Planning
Altitude sickness is a set of conditions caused by ascending faster than the body can adapt to reduced oxygen, and it affects some trekkers above 3,000 m regardless of fitness. The Himalayan Rescue Association's Pheriche clinic recommends ascending no more than 500 m of sleeping altitude per day above 3,000 m, the rule our itineraries are built around. On every departure above 4,000 m our guides log client oxygen saturation twice daily and hold the authority to turn a trekker around.
The pages below cover acute mountain sickness symptoms and prevention, acclimatization scheduling by route, and the helicopter-evacuation insurance that every high trek requires.
Altitude, not fitness, is what turns trekkers back on high routes: a marathon runner who ascends too fast is at more risk than a moderate walker who follows a 500 m-per-day sleeping-altitude schedule. Acute mountain sickness starts as headache, nausea, and poor sleep, and it can progress to the life-threatening lung and brain conditions described below. The only reliable cure is descent, which is why guide-led itineraries build in acclimatization days and keep evacuation options open.
Safety-critical detail sits on two pages reviewed by our senior guide: the altitude sickness prevention guide covers symptoms, the Lake Louise score, and acetazolamide, and the helicopter evacuation insurance coverage guide covers what to carry before you ascend.
Most trekkers underestimate how much of a route sits above the altitude where symptoms begin. Acute mountain sickness can appear from about 2,500 m, and every route in the table below spends multiple nights above 3,000 m.
| Route | Highest point | Nights above 4,000 m |
|---|---|---|
| Poon Hill | 3,210 m | 0 |
| Langtang Valley | 4,984 m at Kyanjin Ri | 1 to 2 |
| Annapurna Base Camp | 4,130 m | 1 |
| Everest Base Camp | 5,545 m at Kala Patthar | 4 to 5 |
| Gokyo Lakes | 5,357 m at Gokyo Ri | 4 to 5 |
| Annapurna Circuit | 5,416 m at Thorong La | 3 to 4 |
| Three Passes | 5,545 m | 8 to 10 |
| Island Peak | 6,189 m | 7 to 9 |
Sleeping altitude gains stay under 500 m per day above 3,000 m, with a rest day for every 1,000 m of net ascent. This is the Himalayan Rescue Association's Pheriche clinic guidance, and it governs the shape of every itinerary on this site before anything else is scheduled.
What you climb during the day does not count against the rule; what you sleep at does. That is why acclimatization days are built as walks rather than rest in bed: on the Everest route the Namche day climbs to about 3,880 m at the Everest View Hotel before dropping back to sleep at 3,440 m, and the Dingboche day climbs Nangkartshang before sleeping lower. A route that cannot fit the 500 m rule into its available days gets more days, not a compressed schedule.
Headache is the entry symptom, and it rarely arrives alone. The Lake Louise scoring system turns "feeling off" into a number the guide can track across days, by adding a headache score to scores for gastrointestinal upset, fatigue, and dizziness.
| Severity | What it looks like | What we do |
|---|---|---|
| Mild AMS | Headache, mild nausea, poor sleep | Stop ascending, hydrate, reassess in 24 hours |
| Moderate AMS | Persistent headache, vomiting, no appetite | Descend 500 m to 1,000 m |
| HAPE | Breathlessness at rest, cough, tightness in the chest | Immediate descent, oxygen, evacuation |
| HACE | Confusion, stumbling walk, loss of coordination | Immediate descent, oxygen, evacuation |
Breathlessness at rest, a stumbling walk, or confusion are never wait-and-see symptoms. HAPE and HACE both kill within hours if a party keeps climbing, and both improve quickly on descent, which is the entire reason our guides hold turnaround authority that is not open to negotiation.
Above 4,000 m, guides log each client's oxygen saturation twice daily, at breakfast and dinner, using a pulse oximeter carried on every departure. Readings fall with altitude in everyone, so the number matters less than its direction: a saturation that keeps dropping across two checks at the same altitude, or that pairs with symptoms, triggers a conversation about descending.
Guides also watch for what clients do not report. Skipping dinner, an unusually slow walk into the lodge, and a second night of broken sleep are the practical warning signs that show up before anyone volunteers a symptom.
Anyone who has had altitude sickness before is the clearest risk group, because a previous episode at a given altitude and ascent rate predicts the next one better than any fitness measure. Beyond that, risk rises with ascent rate rather than with age or ability, which is why a trekker flying straight into Lukla at 2,860 m and walking hard for two days is more exposed than a fitter trekker on a longer profile.
Some conditions need a conversation with your own doctor well before booking, not at the trailhead: heart and lung disease, sickle cell trait, uncontrolled high blood pressure, pregnancy, and recent surgery. Children can trek at altitude but cannot always describe what they are feeling, so family departures need slower profiles and a lower threshold for turning around. Our family packages are built to that shape, and the Everest Base Camp with children guide covers the age and pacing questions in detail.
Fitness helps you enjoy the walking. It does not protect you from altitude, and treating it as protection is the most common mistake we see in fast, strong trekkers on their first Himalayan route. The acclimatization schedule guide shows how the rest days are placed on each route, and our safety protocols page covers the monitoring and evacuation procedure in full.
Planning
