Altitude Sickness and Acclimatisation on Everest Treks

The air at Namche Bazaar holds the same proportion of oxygen as the air at sea level, about 21 percent, but the pressure is lower, so every breath carries less oxygen into your lungs. At Namche, at 3,440 m, each breath brings in roughly two-thirds of what it would at sea level. At Everest Base Camp, at 5,364 m, it is about half. Your body adapts by breathing faster and deeper, raising your heart rate and, over days and weeks, making more red blood cells.

Acclimatisation is that process of adjustment. It works for almost everyone, but it takes time, and it cannot be hurried by fitness, willpower or youth. Fit people sometimes get sicker because they walk too fast. The aim of a trekking itinerary is to give your body time to catch up before you climb again.

Acute mountain sickness, AMS, is the common form of altitude illness. It usually starts six to twelve hours after arriving at a new height, and it feels like a hangover: headache, plus one or more of nausea or loss of appetite, tiredness or weakness, dizziness and poor sleep. On the Everest trail it most often appears at Namche, Dingboche or Lobuche, especially after a big gain in sleeping altitude.

Mild AMS is common and not dangerous if you respond to it. The rule is to stop climbing until the symptoms go. That may mean an extra night at the same village, rest, fluids and simple painkillers such as paracetamol or ibuprofen. If symptoms get worse despite rest, you go down, usually 300 to 500 m is enough to feel better. Going up with worsening symptoms is the mistake that turns a headache into an emergency.

Two rarer conditions are life-threatening, and every trekker should know the signs. High altitude pulmonary oedema, HAPE, is fluid in the lungs. Look for breathlessness at rest, not just on hills, a dry cough that becomes wet, a rattling sound in the chest, unusual tiredness and blue lips or fingertips. It can develop without much headache.

High altitude cerebral oedema, HACE, is swelling of the brain, usually a progression from severe AMS. The key signs are confusion, unusual behaviour, drowsiness and loss of balance: the person cannot walk heel to toe in a straight line. Both need immediate descent, oxygen if available, and medical help. In the Khumbu, helicopters can reach most villages in good weather, which is why insurance that covers helicopter rescue is essential.

• Breathless at rest: suspect HAPE, descend now.
• Confused, drowsy or staggering: suspect HACE, descend now.
• Never leave a sick trekker alone, and never send them down alone.
• Descend at night if needed; waiting for morning can be fatal.

The guidelines most mountain medicine bodies use are simple. Above about 3,000 m, increase the altitude you sleep at by no more than 300 to 500 m a night, and take an extra night at the same altitude every three to four days or every 1,000 m. Climbing higher during the day is fine, and helpful, as long as you come down to sleep. That is the logic of climb high, sleep low.

On the Everest Base Camp trek, that is why the rest days fall at Namche and Dingboche. Namche to Tengboche is a gain of about 430 m, Tengboche to Dingboche about 540 m, Dingboche to Lobuche about 530 m. These steps are already at the upper end of the guideline, so skipping a rest day pushes them well beyond it. On the Gokyo route the steps are shorter, which is one reason it suits slower acclimatisers.

Most of acclimatisation is simply living well at altitude. Walk slowly enough that you can talk in full sentences. Drink three to four litres of water a day above 3,500 m; your urine should be pale. Eat even when you are not hungry, especially carbohydrates such as dal bhat, potatoes, noodles and porridge. Avoid alcohol and sleeping pills above Namche, as both depress breathing at night.

Many guides now carry a pulse oximeter and take readings morning and evening. Oxygen saturation of 85 to 90 percent at Namche and in the 80s or high 70s at Lobuche and Gorak Shep is normal; the trend and how you feel matter more than one number. A falling reading together with symptoms is a reason to stop. A low reading in someone who feels well is a reason to watch, not to panic.

Anyone can get altitude illness, but some trekkers should plan more carefully. If you have had AMS, HAPE or HACE before, you are more likely to get it again, so tell your guide and your doctor, and choose a slower itinerary such as the Gokyo route or add a third night at Namche. People with heart or lung conditions, high blood pressure, sickle cell disease or pregnancy should get specialist medical advice before booking any trek above 3,500 m.

Children acclimatise about as well as adults but may not describe how they feel. Watch for crankiness, poor appetite and unusual sleepiness, and treat these as possible symptoms. Older trekkers often do well because they walk slowly; the main risk is existing health conditions rather than age itself. Whoever you are, the rule is the same: tell someone early, and do not go higher with symptoms.

Acetazolamide, often sold as Diamox, speeds up acclimatisation by making your body breathe more. Wilderness Medical Society guidelines suggest 125 mg twice a day as a preventive dose for people at moderate or high risk, started the day before ascent. It is widely available in Kathmandu pharmacies without a prescription, but you should discuss it with your own doctor before you travel, especially if you have a sulfa allergy, kidney problems or take other medicines.

Common side effects are tingling fingers and toes, more frequent urination and a strange taste in fizzy drinks. Acetazolamide does not mask symptoms; if you feel ill while taking it, you are ill. Other drugs, such as dexamethasone and nifedipine, are used to treat serious altitude illness and should be carried by your group's guide or first-aid kit rather than taken on your own judgement.

The Himalayan Rescue Association, a Nepali non-profit founded in 1973, runs aid posts at Pheriche and Machhermo in the main trekking seasons, staffed by volunteer doctors. They give a free afternoon talk on altitude illness, which every trekker should attend, and treat patients for a fee. In spring, a clinic called Everest ER operates at base camp for expeditions. Namche has a health post, and the Khunde Hospital, founded by the Himalayan Trust in 1966, is a short walk from Khumjung.

Helicopters fly rescues from most villages when weather allows, arranged through your guide, agency and insurer. Before you leave Kathmandu, make sure your guide has your insurance policy number and the insurer's emergency phone number.