Elevation sickness, more commonly called altitude sickness, is a group of symptoms that occur when your body can’t get enough oxygen from the thinner air at high elevations. It can affect anyone, regardless of age or fitness level, and typically begins at elevations above 8,000 feet (2,438 meters). In its mildest form it feels like a bad hangover. In its most severe forms, it can be fatal within hours.
Why Thinner Air Makes You Sick
The air at high elevations contains the same percentage of oxygen as at sea level, about 21%. The difference is pressure. As you climb higher, atmospheric pressure drops, which means each breath delivers fewer oxygen molecules into your lungs. Your blood oxygen levels fall, a state called hypoxia, and your body scrambles to compensate by breathing faster, increasing heart rate, and producing more red blood cells. When you ascend faster than your body can adjust, altitude sickness sets in.
Short-term exposure above 8,000 feet can cause low blood oxygen, sleep disturbances, and exercise intolerance even in healthy people. The higher you go and the faster you get there, the more likely you are to develop symptoms.
Three Forms of Altitude Sickness
Altitude sickness exists on a spectrum with three distinct conditions, each progressively more dangerous.
Acute Mountain Sickness (AMS)
This is the mildest and most common form. Symptoms usually appear 6 to 12 hours after arriving at a new elevation and include headache, nausea, dizziness, fatigue, and poor appetite. It feels remarkably similar to a hangover or the early stages of the flu. Most cases resolve on their own within one to three days if you stop ascending and give your body time to acclimatize. Over-the-counter pain relievers can help with the headache, and staying hydrated makes a noticeable difference.
High Altitude Pulmonary Edema (HAPE)
HAPE occurs when fluid builds up in your lungs. It often develops one to four days after reaching a high elevation and is a medical emergency. Early signs include a dry cough that may progress to coughing up pink, frothy mucus, along with shortness of breath that worsens even at rest. Your heart rate and breathing rate increase, you may develop a low fever, and your lips or fingernails can turn blue. While HAPE isn’t technically the most severe form of altitude sickness, it can kill faster than any other because fluid in the lungs directly blocks oxygen exchange. Without treatment, it can be fatal within hours.
High Altitude Cerebral Edema (HACE)
HACE is the most dangerous form. It happens when the brain swells due to oxygen deprivation and fluid leakage from blood vessels. The hallmark symptom is a loss of coordination: you can’t walk in a straight line, as if you were heavily intoxicated. Confusion, disorientation, and altered consciousness follow. HACE often develops alongside or shortly after AMS or HAPE and requires emergency evacuation to a lower altitude immediately.
Altitude Categories and Risk Levels
Altitude-related illness follows rough thresholds that help frame your risk:
- High altitude: 8,000 to 12,000 feet (2,438 to 3,658 meters). This is where most cases of AMS occur. Popular destinations in this range include ski resorts in Colorado, Cusco in Peru, and parts of the Ethiopian highlands.
- Very high altitude: 12,000 to 18,000 feet (3,658 to 5,487 meters). Risk of all three forms increases significantly. Everest Base Camp, at roughly 17,600 feet, sits at the top of this range.
- Extreme altitude: Above 18,000 feet (5,500 meters). The body cannot fully acclimatize at these heights. Prolonged stays cause progressive deterioration no matter how slowly you ascend.
Who Gets It and Why
The single biggest predictor of altitude sickness is your rate of ascent. Fly from sea level to a city at 11,000 feet and your risk is high. Drive up over several days, sleeping at gradually increasing elevations, and your risk drops dramatically.
A previous episode of altitude sickness is a strong indicator that you’re susceptible to it again. There appears to be a significant genetic component governing how efficiently your body responds to low oxygen, though no reliable test exists to predict who will be affected. One counterintuitive finding: physical fitness does not protect you. Fit, young hikers sometimes push through early symptoms and ascend faster, which actually increases their risk compared to someone who moves more slowly.
Living at low elevation means your body has no baseline adaptation. If you normally live near sea level, you’re more vulnerable than someone who lives at 5,000 feet year-round, even if you exercise regularly.
How to Prevent It
Gradual ascent is the most effective prevention strategy. Above 10,000 feet, the standard recommendation is to increase your sleeping elevation by no more than about 1,000 to 1,500 feet per day, with a rest day (no gain in sleeping elevation) every three to four days. You can hike higher during the day as long as you come back down to sleep. This “climb high, sleep low” approach gives your body hours of recovery at a more manageable altitude each night.
For situations where a gradual ascent isn’t possible, such as flying directly into a high-altitude city, a prescription medication called acetazolamide (brand name Diamox) can help. It works by making your blood slightly more acidic, which triggers faster, deeper breathing and speeds up acclimatization. The typical preventive dose is 125 mg twice daily, starting the day before ascent and continuing for the first two days at altitude. The most common side effect is tingling in your fingers, toes, and lips, which is harmless but noticeable. It also changes the taste of carbonated drinks, making them taste flat or metallic.
Hydration matters, but not for the reasons most people assume. Water doesn’t treat altitude sickness directly. You lose more moisture through rapid breathing at altitude, so dehydration adds symptoms like headache and fatigue on top of whatever the altitude itself is doing. Alcohol, on the other hand, worsens dehydration and impairs your breathing response during sleep, making it worth avoiding during your first day or two at elevation.
What to Do if Symptoms Appear
For mild AMS, the most important step is to stop ascending. Rest at your current elevation for a day or two. If symptoms don’t improve or they worsen, descend. Even dropping 1,000 to 2,000 feet often brings noticeable relief within hours.
For HAPE or HACE, descent is the treatment. Every hour at the same elevation allows the condition to worsen. If you’re with someone who becomes confused, can’t walk straight, or is coughing up pink frothy mucus, get them to a lower altitude as quickly as possible. Supplemental oxygen, if available, buys time during evacuation. In remote settings where descent is impossible or delayed, emergency medications like dexamethasone (for brain swelling) and nifedipine (for fluid in the lungs) can stabilize the situation temporarily, but they are not substitutes for getting lower.
The encouraging part: mild to moderate altitude sickness resolves completely once you descend or acclimatize. Most people with AMS feel significantly better within 12 to 24 hours of stopping their ascent. Even HAPE, if caught early and treated with descent and supplemental oxygen, typically resolves without lasting damage. HACE is the exception. Delayed treatment can lead to permanent neurological problems, which is why recognizing the warning signs early, particularly loss of coordination and confusion, is so important.