Altitude Sickness (Mountain Sickness): Complete Guide to Symptoms, Risks, and Recovery
Updated: 2 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: September 2026
TL;DR
Altitude sickness happens when your body doesn't have time to adjust to the thinner air at high elevation, where every breath contains less oxygen. The most common form, acute mountain sickness (AMS), is usually mild and resolves within a day or two if you stop ascending and rest. However, two severe forms — high altitude pulmonary edema (HAPE) and high altitude cerebral edema (HACE) — can become life-threatening within hours. The single most effective treatment is descending to a lower altitude: at least 984 feet (300 meters), and up to 3,281 feet (1,000 meters) if symptoms worsen. With a flexible ascent schedule of no more than 1,640 feet (500 meters) per day above 8,000 feet, most cases are preventable.
Quick Answer
What is altitude sickness?
Altitude sickness occurs when the body can't adapt quickly enough to lower oxygen availability at high elevation — and it affects almost everyone who ascends rapidly to 11,000 feet (3,352 meters).
The most common symptom is headache, which usually appears within the first day of reaching high altitude; nausea, fatigue, and trouble sleeping often accompany it.
Emergency signs — shortness of breath at rest, chest pain, clumsiness, confusion, slurred speech, or decreased consciousness — require immediate descent and medical care.
The cure is to descend to a lower altitude: at least 984 feet (300 meters), up to 3,281 feet (1,000 meters) if symptoms worsen; supplemental oxygen and medications such as acetazolamide, dexamethasone, and nifedipine can support treatment.
What Is Altitude Sickness?
Altitude sickness is an illness that develops when your body doesn't have enough time to adjust to the lower oxygen availability found higher in the atmosphere. As you climb, the atmosphere becomes thinner. Each breath still contains the same amount of air, but less oxygen.
The condition is usually preventable and treatable. But when it becomes severe, it can rapidly turn life-threatening. Recognizing the symptoms early — and pausing your ascent at the first signs — is what keeps most cases from developing into dangerous complications.
Altitude sickness is also called mountain sickness or acute mountain sickness (AMS). The severe forms carry specific medical names, and knowing the differences between them matters in the mountains.
What Are the Three Types of Altitude Sickness?
There are three distinct forms of altitude sickness, ranging from mild and common to severe and life-threatening.
Type | Full Name | What Happens | Severity |
AMS | Acute Mountain Sickness | The body reacts to low oxygen with headache and flu-like symptoms | Mildest and most common |
HAPE | High Altitude Pulmonary Edema | Fluid fills the lungs, making breathing increasingly difficult | Medical emergency; can cause death faster than other forms |
HACE | High Altitude Cerebral Edema | The brain swells from fluid buildup at high altitude | Most severe form; requires emergency care |
HAPE is worth understanding precisely: it is not the most severe form, but it can kill more quickly than the others. HACE is the most dangerous overall, and both demand immediate action.
How Common Is Altitude Sickness?
Altitude sickness is common among people who have not had time to acclimate to high elevation. Prevalence depends heavily on how high you go and how fast you get there.
Elevation | Risk Level |
Below 8,000 feet (2,438 meters) | AMS is rare |
Around 8,000 feet (2,438 meters) | Risk begins to rise; the safe daily ascent limit applies above this line |
11,000 feet (3,352 meters), ascended quickly | Almost everyone develops AMS |
In practical terms, a drive up to a mountain town around 8,000 feet carries some risk, while flying directly to a destination near 11,000 feet carries risk for nearly every traveler. The speed of your arrival matters as much as the altitude itself.
What Are the Symptoms of Altitude Sickness?
Symptoms of acute mountain sickness usually appear within the first day or so of reaching high altitude. The severe forms, HAPE and HACE, typically develop later — usually two to five days after arrival.
Symptom | Details |
Headache | The most common symptom |
Nausea and vomiting | Often accompanies the headache |
Loss of appetite | Food suddenly seems unappealing |
Fatigue | Tiredness even when resting |
Malaise | A general feeling of being unwell |
Trouble sleeping | Restless nights at altitude |
Dizziness or lightheadedness | Feeling off-balance |
Vision changes | Rare; signals more severe illness caused by ruptured blood vessels in the retinas |
A key warning: vision changes at altitude are rare and indicate more serious altitude sickness. Treat them as an urgent signal, not a passing inconvenience.
What Are the Emergency Signs of Severe Altitude Sickness?
Some signs mean the situation has escalated beyond mild AMS into HAPE or HACE — territory where time is measured in hours. HAPE can be deadly within 12 hours, and HACE within 24 hours.
Emergency Sign | Associated With |
Shortness of breath, even when resting | HAPE |
Chest tightness or pain | HAPE |
Clumsiness or coordination problems | HACE |
Confusion | HACE |
Slurred speech | HACE |
Decreased consciousness | HACE |
If you or a companion shows any of these signs, go down the mountain immediately and seek emergency medical care. Do not wait to see if symptoms improve on their own.
What Causes Altitude Sickness?
The cause is simple: given enough time, your body can adapt to high altitude. Without time to adapt, altitude sickness develops.
Climbing too fast removes the window your body needs to make physiological adjustments to thinner air. This is why acclimatization — spending extra days gaining altitude gradually — is the foundation of all prevention advice.
What Are the Risk Factors?
Several factors measurably increase your odds of developing altitude sickness. One common assumption, however, is wrong.
Risk Factor | Why It Matters |
Higher altitude | The higher you go, the greater the risk |
Speed of ascent | Flying to altitude is much riskier than hiking or driving there slowly |
Ascending faster than 1,640 feet (500 meters) per day above 8,000 feet | Exceeds the body's ability to acclimate |
Alcohol and sedatives | Medical and nonmedical drugs — especially sedatives — greatly increase your chances |
Previous altitude sickness | A history of altitude sickness, especially HAPE or HACE, raises future risk |
The misconception: physical fitness is not a risk factor. Being in excellent shape does not protect you. Fit climbers get altitude sickness just as often as everyone else.
One nuance: while fitness doesn't cause altitude sickness, the condition can worsen certain existing health problems, such as anemia or chronic obstructive pulmonary disease (COPD). People with these conditions should consult a clinician before traveling to high altitude.
What Are the Complications?
The primary complication is AMS progressing into HAPE or HACE — and either form can be deadly.
Form | Time to Death if Untreated |
HAPE | Within 12 hours |
HACE | Within 24 hours |
This is why AMS must never be dismissed as a nuisance. A headache today can become a life-threatening emergency tomorrow if you keep ascending despite symptoms.
How Is Altitude Sickness Diagnosed?
Diagnosis is based primarily on your symptoms and a physical exam. A clinician will check your breathing, blood pressure, and heart rate, and may test your coordination to look for early signs of HAPE or HACE.
Diagnosis in remote mountain settings has a practical advantage: medical equipment is usually unavailable, but by the time a sick person descends, symptoms have often improved or disappeared entirely — which itself supports the diagnosis.
Test | Purpose |
Physical exam (breathing, blood pressure, heart rate, coordination) | Primary diagnosis; screens for HAPE and HACE |
X-ray | Checks the lungs for the fluid characteristic of HAPE |
EKG (electrocardiogram) | Rules out a heart attack, which can mimic symptoms |
MRI | Looks for brain swelling and can detect longer-lasting changes from past altitude sickness |
What Is the Treatment for Altitude Sickness?
There is essentially one cure for altitude sickness: go lower. Everything else supports or buys time until you can descend.
Step | What It Involves |
1. Stop and acclimate | Rest at the first signs of AMS instead of continuing upward |
2. Descend | If symptoms are severe or worsening, descend at least 984 feet (300 meters) — up to 3,281 feet (1,000 meters); most experts advise continuing down until symptoms improve |
3. Supplemental oxygen | Higher oxygen concentrations offset the symptoms |
4. Medications | Acetazolamide, dexamethasone, and nifedipine can treat symptoms and delay worsening; dexamethasone and nifedipine are reserved for moderate or worse cases |
5. Hyperbaric therapy | Portable hyperbaric chambers called Gamow bags work like treatment for decompression sickness in scuba divers, delaying worsening until evacuation is possible |
How Soon Will I Feel Better?
Recovery speed depends on what you do. Descending is by far the fastest path to relief — most people feel progressively better as they go down.
Approach | Typical Recovery Time |
Descending | Fastest relief; progressive improvement during descent |
Stopping to acclimate | Symptoms last several hours up to a day |
Minor symptoms left to resolve | Usually gone within a day or two |
What Is the Outlook for Altitude Sickness?
The outlook is generally very good. Most people never get beyond AMS, and if they stop ascending when symptoms begin, the headache typically stays at the same level and resolves within a day or two.
Scenario | Outlook |
Mild AMS, no further ascent | Not dangerous; unpleasant but resolves in 1–2 days |
Milder forms over a few days | Symptoms disappear as the body acclimates |
HAPE or HACE | Life-threatening emergencies that worsen quickly; deadly within 24 hours without treatment |
Prevention remains the best medicine: keep a flexible schedule that allows gradual acclimatization, and never ignore severe symptoms.
When Should I Go to the Emergency Room?
Seek emergency care immediately if you experience any of the following at altitude: shortness of breath even at rest, chest tightness or pain, clumsiness or coordination problems, confusion, slurred speech, or decreased consciousness. These are the warning signs of HAPE or HACE, and both conditions are measured in hours, not days.
Conclusion
Altitude sickness is a predictable and largely preventable condition — but only if you respect the numbers. Below 8,000 feet (2,438 meters), AMS is rare. Ascend no more than 1,640 feet (500 meters) per day once you're above that line, keep alcohol and sedatives out of the equation, and be ready to stop or turn back at the first headache.
If severe warning signs appear — trouble breathing at rest, chest pain, confusion, or slurred speech — descend immediately and get emergency help. The mountains will be there another day; your body's ability to recover may not be if you ignore the signals.
Your next step: Planning a high-altitude trip? Talk to your healthcare provider beforehand about a prevention plan — including whether acetazolamide is right for you — and build rest days into your itinerary. Then share this guide with every member of your group before you climb.
Frequently Asked Questions
What altitude does altitude sickness start at?
Altitude sickness can begin as low as 6,000–8,000 feet, but acute mountain sickness is rare below 8,000 feet (2,438 meters). Risk climbs steeply with elevation: almost everyone who ascends quickly to 11,000 feet (3,352 meters) develops AMS.
How long does it take for altitude sickness to kick in?
Symptoms of acute mountain sickness usually appear within the first day of reaching high altitude. The severe forms — HAPE and HACE — typically take longer, usually developing two to five days after arrival.
What are the first signs of altitude sickness?
Headache is the most common and often earliest symptom. It is frequently joined by nausea, loss of appetite, fatigue even at rest, trouble sleeping, and dizziness.
How do you get rid of altitude sickness fast?
The fastest way to feel better is descending to a lower altitude — at least 984 feet (300 meters), up to 3,281 feet (1,000 meters) if symptoms worsen. Supplemental oxygen and medications can help, but nothing replaces going down.
Can you get altitude sickness flying?
Yes. Flying to a high-altitude destination is much riskier than hiking or driving there gradually, because your body gets no time to acclimatize. Almost everyone who flies quickly to 11,000 feet (3,352 meters) will develop AMS.
Does fitness protect against altitude sickness?
No. Physical fitness is not a risk factor — fit climbers get altitude sickness at the same rates as everyone else. However, altitude sickness can worsen existing conditions such as anemia or COPD.
What are the warning signs of HAPE and HACE?
Watch for shortness of breath at rest and chest tightness (HAPE), and clumsiness, confusion, slurred speech, and decreased consciousness (HACE). Both can be deadly within hours — descend immediately and seek emergency care.
What medications treat altitude sickness?
Acetazolamide, dexamethasone, and nifedipine are used to treat altitude sickness and delay severe symptoms. Dexamethasone and nifedipine are reserved for moderate or worse cases.
References
This article is based on medically reviewed clinical information from Basnyat B, Tabin G, "Altitude Illness," Harrison's Principles of Internal Medicine, 21st ed. (2022), Burtscher J, et al., "Flying to high-altitude destinations risk," Journal of Travel Medicine (2023), the Center for Wilderness Safety's guide to oxygen levels at altitude, Jensen JD, Vincent AL, "High Altitude Cerebral Edema," StatPearls (2022), Jensen JD, Vincent AL, "High Altitude Pulmonary Edema," StatPearls (2022), Peacock AJ, "ABC of oxygen: oxygen at high altitude," BMJ (1998), and Prince TS, Thurman J, Huebner K, "Acute Mountain Sickness," StatPearls (2022).
This article is for general educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for guidance specific to your health and travel plans.
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