Altitude sickness basics

Altitude sickness can affect anyone, however fit, young or experienced. It is the most common serious problem on Himalayan treks and almost entirely preventable with a sensible itinerary. This article is general information, not medical advice. Talk to a doctor who knows travel medicine before you go.

The short answer to "how do I avoid it?" is to climb slowly, sleep only about 500 m higher per night once you are above 3,000 m, rest before going higher when you feel unwell, and descend if symptoms worsen. The rest of this guide shows what that means on real routes and what to do when it goes wrong.

Why altitude affects you

As you climb, air pressure falls and every breath carries less oxygen. Your body adapts (acclimatises) over days, not hours: you breathe faster, your blood chemistry shifts and you make more red blood cells. Problems start when you climb faster than your body can adapt, usually above about 2,500 to 3,000 m. Kathmandu sits at roughly 1,400 m, so almost every trek in Nepal takes you into the range where this matters.

The three conditions

Acute Mountain Sickness (AMS)

The most common form. Symptoms usually appear 6 to 24 hours after arriving at a new altitude:

• headache, plus one or more of:
• nausea or loss of appetite
• fatigue or weakness
• dizziness
• poor sleep

Mild AMS feels like a bad hangover. Do not go higher until the symptoms are gone. If they get worse despite rest, descend.

High Altitude Pulmonary Edema (HAPE)

Fluid in the lungs. Life-threatening. Watch for:

• breathlessness at rest
• a persistent cough, sometimes with frothy or pink sputum
• a crackling sound in the chest
• blue lips or fingertips
• performance dropping far more than your companions'

High Altitude Cerebral Edema (HACE)

Swelling of the brain. Life-threatening. Watch for:

• confusion, strange behaviour or drowsiness
• loss of coordination: the person cannot walk heel to toe in a straight line
• a severe headache that painkillers don't help

HAPE and HACE are emergencies. Descend immediately, even at night, and call for rescue.

The ascent rule, with real numbers

The CDC Yellow Book, citing Wilderness Medical Society guidance, says that above 3,000 m you should not raise your sleeping altitude by more than about 500 m per night, and should add an extra night of acclimatisation for every 1,000 m of sleeping altitude gained. It calls these figures a reasonable starting point that can be too fast for some people and too slow for others.

Here is how a classic itinerary compares, using the overnight stops on our route pages.

Two lessons stand out. First, the big jump from Phakding to Namche is below 3,000 m at the start and is followed by a rest day, which is exactly why the rest day is there. Second, the Annapurna Circuit crosses a 5,416 m pass in one day and then sleeps 1,600 m lower, which is "climb high, sleep low" in its purest form, but only works if you arrive at Thorong Phedi well acclimatised. Read the full day plans for Everest Base Camp and the Annapurna Circuit to see where each rest day sits.

If your itinerary has you gaining more than 500 m a night above 3,000 m with no spare day, that is a reason to ask the agency to change it, not a reason to be tougher.

Prevention in practice

• Climb high, sleep low. Use rest days for a day hike to a higher viewpoint, such as the Nangkartshang ridge above Dingboche, then come back down to sleep.
• Add days, not speed. A spare day built in at 3,500 to 4,500 m is the cheapest insurance you can buy.
• Drink regularly and eat enough. Dehydration mimics and worsens AMS, though over-drinking does not prevent it.
• Avoid alcohol early on. The CDC advises avoiding alcohol for the first 48 hours at altitude, and sleeping pills can suppress breathing at night.
• Medication. Some trekkers use acetazolamide (Diamox) for prevention. It is prescription-only in many countries, has side effects, and does not replace a slow ascent. Discuss it with a doctor before your trip and trial it at home.
• Arrive acclimatised. A night or two above 2,750 m in the month before you travel, for example a short trek in the foothills, gives your body a head start.

What a normal day feels like, and what does not

Expect to be a little breathless on stairs, to wake several times at night and to have less appetite above 4,000 m. Those are normal. What is not normal is a headache that does not ease with rest, vomiting, stumbling, or being much slower than everyone else on flat ground.

The golden rules

1. Any illness at altitude is altitude sickness until proven otherwise.
2. Never go higher with symptoms.
3. If symptoms get worse, go down.
4. Never leave someone with symptoms alone.

Trekking in a group helps

It is hard to judge your own confusion or breathlessness. Partners and a licensed guide will notice changes you miss. Agree before you start that:

• anyone can call a rest day without pressure,
• the group changes plans for the slowest acclimatiser, or splits safely with the guide,
• you will descend with anyone who is getting worse.

Where to get help on the trail

The Himalayan Rescue Association (HRA) runs seasonal aid posts at Pheriche on the Everest route and at Manang on the Annapurna Circuit, with afternoon altitude talks that are worth attending even if you feel well. Teahouse owners and guides know where the nearest post and the nearest helicopter landing site are. Ask on your first evening.

Insurance and rescue

• Buy insurance that covers helicopter rescue to at least 6,000 m, and carry the policy number and the insurer's emergency phone number.
• Check that the policy covers trekking at the altitude of your route, and that the insurer will pay the rescue company directly or reimburse quickly.
• Save the numbers for the Himalayan Rescue Association and the Tourist Police, shown on every region page.

Quick reference

Go slow, listen to your body and look after each other. If you are still choosing a route, our guide to first-time trekking in Nepal explains which treks give the most forgiving profile, and how to find a trekking partner safely covers who to walk with.