Altitude Sickness Guide
Altitude Safety for Trekkers
AMS, HAPE, and HACE: what they are, how to prevent them, and when to descend. From 15 years of guiding above 4,000m in Nepal.
Last updated · June 2026
AMS, HAPE, and HACE: what they are, how to prevent them, and when to descend. From 15 years of guiding above 4,000m in Nepal.
Altitude sickness is caused by reduced oxygen pressure in the air as elevation increases. At 5,364m (Everest Base Camp), the air contains roughly 50% of the oxygen available at sea level. Your body can adjust to this, but it needs time. Ascending too fast is the primary cause of altitude-related illness on Nepal treks.
There are three forms of altitude sickness, each progressively more dangerous. Acute Mountain Sickness (AMS) is the mildest and most common, affecting roughly half of all trekkers above 4,000m. High Altitude Pulmonary Edema (HAPE) causes fluid to accumulate in the lungs. High Altitude Cerebral Edema (HACE) causes swelling of the brain. Both HAPE and HACE are medical emergencies that require immediate descent.
The good news: altitude sickness is almost entirely preventable. Proper acclimatization (ascending slowly, taking rest days, and following the "climb high, sleep low" principle) eliminates the vast majority of cases. The trekking industry in Nepal has decades of experience managing altitude, and well-designed itineraries from agencies like Mountain Hawk Trek build acclimatization days into every route above 3,000m.
This page covers the symptoms to recognize, the acclimatization schedule to follow, when to take Diamox, and when to descend. It is written from 15 years of guiding experience across the Everest, Annapurna, Manaslu, and Langtang regions. It is not a substitute for medical advice.
Do not ascend further. Rest at current altitude for 24 hours. If symptoms worsen, descend immediately. Mild AMS affects roughly 50% of trekkers above 4,000m and usually resolves within 1 to 2 days with rest.
Descend immediately, at least 500m to 1,000m. HAPE is life-threatening if untreated. Administer supplemental oxygen if available. Nifedipine (20mg slow-release) can buy time during descent. Do not wait for morning. Helicopter evacuation should be considered.
Descend immediately. HACE is a medical emergency. The person may not recognize their own symptoms. Administer dexamethasone (8mg initial, then 4mg every 6 hours) if available. Use a Gamow bag if descent is not immediately possible. Helicopter evacuation is strongly recommended.
No altitude restrictions. Ascend at any pace. Most trekkers feel normal.
Gain no more than 500m of sleeping altitude per day. Take a rest day for every 1,000m gained.
Same 500m/day rule. Rest days are mandatory, not optional. Trek high, sleep low when possible.
Spend minimum time above 5,000m. Most trekkers summit (Kala Patthar, Thorong La) and descend the same day. Do not sleep above 5,000m without prior acclimatization.
"Climb high, sleep low" means hiking to a higher point during the day and descending to sleep. For example, on an acclimatization day in Dingboche (4,410m), trekkers hike to Chhukung (4,730m) and return. This trains the body without the stress of sleeping at the higher altitude.
Permits close. Weather windows narrow. The best Everest region teahouse beds fill by October, and spring season opens again in March. Our Kathmandu team is already booking for autumn. If you have been planning a trek, this is the window.
