At high elevations, the combination of reduced oxygen and lower atmospheric pressure can produce a variety of unpredictable effects on the body – known collectively as acute mountain sickness (AMS) or, colloquially, “altitude sickness”.

Barraged by medical advice and horror stories, trekkers all too often develop altitude paranoia. The fact is that just about everyone who treks over 4000m experiences some mild symptoms of AMS, but serious cases are very rare and the simple cure – descent – almost always brings immediate recovery. Up to around 4000m, very few people will experience anything worse than slowness, dizziness and headaches. That said, the syndrome varies hugely from one person to the next, and strikes without regard for fitness – in fact, young people seem to be more susceptible. It can be hard to monitor your own symptoms of altitude sickness, too: one reason why it’s a good idea to trek with other people. For further advice on AMS, visit the Himalayan Rescue Association’s aid posts at Manang (on the Annapurna Circuit) and Pheriche (on the Everest trek).


The body can acclimatize to some very high elevations but it takes time and must be done in stages. The golden rule is don’t go too high too fast. Above 3000m, experts recommend that the daily net elevation gain should be no more than 300–400m. Take mandatory acclimatization days at around 3500m and 4500m – more if you’re feeling unwell – and try to spend them day-hiking higher. These are only guidelines, and you’ll have to regulate your ascent according to how you feel. Trekkers who fly directly to high airstrips have to be especially careful to acclimatize.

Drink plenty of liquids at altitude, since the air is incredibly dry – unless you pee clear, you’re probably not drinking enough. Keeping warm, eating well, getting plenty of sleep and avoiding alcohol will also help reduce the chances of developing AMS.


AMS usually gives plenty of warning before it becomes life-threatening. Mild symptoms include headaches, dizziness, racing pulse, nausea, loss of appetite, shortness of breath, disturbed sleep and swelling of the hands and feet. One or two of these symptoms is a sign that your body hasn’t yet adjusted and you shouldn’t ascend further until you start feeling better; if you do keep going, be prepared to beat a hasty retreat if the condition gets worse.

AMS is defined as moderate if the headache becomes severe and medication doesn’t help, the nausea verges on vomiting, and coordination starts to suffer. At this point you want to start descending, as severe AMS symptoms can develop from moderate ones within hours; these can include all of the above, plus shortness of breath even while at rest, difficulty walking, mental confusion or lethargy, bubbly breathing or coughing, and bloody sputum. The worst case scenario is High Altitude Pulmonary or Cerebral Odema (HAPO/HACO), or potentially fatal build-up of fluids in the lungs or brain.

Descent and Diamox

The only cure for AMS is immediate descent. Anyone showing moderate or severe symptoms should be taken downhill immediately, regardless of the time of day or night – hire a porter or pack animal to carry the sufferer if necessary. Recovery is usually dramatic, often after a descent of only a few hundred vertical metres.

Acetazolamide (better known under the brand name Diamox) improves respiration at altitude, and can therefore accelerate acclimatization. (It also stimulates breathing, evening out the disturbing peaks and troughs of “periodic breathing” during sleep at altitude.) Some doctors recommend a preventive dose (125mg twice a day) for people trekking at high elevations, though note that unpleasant side effects such as numbness, tingling sensations (Nepali guides call it jhum jhum) and light-headedness are not uncommon. To treat AMS, the dosage is 250mg every twelve hours. Note that Diamox is a diuretic, so it’s all the more important to keep hydrated while taking it. And note too that while it can accelerate acclimatization, it won’t stop AMS symptoms worsening if you keep on ascending.

Heat stroke, hypothermia and frostbite

Heat stroke is more common than you might think; drink plenty of water, eat something salty, wear a hat and rest. Other altitude-related dangers such as hypothermia and frostbite are encountered less often by trekkers, but can pose real threats on high, exposed passes or in bad weather. Common-sense precautions bear repeating: wear or carry adequate clothing; keep dry; cover exposed extremities in severe weather; eat lots and carry emergency snacks; and make for shelter if conditions get bad.

The symptoms of hypothermia are similar to those of AMS: slurred speech, fatigue, irrational behaviour and loss of coordination. Low body temperature is the surest sign. The treatment, in a word, is heat. Get the victim out of the cold, put him or her in a good sleeping bag (with another person, if necessary) and ply with warm food and drink.

Frostbite appears initially as small white patches on exposed skin, caused by local freezing. The skin will feel cold and numb. To treat, apply warmth (not snow!). Avoid getting frostbite a second time, as this can lead to permanent damage. Snow blindness shouldn’t be a worry as long as you’re equipped with a good pair of sunglasses. On snowy surfaces you’ll need proper glacier glasses with side shields.

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