What are the 3 stages of altitude sickness? The three forms hikers mean are acute mountain sickness (AMS), high-altitude pulmonary edema (HAPE), and high-altitude cerebral edema (HACE). AMS is the common early warning; HAPE affects the lungs; HACE affects the brain. HAPE and HACE are emergencies that require descent and medical care.
What are the 3 stages of altitude sickness?
The 3 stages of altitude sickness are AMS, HAPE, and HACE, but they do not always happen in a neat staircase.
That matters. A hiker can feel lousy with AMS and recover by stopping ascent, resting, and descending if symptoms worsen. Another hiker can develop HAPE without much headache or nausea first. HACE usually grows out of worsening AMS, but in the field, you do not need a perfect label. You need to spot trouble early and move down.
The CDC Yellow Book guidance on high-altitude travel divides altitude illness into AMS, HAPE, and HACE. The Wilderness Medical Society altitude illness guidelines also frame these as the core acute altitude illnesses hikers, climbers, and trip leaders should know.
| Form | What it affects | Red flag | Field response |
|---|---|---|---|
| AMS | Whole body response to altitude | Headache with nausea, dizziness, fatigue, or poor appetite | Stop going higher; monitor closely |
| HAPE | Lungs | Shortness of breath at rest, wet cough, chest tightness, poor performance | Descend and seek urgent medical help |
| HACE | Brain | Confusion, stumbling, behavior changes, severe drowsiness | Descend immediately and treat as an emergency |
What is acute mountain sickness?
AMS is the mildest and most common form of altitude illness.
It often shows up after a fast gain in sleeping elevation, especially above 8,000 ft [2,438 m]. Classic AMS feels like a bad hangover you did not earn: headache, nausea, dizziness, fatigue, poor appetite, and sometimes vomiting. The NIH MedlinePlus page on acute mountain sickness lists gradual ascent as the best prevention strategy.
Here is the trail test: if you have a headache plus other symptoms after gaining altitude, treat it as altitude illness until proven otherwise. Do not push to the next camp because the map says you are “almost there.” That is how mild AMS turns into a rescue problem.
- Headache after recent ascent
- Nausea or loss of appetite
- Dizziness or lightheadedness
- Unusual fatigue or weakness
- Poor sleep with other symptoms
- Vomiting or symptoms that keep getting worse
If symptoms are mild, the safest move is simple: stop ascending. Rest where you are. Hydrate normally. Eat what you can. If symptoms improve, you may be able to continue cautiously later. If symptoms worsen, descend.
What is HAPE?
HAPE is fluid buildup in the lungs at altitude, and it can kill.
High-altitude pulmonary edema does not feel like normal huffing and puffing on a steep climb. Everyone breathes harder at altitude. HAPE is different: your breathing stays wrong even when you stop. You may feel chest tightness, develop a cough, lag badly behind your group, or get short of breath while resting in camp.
Do not wait for dramatic symptoms. A hiker with HAPE may simply look slow, quiet, wiped out, or unable to recover after easy effort. That is your cue to pay attention, not tease them for being out of shape.
- Shortness of breath at rest
- Dry cough that may become wet or frothy
- Chest congestion or tightness
- Fast heart rate or fast breathing at rest
- Blue or gray lips, fingernails, or skin tone
- Extreme drop in performance compared with the rest of the group
HAPE needs descent and medical care. Oxygen and other treatments may help when available, but they do not make “keep climbing” a smart plan. If you suspect HAPE, stop the trip objective. The summit does not matter anymore.
What is HACE?
HACE is brain swelling from altitude illness, and it is a true emergency.
High-altitude cerebral edema can look messy at first. The person may seem drunk, irritable, oddly quiet, confused, or clumsy. They may stumble on flat ground, miss obvious instructions, slur words, or insist they are fine while clearly not fine. That last part is dangerous. A brain under stress does not always make good decisions.
A simple field check is the heel-to-toe walk. Ask the person to walk a straight line, placing one foot directly in front of the other. If they cannot do it and they have been gaining altitude, treat that as a major warning sign.
- Confusion or strange behavior
- Loss of coordination or stumbling
- Severe headache with worsening symptoms
- Slurred speech
- Extreme sleepiness or hard to wake
- Loss of consciousness
HACE requires immediate descent. Not later. Not after coffee. Not after “seeing how they feel in an hour.” Move down, get help, and keep the person protected from cold, falls, and exhaustion.
How fast can altitude sickness happen?
Altitude sickness can start within hours after gaining elevation.
AMS often appears 6 to 12 hours after ascent, though timing varies. HAPE and HACE can develop after a day or more at altitude, but rapid ascents, hard exertion, dehydration, cold stress, and a prior history of altitude illness can stack the deck against you.
The big trap is the drive-up trailhead. You can leave a low-elevation city in the morning, sleep above 9,000 ft [2,743 m] that night, then wonder why your head is pounding before breakfast. Your legs took a car. Your body still has to acclimatize.
What should you do if altitude sickness starts on a hike?
Stop going higher first.
That one move solves many problems before they grow teeth. You cannot out-hustle altitude illness by pushing upward. The body needs time and, when symptoms worsen, lower elevation.
- Stop ascent. Do not climb higher while symptoms are present.
- Check symptoms honestly. Headache plus nausea, dizziness, fatigue, or poor appetite points toward AMS.
- Look for lung and brain signs. Shortness of breath at rest, confusion, stumbling, or severe weakness changes the situation fast.
- Rest and monitor mild AMS. Stay put, reduce effort, eat, drink normally, and reassess often.
- Descend if symptoms worsen. If the person is not improving, go down.
- Treat HAPE or HACE as emergencies. Descend immediately and seek medical care.
The CDC’s traveler page on high altitudes says people with altitude illness should not travel to higher elevations until symptoms are gone. That is clean advice. Follow it.
How can you prevent altitude sickness before a high-elevation trip?
Ascend slowly and give your body time to acclimatize.
Fitness helps you hike, but it does not make you immune to altitude sickness. Strong runners, young backpackers, and experienced hikers can still get AMS, HAPE, or HACE. The mountain does not check your race results.
- Gain sleeping elevation gradually when your route allows it.
- Build in rest days on high trips instead of packing every day with big climbs.
- Keep the first day easier than your ego wants.
- Avoid alcohol and sedating drugs at altitude unless your clinician has cleared them.
- Eat enough, even when your appetite dips.
- Talk with a clinician before travel if you have heart, lung, blood, or sleep-breathing conditions.
- Ask a travel medicine or wilderness medicine clinician about prevention medication if you have a history of altitude illness or a fast itinerary.
Do not self-prescribe or swap pills in the parking lot. Some medications used for altitude illness prevention and treatment can be risky for certain people. Get personal medical guidance before the trip, not while your tent is flapping at 12,000 ft [3,658 m].
When to see a doctor
See a doctor before your trip if you have medical risk factors, and get urgent help during or after a trip for severe or worsening symptoms.
Before heading high, talk with a clinician if you have heart disease, lung disease, sickle cell disease, severe anemia, pulmonary hypertension, sleep apnea, a history of HAPE or HACE, or previous severe AMS. You should also ask for guidance if your itinerary forces a rapid ascent with no easy rest day.
During a trip, seek emergency medical help for shortness of breath at rest, confusion, trouble walking straight, blue lips or fingernails, coughing up frothy or bloody fluid, fainting, severe weakness, or symptoms that worsen despite stopping ascent. This article cannot diagnose you. When in doubt, descend and get medical care.
Related reading
- What Is Altitude Sickness? Symptoms and Causes
- Diamox for Altitude Sickness: What Hikers Should Know
- How to Prevent Altitude Sickness
- How Long Does Altitude Sickness Last?
FAQ
Can you get altitude sickness at 8,000 ft?
Yes. Altitude sickness can happen around 8,000 ft [2,438 m], especially after a fast ascent from low elevation. Risk rises as sleeping elevation increases and acclimatization time drops.
Are HAPE and HACE the same as severe AMS?
No. HAPE affects the lungs, while HACE affects the brain. HACE is often linked to worsening AMS, but HAPE can appear with or without clear AMS symptoms.
Should you keep hiking with mild altitude sickness?
No, do not keep ascending with symptoms. Stop, rest, and monitor; if symptoms get worse or do not improve, descend.
Does drinking water prevent altitude sickness?
No. Normal hydration helps you feel better and perform better, but water does not guarantee protection from AMS, HAPE, or HACE. Overdrinking can also create problems, so drink to thirst and keep eating.
What is the fastest way to feel better from altitude sickness?
Descending is the most reliable way to improve altitude illness, especially if symptoms are worsening. Mild AMS may improve with rest at the same elevation, but HAPE or HACE signs call for immediate descent and medical care.
Can fit hikers get altitude sickness?
Yes. Fitness does not prevent altitude sickness. A strong hiker can still get AMS, HAPE, or HACE after a rapid ascent or poor acclimatization.