What is altitude sickness? It’s your body’s reaction to gaining elevation faster than it can adapt to thinner air, usually above 8,000 feet [2,400 meters]. It can start with headache, nausea, dizziness, fatigue, or poor sleep, and it can become dangerous if you keep climbing.
What is altitude sickness?
Altitude sickness is illness caused by rapid exposure to high elevation and lower oxygen pressure.
At altitude, the air still contains oxygen, but the lower air pressure makes it harder for your lungs to move enough oxygen into your blood. Your body tries to catch up by breathing faster, raising your heart rate, shifting fluids, and changing blood chemistry. If you ascend too quickly, those adjustments lag behind.
Hikers often use “altitude sickness” as a catch-all term, but medical sources separate it into three main problems: acute mountain sickness, high-altitude pulmonary edema, and high-altitude cerebral edema. The CDC Yellow Book guide to high-altitude travel describes these as the major altitude illness syndromes.
Acute mountain sickness, or AMS, is the common one. It feels like a mean hangover you did not earn. HAPE affects the lungs. HACE affects the brain. Those last two are emergencies.
What are the altitude sickness symptoms?
The most common altitude sickness symptoms are headache, nausea, dizziness, fatigue, appetite loss, and poor sleep.
Symptoms often show up within 2 to 12 hours after arriving at a higher elevation or after climbing higher, according to the CDC. They commonly appear during or after the first night up high. That timing matters. If you felt fine at the trailhead and miserable by camp, altitude belongs high on the suspect list.
Common mild to moderate symptoms include:
- Headache
- Nausea or vomiting
- Dizziness or lightheadedness
- Fatigue that feels bigger than the effort
- Loss of appetite
- Trouble sleeping
- Shortness of breath with exertion
- Rapid pulse
Severe warning signs are different. They are not “tough it out” problems. The NIH MedlinePlus guide to acute mountain sickness lists confusion, shortness of breath at rest, chest tightness, blue or gray skin color, coughing blood, and inability to walk straight as serious signs that need urgent action.
What causes altitude sickness?
Altitude sickness is caused by going too high too fast for your body to acclimatize.
The engine behind it is simple: lower air pressure means your body gets less usable oxygen with each breath. The faster you climb, the less time your body has to adapt. Hard effort, cold, dehydration, poor sleep, alcohol, and an aggressive itinerary can make the day feel worse, but speed of ascent is the big lever.
Your home elevation matters too. If you live near sea level and drive or fly straight to a mountain town, then hit a high trail the next morning, you have given your body a short fuse. Fitness helps you hike faster. It does not make you immune to altitude sickness.
Risk climbs when you:
- Sleep much higher than the night before
- Ascend quickly above 8,000 feet [2,400 meters]
- Have had altitude sickness before
- Push hard before acclimatizing
- Use alcohol or sedatives at altitude
- Have certain heart, lung, nervous system, or anemia-related conditions
How high do you have to be to get altitude sickness?
Altitude sickness usually appears above 8,000 feet [2,400 meters], but some people feel symptoms lower.
There is no magic line painted across the mountain. One hiker can sleep at 10,000 feet [3,050 meters] and feel normal. Another can get a pounding headache at the same campsite. What changed? Ascent rate, sleep elevation, effort, prior history, hydration, and individual biology.
For hikers in the United States, common trip settings include Colorado fourteeners, Sierra Nevada passes, Wyoming high country, the White Mountains in California, and volcano climbs in the Pacific Northwest. You do not need Himalayan elevation to feel bad. A fast weekend jump from sea level to a trailhead near 10,000 feet [3,050 meters] can be enough.
What is the difference between AMS, HAPE, and HACE?
AMS is the common form; HAPE and HACE are life-threatening forms that need immediate descent and medical help.
| Type | What it affects | Red flags |
|---|---|---|
| Acute mountain sickness (AMS) | Whole body, especially the head and stomach | Headache with nausea, dizziness, fatigue, poor sleep, or appetite loss |
| High-altitude pulmonary edema (HAPE) | Lungs | Shortness of breath at rest, cough, chest tightness, weakness, blue lips or skin |
| High-altitude cerebral edema (HACE) | Brain | Confusion, clumsiness, inability to walk straight, extreme drowsiness, behavior changes |
Think of AMS as a stop sign. HAPE and HACE are flashing red lights. The Wilderness Medical Society altitude illness guidelines emphasize descent, oxygen when available, and urgent treatment for severe altitude illness. On a hike, the safest plain-English rule is this: if symptoms are getting worse, do not climb higher.
How can you tell altitude sickness from dehydration or being out of shape?
You cannot always tell with certainty on the trail, so treat suspicious symptoms conservatively.
Dehydration can cause headache, fatigue, dizziness, and nausea. So can heat, cold, poor sleep, low food intake, infection, migraine, carbon monoxide exposure in enclosed spaces, and simply going too hard. Altitude sickness overlaps with all of them.
Use the pattern. If symptoms begin after a gain in elevation, include headache, and improve when you stop ascending or descend, altitude is a strong possibility. If a hiking partner becomes confused, cannot walk straight, or is short of breath while resting, do not argue about labels. Go down and get help.
What should you do if you get altitude sickness while hiking?
Stop ascending, rest, and descend if symptoms are severe, worsening, or not improving.
- Stop going up. Climbing higher with symptoms is how a rough day becomes a rescue.
- Rest at the same elevation. Keep the pace low, add layers, eat simple food, and drink normally.
- Watch the trend. Mild symptoms that improve with rest are different from symptoms that worsen over hours.
- Descend if symptoms worsen. Even dropping 1,000 feet [300 meters] can make a major difference for many people.
- Use oxygen if available. Guided climbs, huts, clinics, and some expeditions may have it.
- Get medical help for severe signs. Confusion, trouble walking, breathlessness at rest, chest symptoms, blue skin, or coughing blood are emergency signals.
Do not sleep higher if you feel worse. Do not split the group and leave a sick hiker alone. Do not rely on grit, caffeine, or a summit deadline.
How do you prevent altitude sickness?
The best prevention is a slow ascent with conservative sleep elevations.
If your trip gives you the option, build in time. Sleep lower before sleeping higher. Keep the first day easy. Carry less ego than water. MedlinePlus recommends gradual ascent and, above 9,850 feet [3,000 meters], increasing sleeping elevation slowly rather than making big jumps.
Use this checklist before a high hike:
- Spend a night or two at moderate elevation before going higher.
- Avoid big sleeping-elevation jumps when you can.
- Plan a rest day after major elevation gain.
- Keep early effort mellow, even if your legs feel strong.
- Eat regular meals with easy carbohydrates.
- Drink to thirst; do not force extreme water intake.
- Skip alcohol before and during the first high nights.
- Know the descent route before symptoms start.
- Talk with a clinician before the trip if you have heart, lung, sleep-breathing, neurologic, or anemia concerns.
Some hikers use prescription medication to prevent or treat altitude illness. That decision belongs with a qualified medical professional who knows your health history and your route. Do not borrow pills from a friend at the trailhead.
When to see a doctor
See a doctor before your trip if you have medical risks, and seek urgent care if severe symptoms appear at altitude.
Before you go, talk with a clinician if you have had altitude sickness before, plan a rapid ascent, have heart or lung disease, use oxygen or CPAP, are pregnant, have anemia, or take medications that affect breathing or alertness. A travel medicine clinician can help you plan a safer ascent and discuss prescription options when appropriate.
On the mountain, call emergency services or evacuate for medical care if you or a hiking partner has confusion, fainting, inability to walk straight, shortness of breath at rest, chest tightness, blue lips or skin, coughing blood, or symptoms that keep getting worse after stopping ascent. The Mayo Clinic overview of shortness of breath causes also treats sudden breathing trouble as a symptom that can need urgent evaluation.
Related reading
- Diamox for Altitude Sickness: What Hikers Should Know
- How to Prevent Altitude Sickness
- How Long Does Altitude Sickness Last?
- The 3 Stages of Altitude Sickness: AMS, HAPE, HACE
FAQ
Can you get altitude sickness at 7,000 feet?
Yes, but it is less common than above 8,000 feet [2,400 meters]. Sensitive hikers, rapid travel from low elevation, hard exertion, poor sleep, or medical conditions can make symptoms show up lower.
How long does altitude sickness last?
Mild symptoms often improve within a day or two if you stop ascending and allow your body to acclimatize. Symptoms that worsen, last despite rest, or include breathing or brain-related signs need descent and medical care.
Can you sleep off altitude sickness?
No, not if symptoms are moderate, worsening, or severe. Sleeping higher while sick can make altitude illness worse, especially if breathing problems or confusion are starting.
Does drinking more water prevent altitude sickness?
No, water alone does not prevent altitude sickness. Normal hydration helps you feel and perform better, but gradual ascent is the main prevention strategy.
Can fit hikers still get altitude sickness?
Yes. Strong legs and good cardio do not guarantee acclimatization, and fit hikers sometimes get into trouble because they climb fast and ignore early symptoms.
What is the fastest way to feel better from altitude sickness?
The fastest reliable move is to stop ascending and descend if symptoms are significant or worsening. Oxygen and prescription medicines may help in the right setting, but descent is the trail fix you can usually control.