Altitude sickness is the single biggest medical risk on any Nepal trek above 3,000 metres. It can affect anyone regardless of fitness, age, or previous experience. The good news: with proper acclimatisation, careful monitoring, and the discipline to descend when symptoms warrant, severe altitude sickness is almost always preventable. This guide covers what altitude sickness actually is, how to recognise it, how to prevent it, and exactly what to do at each level of symptom severity.
What altitude sickness actually is
At sea level, every breath you take contains a certain amount of oxygen. As you ascend, atmospheric pressure drops and the amount of oxygen available per breath decreases. At 3,500 metres (the altitude of Namche Bazaar or Manang), each breath contains roughly 65% of the oxygen you'd inhale at sea level. At 5,400 metres (Thorong La, Kala Patthar), it's about 50%.
Your body responds by making physiological adjustments: increased breathing rate, increased heart rate, production of additional red blood cells, changes to blood pH. These adjustments take time. The standard rule is that above 3,000m, your body needs roughly 24–48 hours to acclimatise to each new 1,000-metre gain in sleeping altitude.
Altitude sickness — properly called Acute Mountain Sickness (AMS) — happens when you ascend faster than your body can adapt. It is not a sign of weakness, poor fitness, or lack of preparation. Olympic athletes get altitude sickness. Sherpa porters get altitude sickness. Anyone can get it, and the only reliable defence is careful pacing.
The three categories of altitude sickness
Altitude illness exists on a spectrum from mild discomfort to life-threatening emergency:
Acute Mountain Sickness (AMS) — mild to moderate
The most common form. Symptoms include:
- Headache (the cardinal symptom)
- Loss of appetite
- Nausea or vomiting
- Fatigue or weakness
- Dizziness or light-headedness
- Sleep disturbance (you wake up unable to breathe)
Mild AMS is uncomfortable but not dangerous. Roughly 50% of trekkers above 4,000 metres experience some symptoms. The standard threshold for "concerning" AMS is a moderate to severe headache combined with one other symptom.
High Altitude Cerebral Edema (HACE) — life-threatening
Fluid accumulation in the brain. Symptoms:
- Severe headache that doesn't respond to painkillers
- Loss of coordination (ataxia) — you can't walk in a straight line
- Confusion, irrational behaviour, or hallucinations
- Drowsiness leading to unconsciousness
HACE is a medical emergency. Without immediate descent and treatment, it is rapidly fatal. The classic test is the "heel-to-toe" walk: ask the affected person to walk a straight line putting one foot directly in front of the other. If they can't do it, they have HACE until proven otherwise.
High Altitude Pulmonary Edema (HAPE) — life-threatening
Fluid accumulation in the lungs. Symptoms:
- Shortness of breath even at rest (not just on exertion)
- Wet, gurgling cough
- Pink or blood-tinged sputum
- Chest tightness
- Cyanosis (blue tinge to lips and fingernails)
- Rapid heart rate
HAPE is also a medical emergency. Like HACE, it requires immediate descent and is rapidly fatal if untreated.
How we prevent it on our treks
Our acclimatisation protocol is non-negotiable:
1. The 500-metre rule. Above 3,000m, we never gain more than 500 metres of sleeping altitude per day. Day hikes higher are encouraged ("climb high, sleep low") because they expose your body to thin air without making you sleep in it.
2. Rest days every 1,000 metres. A full rest day at the same altitude every 1,000 metres of cumulative gain. On the Annapurna Circuit, we rest at Manang (3,500m). On Everest Base Camp, we rest at Namche (3,440m) and Dingboche (4,410m).
3. Pulse oximetry monitoring. We carry pulse oximeters and check oxygen saturation on every group member at every camp. Normal SpO2 at sea level is 98–100%. At 3,500m, it should be above 85%. At 4,500m, above 80%. Below those thresholds, we don't go higher.
4. Hydration. Three to four litres of water per day. Altitude causes increased breathing which dehydrates faster than people expect.
5. No alcohol. Above 3,000m, alcohol significantly impairs acclimatisation and dehydrates the body. We strongly advise against it.
6. Going slow. Trekking pace at altitude should be deliberately slower than it feels comfortable. If you can't have a normal conversation while walking, you're walking too fast.
7. Diamox prophylaxis. For trekkers with a history of altitude sickness or for routes with rapid ascent profiles, we recommend acetazolamide (Diamox) — 125mg twice daily starting one day before reaching 3,000m. Consult your doctor before your trip. Side effects include tingling fingers and increased urination.
What to do if symptoms appear
Mild AMS (headache, mild nausea, fatigue):
- Stop ascending. Stay at current altitude.
- Rest. Hydrate (1–2 extra litres of water).
- Take ibuprofen or paracetamol for headache.
- Eat carbohydrates, even if appetite is low.
- If symptoms resolve within 24 hours, continue ascending the next day.
- If symptoms persist or worsen, descend at least 500 metres.
Moderate AMS (severe headache, persistent nausea, breathing difficulty):
- Descend immediately, at least 500 metres.
- Do not wait until morning. Descend in the dark with headtorches if necessary.
- Consider Diamox 250mg twice daily.
- If symptoms don't improve at lower altitude, descend further.
HACE or HAPE (any of the symptoms listed above):
- Immediate descent, by helicopter if available, by litter if necessary.
- Lose at least 1,000 metres of altitude as quickly as possible.
- Supplemental oxygen if available.
- Dexamethasone for HACE (8mg loading dose then 4mg every 6 hours).
- Nifedipine for HAPE (30mg slow-release every 12 hours).
- Medical evaluation at the earliest opportunity.
Our guides carry emergency oxygen, dexamethasone, and Diamox on every trek above 4,000m. We have helicopter evacuation protocols and direct relationships with Kathmandu-based rescue services. Your travel insurance must cover helicopter evacuation at altitude — confirm this before you book your trek.
The signs we look for
Our guides watch for early warning signs that trekkers themselves often dismiss:
- Group members lagging behind the pace they've been maintaining
- Repeated requests to stop "for photos" that are really rest stops
- Decreased verbal communication at the dinner table
- Refusing food, especially carbohydrate-heavy items
- Going to bed earlier than usual
- Coughing, especially at night
- Visible decrease in alertness
If your guide pulls you aside and suggests descending, take them seriously. They are watching for things you may not notice in yourself. Their judgement is the single most important safety factor at altitude.
Things people get wrong
"I'm fit, I won't get altitude sickness." Fitness is irrelevant. Altitude sickness affects everyone equally regardless of athletic ability. We have seen marathon runners get severe AMS while sedentary 70-year-olds completed the same trek without issue.
"I've been to altitude before, I'll be fine." Previous successful trips at altitude do not predict the next one. You can do EBC without symptoms one year and get HAPE on the same trail the next year. Don't assume.
"I'll just push through the headache." Pushing through mild symptoms at altitude is how mild symptoms become severe. The rule is: if you have symptoms, do not ascend. Wait or descend.
"I'll take Diamox so I can climb faster." Diamox doesn't change the physiology of acclimatisation; it just makes you more comfortable while your body adapts. You still need to follow the 500m rule.
"I don't want to ruin my trip by descending." Trips are made worse, not better, by ignoring altitude symptoms. A 24-hour descent and re-attempt is annoying but survivable. A helicopter evacuation is expensive and ends the trip immediately. A failure to descend in time can be fatal.
Final thoughts
The Himalaya rewards humility. Every year, trekkers get into serious trouble because they didn't take altitude seriously enough — they raced their itinerary, ignored symptoms, refused to descend, drank alcohol, didn't drink water, didn't communicate with their guide. Every year, other trekkers complete the same routes safely because they walked slowly, ate properly, hydrated, slept well, and trusted their guide's judgement.
Be the second kind of trekker. Walk slowly. Drink water. Listen to your body. Listen to your guide. The mountain will still be there next year if you need to descend this year.
