Altitude sickness: prevent it, spot it, act on it

What happens up high

The air at 4,000 m holds about 40% less oxygen per breath than at sea level. Your body copes by breathing faster, raising your heart rate and, over days, making more red blood cells. That adjustment is acclimatisation, and it can't be rushed.

Altitude sickness is what happens when you climb faster than your body adjusts. It can start from about 2,500 m, and it's common on Himalayan treks: most treks on Trail to Tides sleep above 3,000 m.

Who gets it

Anyone. Marathon runners get it; first-time trekkers often don't. The biggest risk factors are going up too fast, having had it before, and arriving straight at height (flying into Leh at 3,500 m, for example).

• Fitness doesn't prevent it. Fit people often climb faster, which raises the risk.
• Age and sex make little difference. Young, fit men get it as often as anyone.
• A past episode is the best predictor: plan an even slower ascent and talk to a doctor.
• Smoking, alcohol and sleeping pills make things worse: they blunt breathing, especially at night.

Acclimatise: the rules that work

1. 1

   Above 3,000 m, raise your sleeping height by no more than 300 to 500 m a night. Where you walk during the day matters less than where you sleep.

2. 2

   Take a rest day every 3 to 4 days, or for every 1,000 m you gain. Good itineraries build these in; check yours does.

3. 3

   Climb high, sleep low. On rest days, walk a few hundred metres higher, then come back down to sleep.

4. 4

   Flying to Leh or another high town? Rest completely for the first 48 hours. No sightseeing, no stairs if you can avoid them.

5. 5

   Drink to stay hydrated (pale urine), eat well, and keep alcohol for the way down. Dehydration feels like altitude sickness and makes it harder to spot.

6. 6

   Walk slowly enough to talk. If you can't speak a full sentence, slow down.

Know the signs

There are three conditions. The first is common and manageable; the other two kill if ignored.

Mild AMS often appears 6 to 12 hours after arriving at a new height, and is usually worst on the first night. Tell your trek leader about every headache. It's not complaining; it's information they need.

What to do

• Mild AMS: stop going up. Rest at the same height, drink, eat, take paracetamol or ibuprofen for the headache. If you're fine after a day or two, you can carry on slowly.
• Getting worse, or not better after 24 hours at the same height: go down. Even 300 to 500 m makes a real difference.
• Any sign of HACE or HAPE: descend now, day or night, and never alone. Give oxygen if the team carries it. Call for evacuation if the person can't walk.
• Never leave someone with symptoms alone in a tent while the group goes higher.

Medicines

Acetazolamide (Diamox) speeds up acclimatisation and is widely used to prevent AMS on fast ascents (like flying to Leh) or by people who've had it before. It's prescription medicine: talk to your doctor before the trip, especially if you're allergic to sulfa drugs. Tingling fingers and needing to pee more are normal side effects.

Drugs like dexamethasone and nifedipine are used for HACE and HAPE emergencies by trained leaders and doctors. They are not something to self-prescribe, and they don't replace going down.

A pulse oximeter (the finger clip) is useful, but readings at altitude are naturally low and vary a lot. How someone looks and feels, and whether it's getting worse, matters more than any single number.