Every trekking season in Nepal, Ladakh and the Karakoram, people who are fit, experienced and sensible are carried down valleys or flown out by helicopter because they climbed too fast. Acute mountain sickness is not a sign of weakness, and it is not something you can train away at home. It is the body’s ordinary response to thinner air, and the only reliable defence is time. This guide sets out the rules we follow above 3,000 m, why they exist, and what to do when they are broken by circumstance.
What thin air actually does
The proportion of oxygen in the air stays at about 21 percent at every altitude. What changes is the pressure. At 3,500 m, roughly the height of Namche Bazaar or Leh, the air pressure is about two thirds of sea level, so each breath delivers about two thirds of the oxygen. At 5,364 m, the altitude of Everest Base Camp, it is close to half. Your body responds by breathing faster, raising the heart rate, and over several days producing more red blood cells. This process, called acclimatisation, cannot be hurried. It takes place mostly while you sleep, which is why the sleeping altitude, not the highest point reached during the day, is what matters.
Acute mountain sickness begins when ascent outpaces acclimatisation. Fluid shifts in the brain and lungs cause the symptoms, and in a small minority of cases those shifts progress to the two life-threatening forms: high altitude cerebral edema, which affects the brain, and high altitude pulmonary edema, which floods the lungs. Both can kill within hours. Both are almost always preceded by ordinary altitude sickness that was ignored.
The ascent rule: 300 to 500 m a day
Above 3,000 m, the widely accepted guideline is to raise your sleeping altitude by no more than 300 to 500 m per day, and to add a rest day for every 1,000 m gained, or roughly every third or fourth day. The classic Everest Base Camp itinerary is built around this: two nights in Namche Bazaar at 3,440 m, two nights in Dingboche at 4,410 m, then Lobuche at 4,940 m and Gorak Shep at 5,164 m. Skip the rest days and you are stacking risk that will not show until two days later.
A rest day does not mean lying in a sleeping bag. The principle is to climb high and sleep low: walk up a nearby ridge for two or three hours, gain 300 to 500 m, then come back down to sleep. From Namche, the ridge above the village or the trail toward Khumjung serves this purpose. From Dingboche, the hill behind the lodges rises to over 5,000 m. Your body gets a preview of the altitude to come, then recovers overnight at a height it already tolerates.
Flying straight to altitude changes the arithmetic. Anyone arriving in Leh by air lands at about 3,500 m without any walk-in, and the first two days should involve nothing more strenuous than a gentle wander through town. The same applies at Lukla, at 2,846 m: the first day’s walk to Phakding actually descends, and the second day’s climb to Namche is the first real test.
Recognising the early signs
The defining symptom is a headache that appears within six to twelve hours of arriving at a new altitude. On its own, a mild headache that eases with rest, fluids and simple painkillers is common and usually harmless. Acute mountain sickness is diagnosed when the headache comes with at least one other symptom:
- Loss of appetite, nausea or vomiting
- Unusual tiredness or weakness out of proportion to the day’s effort
- Dizziness or light-headedness
- Poor sleep, often with a pattern of stopping and restarting breathing
Two rules follow directly. First, never ascend to a higher sleeping altitude with symptoms. Stay where you are, for as many nights as it takes, until they clear. Second, if symptoms are getting worse while you rest, descend. Five hundred metres is often enough to bring dramatic relief, and there is no shame in it. The trail will still be there in two days.
A pulse oximeter is a useful supplement, not a replacement for judgement. Readings in the low 80s are normal at 4,500 m and readings in the 70s are common at 5,000 m. What matters is a reading that is much lower than the rest of the group’s, or that drops sharply from one evening to the next.
The two emergencies
High altitude cerebral edema announces itself through the brain. The person becomes clumsy and cannot walk a straight line heel to toe, becomes confused, irritable or oddly withdrawn, and may not recognise that anything is wrong. This is why nobody on a trek should ever be left alone in a lodge to sleep it off. Somebody must watch, and somebody must be willing to make the decision to go down at night if necessary.
High altitude pulmonary edema shows in the lungs. Breathlessness at rest, not just on the climb, is the key sign, along with a persistent cough, a crackling sound in the chest, a blue tinge to the lips and a sudden collapse in stamina. It can develop without any headache at all, which catches people out.
For both, the treatment is the same and non-negotiable: descend immediately, as far as possible, with assistance. Supplemental oxygen and a portable pressure bag buy time if they are available, but nothing replaces going down. Helicopter evacuation from the Khumbu and Annapurna regions is routine in good weather, which is why insurance covering rescue to at least 6,000 m is not optional.
Medication, hydration and the myths
Acetazolamide is the drug most commonly used to speed acclimatisation. Taken in a low dose starting a day before ascent, it makes the blood slightly more acidic and encourages deeper breathing, especially at night. It does not mask symptoms, so a headache that appears while taking it still means what it always means. Whether to use it is a decision to take with a doctor before leaving home.
Drink enough to keep your urine pale, which at altitude typically means three to four litres a day, but do not overdo it; excessive water does not prevent sickness. Avoid alcohol and sleeping tablets, which both suppress breathing at night. Eat even when you have no appetite. And be sceptical of anyone who claims that garlic soup or sheer determination will keep sickness away. They are pleasant, but they are not medicine.
Building a plan you can keep
Before leaving, we write the sleeping altitude for every night on a single sheet and check the daily gains against the 500 m rule. We build in at least one spare day for the group, so that stopping for a night does not wreck the flight home. We agree in advance that any member can call a halt without argument. On the trail we ask each other the same questions every evening: headache, appetite, sleep, breathing. It takes thirty seconds, and over many years it has kept our worst experience at altitude to a lost day in a lodge.