prolonged stays dangerous. No full acclimatization possible. |
Further reading
Altitude sickness prevention and symptom checker. Learn about AMS, HACE, and HAPE with acclimatization schedules and risk assessment tools.
Related tools
Sources
Altitude illness risk rises above 8,000 feet (2,400 m). The guidance below on gradual ascent and warning signs comes from the CDC's travellers' health pages.
Links checked 2026-09-09.
Frequently asked questions
At what altitude does altitude sickness start?
Acute mountain sickness typically appears above 8,000 feet (2,500 m), which is also the altitude the CDC's travellers' health guidance treats as the point where risk rises. HAPE can start at the same height; HACE usually appears higher, above about 12,000 feet.
How do I tell mild altitude sickness from something dangerous?
Mild AMS is a headache with nausea, fatigue, dizziness or poor sleep, and it usually settles in one to three days without descending. The signs that change the answer are neurological or respiratory: confusion, being unable to walk straight, breathlessness while resting, a persistent cough, gurgling breathing, or blue lips. Those are HACE and HAPE.
What do I do if the serious signs appear?
Descend immediately. This guide lists descent first for both HACE and HAPE, ahead of medication and oxygen, because untreated HACE can be fatal within 24 hours. Do not wait to see whether it improves overnight, and do not continue ascending while any symptom is still present.
Is this guide a substitute for medical advice?
No, and the page says so at the top. Altitude illness overlaps with other conditions, medication and pre-existing illness change the risk, and a symptom checklist cannot examine you. Speak to a healthcare provider before travelling to altitude if you have any pre-existing condition.