Altitude sickness is the biggest health risk on most Himalayan treks, and it can happen to anyone. It doesn’t care how fit you are, how young you are, or how many treks you’ve done. The good news is that it’s mostly preventable, and when it does happen, the right action is simple: don’t go higher, and go down if it gets worse.
This guide explains what altitude sickness is, how to spot it, how to lower your risk, and what to do if you or a trekking partner feels unwell. It also shows how the risk differs from trek to trek.
This guide is general information, not medical advice. Talk to your doctor before a high-altitude trek, especially if you have a heart, lung, or other health condition.
Quick Answer
- Altitude sickness happens when your body doesn’t get enough time to adjust to thin air as you climb.
- Mild symptoms are a headache plus nausea, tiredness, or dizziness. They usually start 2 to 12 hours after you reach a new height.
- Serious symptoms are confusion, trouble walking in a straight line, or breathlessness while resting. These need immediate descent.
- Prevention: climb slowly, take rest days, sleep lower than you hike, and tell your guide how you feel.
What Causes Altitude Sickness?
The higher you go, the less oxygen each breath gives you. Your body can adjust, but it needs time. If you climb faster than your body can adapt, you get sick. This usually becomes a risk above about 2,500 m.
Three things matter most:
- How fast you climb. Speed is the main factor.
- How high you sleep. The height where you spend the night counts more than the height you reach in the daytime.
- Your own body. Some people are just more sensitive.
Fitness does not protect you. Strong, fit trekkers get altitude sickness as often as anyone else, and sometimes more, because they tend to climb faster.
The Three Types of Altitude Sickness
| Type | What it is | Main signs | How serious |
|---|---|---|---|
| AMS (Acute Mountain Sickness) | The common, mild form | Headache, plus nausea, tiredness, dizziness, or poor appetite | Mild, but can get worse if you keep climbing |
| HAPE (High Altitude Pulmonary Edema) | Fluid in the lungs | Breathless at rest, cough, tight chest, very low energy, sometimes frothy or pink spit | Life-threatening |
| HACE (High Altitude Cerebral Edema) | Swelling of the brain | Confusion, strange behavior, loss of balance, trouble walking straight, severe drowsiness | Life-threatening |
AMS usually starts within the first day at a new height. HAPE and HACE tend to appear later, often after 2 to 5 days, and they can get serious quickly.
Symptoms to Watch For
Common signs of mild altitude sickness (AMS):
- A headache (the main sign)
- Nausea or vomiting
- Poor appetite
- Feeling very tired or weak
- Dizziness or light-headedness
- Poor sleep
Doctors often use a Lake Louise score to measure AMS. A score of 3 or more, including a headache, after rapid ascent to over 2,500 m points to AMS.
Danger signs that need immediate action:
- Confusion or acting unlike yourself
- Stumbling or being unable to walk a straight line
- Breathlessness while resting
- A cough that won’t stop, or frothy, pink spit
- Blue or gray lips
- Symptoms that keep getting worse even after resting
If you see any of these, tell your guide immediately and start going down.
The Golden Rules
The Himalayan Rescue Association teaches three simple rules, and they apply on every Nepal trek:
- Learn to recognize the symptoms. Mild altitude sickness is common.
- Don’t go higher if you have symptoms. Rest until they pass.
- If you’re getting worse, go down. Descent is the best treatment.
How to Prevent Altitude Sickness
1. Climb slowly.
Above about 3,000 m, don’t raise your sleeping height by more than about 500 m per day. Experts also suggest a rest day for about every 1,000 m you gain.
2. Climb high, sleep low.
Take a hike to a higher spot during the day, then return to sleep lower. This helps your body adjust.
3. Don’t skip acclimatization days.
Rest days aren’t wasted time. They’re the main reason many trekkers reach the top safely.
4. Tell your guide how you feel.
Don’t hide a headache to keep up with the group.
5. Eat well and stay comfortably hydrated.
Dehydration can make symptoms worse.
6. Go easy on alcohol.
Ask your doctor before using sleeping pills at altitude.
7. Ask your doctor about medication.
For some trekkers, a doctor may suggest a preventive medicine such as acetazolamide (Diamox). Don’t take it, or any altitude medicine, without medical advice.
8. Choose an itinerary that gives you time.
Very fast itineraries leave less time to adjust. This is a big reason we recommend a gentler version of treks like Mardi Himal.
Who Is at Higher Risk?
You may be more likely to get altitude sickness if you:
- Climb faster than about 400–500 m a day
- Have had altitude sickness before
- Get migraines
- Fly or drive straight to a high place without time to adjust
- Have a heart or lung condition
If any of these apply, talk to your doctor before you go.
What to Do If You Feel Unwell
- Stop going higher. Rest where you are.
- Tell your guide right away.
- Rest and drink fluids. Mild symptoms often improve in 12 to 48 hours if you don’t climb further.
- Don’t take risks. If symptoms don’t improve or get worse, go down.
- Descend if you have danger signs. Experts suggest going down about 500 to 1,000 m, or until symptoms resolve. Don’t go down alone.
- Let your guide and medical help take over. Oxygen and emergency medicines should only be used with guidance.
Altitude Risk by Trek
Here’s how the heights compare on treks Snowbank offers:
| Trek | Highest point | What to know |
|---|---|---|
| Poon Hill · Poon Hill trek | About 3,210 m | Low risk, but mild symptoms are still possible |
| Langtang Valley · Langtang trek | About 3,870 m (Kyanjin Gompa) | Moderate. Gosaikunda adds about 4,380 m |
| Tsum Valley · Tsum Valley trek | About 3,700 m (Mu Gompa) | Moderate and gradual |
| Annapurna Base Camp · ABC trek | 4,130 m | Moderate, with a slower climb than Mardi |
| Mardi Himal · Mardi Himal trek | About 4,200–4,500 m | Gains height fast, so less time to adjust |
| Larkya La (Manaslu) · Manaslu trek | About 5,106 m | High pass after several days at altitude |
| Kang La Pass (Nar Phu) · Nar Phu trek | About 5,320 m | High pass, with acclimatization days at Phu and Nar |
| Everest Base Camp · EBC 15 days | 5,364 m (Kala Patthar 5,545 m) | Highest risk. Built-in rest days matter most |
Heights vary slightly by source, so treat them as close estimates.
What Your Guide and Operator Should Do
A good operator takes altitude seriously. Ask before you book:
- Does the itinerary include acclimatization days?
- Does the guide have first-aid and altitude training?
- Does the team carry a first-aid kit, and oxygen on higher treks? For example, Snowbank’s Tsum Valley package lists supplementary oxygen and a first-aid kit.
- What’s the plan if someone gets sick, including descent and evacuation?
- Is there a spare day built into the schedule?
What to Pack for Your Health
- A personal first-aid kit and any medicines your doctor recommends
- Your own prescription medicines, with extra for delays
- Simple pain relief for headaches, as advised by your doctor
- Water purification and a refillable bottle
- Optional: a pulse oximeter. It can be a useful extra, but how you feel matters more than any number.
FAQs
What is altitude sickness?
It’s an illness that happens when you climb too fast for your body to adjust to thin air. It ranges from a mild headache (AMS) to life-threatening lung or brain swelling (HAPE and HACE).
At what height does altitude sickness start?
It can start above about 2,500 m, and the risk rises with height and speed of ascent.
What are the first signs of altitude sickness?
A headache, plus nausea, tiredness, dizziness, or poor appetite, usually within 2 to 12 hours of reaching a new height.
Can fit people get altitude sickness?
Yes. Fitness does not protect you. Climbing slowly matters far more.
How can I prevent altitude sickness?
Climb slowly, add rest days, sleep lower than you hike, tell your guide how you feel, and don’t go higher if you have symptoms.
How fast should I climb on a trek?
Above about 3,000 m, aim for no more than about 500 m higher sleeping height per day, with a rest day for about every 1,000 m gained.
Should I take Diamox (acetazolamide)?
Only if your doctor recommends it. It helps some trekkers prevent altitude sickness, but it isn’t right for everyone, and it doesn’t replace slow climbing.
What should I do if I get a headache at altitude?
Stop climbing, rest, drink fluids, and tell your guide. If it gets worse or you develop other symptoms, go down.
What are the signs of a medical emergency?
Confusion, trouble walking straight, breathlessness at rest, a cough with frothy or pink spit, or blue lips. These need immediate descent.
How far down should I go if I’m sick?
Experts suggest descending about 500 to 1,000 m, or until symptoms resolve, with a guide.
Is altitude sickness common on Everest Base Camp?
Mild symptoms are common, since the trek reaches 5,364 m. Taking the built-in rest days and climbing slowly greatly lowers your risk.
Do I need travel insurance for altitude?
Yes. Choose a policy that covers high-altitude trekking and helicopter rescue.
Planning a high-altitude trek? Browse our Snowbank Adventure treks , each built with acclimatization in mind. For medical guidance, see the Himalayan Rescue Association , the Wilderness Medical Society altitude illness guidelines , and the Merck Manual on altitude illness .