Altitude sickness is the single most important safety topic for anyone trekking in the Himalayas. It is not about fitness — a marathon runner is just as vulnerable as a casual walker. It is purely about how quickly you ascend and how well your body adapts to lower oxygen pressure. Here is what every trekker needs to know.
Acute Mountain Sickness (AMS) occurs when you ascend faster than your body can adjust to the reduced partial pressure of oxygen at altitude. At 3,500 m, there is roughly 35% less oxygen available than at sea level. At 5,000 m, it drops to 50%.
Symptoms typically begin within 6–12 hours of arriving at a new altitude.
Mild AMS (act now, do not ascend further):
Moderate AMS (descend or rest, do not ascend):
Severe AMS — High Altitude Pulmonary Edema (HAPE) or High Altitude Cerebral Edema (HACE) — MEDICAL EMERGENCY:
If HAPE or HACE is suspected: descend immediately, regardless of time of day or weather. Do not wait until morning.
1. Ascend slowly. Above 3,000 m, do not gain more than 300–500 metres of sleeping altitude per day. This is why reputable trek operators build acclimatisation rest days into their itineraries.
2. Climb high, sleep low. On acclimatisation days, hike to a higher altitude in the afternoon and return to a lower camp to sleep. This stimulates your body's adaptation without over-stressing it.
3. Hydrate aggressively. Drink 3–4 litres of water per day on the trail. Dehydration intensifies AMS symptoms dramatically. Avoid alcohol and sleeping pills at altitude — both suppress breathing.
4. Never ignore a headache. A persistent headache is your body's alarm system. Rest, drink water, take ibuprofen, and do not ascend until the headache resolves completely.
5. Know when to descend. Descending even 300–500 metres can dramatically improve symptoms. Pride is not worth your life.
Diamox is a carbonic anhydrase inhibitor that accelerates acclimatisation by stimulating faster and deeper breathing. It is widely used for AMS prevention and is effective.
Dosage for prevention: 125 mg twice daily, starting 24 hours before ascending above 2,500 m, continued for 48 hours at maximum altitude. Consult your doctor before taking Diamox — it is contraindicated in sulpha drug allergies and has interactions with other medications. Side effects include increased urination and tingling in the hands and feet.
Diamox treats AMS but does not eliminate the need to acclimatise or descend if symptoms are severe.
Every HeyHikers batch operates with trained guides carrying pulse oximeters. We check blood oxygen saturation (SpO2) at every high camp — healthy SpO2 at altitude is above 85%; values below 75% or dropping rapidly are flagged immediately. Our guides are trained in Wilderness First Response (WFR) and carry portable altitude chambers (Gamow bags) on high-altitude routes.
Our itineraries are designed with rest days at key altitude thresholds. We never push batches to ascend if a member is symptomatic.
The mountains are not going anywhere. Acclimatise properly and you will summit safely. Ignore the signs and the mountain wins.