Altitude Sickness Guide for Indian Trekkers: AMS, HAPE and HACE Explained

01Why Altitude Makes You Ill

As you go higher, air pressure falls, so each breath carries less oxygen. Above about 2,500 to 3,000 m most people notice it: you breathe harder, your heart rate rises, you sleep badly. Given time, the body adapts by breathing more deeply, producing more red blood cells and adjusting its chemistry. Altitude illness happens when you climb faster than that adaptation can keep up.

Susceptibility varies from person to person and is hard to predict. Fitness does not protect you; fit people sometimes do worse because they climb too fast. The single biggest factor you control is your rate of ascent.

02Acute Mountain Sickness (AMS)

AMS is the common, milder form. It usually begins 6 to 12 hours after arriving at a new altitude. The hallmark is a headache, together with one or more of: nausea or loss of appetite, fatigue or weakness, dizziness, and poor sleep. It feels a lot like a hangover.

Mild AMS usually improves if you stop climbing, rest, drink enough and allow a day or two to adapt. It gets worse if you keep going up. The rule is simple: never ascend with symptoms of AMS.

03The Danger Signs: HAPE and HACE

High-altitude pulmonary oedema (HAPE) is fluid in the lungs. Warning signs are breathlessness at rest, not only when walking; a persistent cough, sometimes with frothy or pink sputum; unusual fatigue; and a gurgling or crackling chest. It can develop even without prior AMS.

High-altitude cerebral oedema (HACE) is swelling of the brain, usually a progression of severe AMS. Warning signs are confusion, unusual behaviour, extreme drowsiness and loss of coordination. A simple field test is to ask the person to walk heel-to-toe along a straight line; if they cannot, treat it as HACE.

04How to Prevent It: Ascent Rules

These figures follow widely used wilderness medicine guidance. Good trek itineraries are built around them, which is why treks like Everest Base Camp include acclimatisation days. Be suspicious of any itinerary that gains a lot of sleeping altitude very quickly to save a day.

• Above 3,000 m, increase your sleeping altitude by no more than about 300-500 m per day
• Take a rest day every 3 to 4 days, or every 1,000 m of height gained
• Climb high, sleep low: day hikes above camp help, but return to sleep
• Avoid flying or driving straight to very high altitude when you can
• Drink to stay hydrated, but extra water does not prevent AMS on its own
• Avoid alcohol and sleeping pills at altitude, especially in the first days

05Diamox (Acetazolamide)

Acetazolamide, sold in India as Diamox, speeds up acclimatisation and is used both to prevent AMS on rapid ascents and to treat mild AMS. It is a prescription medicine with side effects, commonly tingling in the fingers and toes, more frequent urination and making fizzy drinks taste flat. People with a sulfa drug allergy and certain other conditions should not take it.

Do not take it on the advice of a fellow trekker. See a doctor before your trip, discuss whether it is appropriate for your itinerary, and ideally try a dose at home first. Remember that medication does not replace a sensible ascent rate, and it does not make it safe to keep climbing with symptoms.

06Ladakh: The Fly-In Risk

Leh sits at about 3,500 m. Flying in from Delhi takes you from near sea level to that altitude in about an hour, with no chance to adapt on the way. Ladakh's administration has long advised visitors to rest completely for the first 24 to 48 hours after arrival. Many tourist health problems in Ladakh come from ignoring that: going sightseeing the first afternoon, then driving to passes above 5,000 m on day two.

If you are trekking in Ladakh, build in at least two full days in Leh before going higher, and treat the high road passes as serious altitude, not just a photo stop.

07What to Do on the Trail

• Know the symptoms before you go, and talk about them openly in your group
• Report headaches and nausea to your trek leader early; do not hide them
• Never leave someone with altitude symptoms alone
• If symptoms worsen despite rest, descend, ideally by 500 m or more
• Signs of HAPE or HACE mean immediate descent and evacuation
• Pulse oximeter readings are useful but not decisive; trust symptoms