Altitude sickness typically starts with a headache, nausea, fatigue, and dizziness that develop within hours to three days after reaching elevations above about 8,000 feet (2,450 meters). Around 25% of visitors sleeping above that altitude in Colorado experience it. The condition ranges from a mild, hangover-like feeling to two rare but life-threatening emergencies involving the lungs or brain, and knowing which symptoms fall into which category can help you respond quickly.
Common Symptoms of Mild Altitude Sickness
The most frequent form, called acute mountain sickness (AMS), is diagnosed entirely by symptoms. There’s no blood test or physical exam finding that confirms it. The core symptoms are:
- Headache: the hallmark symptom, present in nearly every case. It often feels like a dull, throbbing pressure that worsens with exertion.
- Nausea or loss of appetite: some people also vomit, especially if they keep pushing higher.
- Fatigue: a heavy, disproportionate tiredness beyond what you’d expect from the physical effort alone.
- Dizziness or lightheadedness: particularly noticeable when standing up or changing position.
Current Wilderness Medical Society guidelines emphasize headache as the defining symptom and have downgraded sleep disruption, which was previously considered a key indicator. Poor sleep at altitude is extremely common even in people who aren’t sick, so it’s no longer a reliable sign on its own. The better way to gauge whether you have AMS is to assess your overall well-being and how well you can function: if you feel genuinely unwell and can’t enjoy what you’re doing, that matters more than checking off individual symptoms on a list.
Symptoms typically appear 6 to 12 hours after arrival at a new altitude but can show up as late as three days in. They tend to be worst on the first or second night, because your body experiences its lowest oxygen levels during sleep.
When Altitude Sickness Affects the Lungs
High-altitude pulmonary edema (HAPE) occurs when fluid leaks into the lungs. It’s uncommon at ski-resort elevations (roughly 1 in 10,000 skiers in Colorado) but affects up to 1 in 100 travelers above 14,000 feet. HAPE can develop alongside regular altitude sickness or entirely on its own, which makes it easy to miss if you’re only watching for headache and nausea.
The symptoms follow a recognizable progression:
- Early: you tire more easily during physical activity than you’d expect, and you feel slightly breathless on exertion.
- Moderate: a dry cough develops, and breathlessness starts appearing even during light activity or rest.
- Advanced: the cough produces pink, frothy mucus. Your heart rate climbs, your breathing becomes rapid, and your lips or fingernails may turn blue.
A low-grade fever can also accompany HAPE, which sometimes leads people to mistake it for a chest infection. The key distinction is timing: if respiratory symptoms worsen within a day or two of gaining altitude, HAPE should be the first concern, not a cold.
When Altitude Sickness Affects the Brain
High-altitude cerebral edema (HACE) is swelling of the brain caused by fluid accumulation. It’s rare below 14,000 feet and almost always develops after AMS symptoms have already been present for a while, though it can escalate fast.
The earliest and most reliable warning sign is loss of coordination. A simple test: try walking heel-to-toe in a straight line. If you stumble or can’t stay balanced, that’s a red flag. Other symptoms include slurred speech, confusion, memory gaps, hallucinations, deep apathy, irritability, and an inability to take care of yourself (like not being able to put on your own boots). Left untreated, HACE progresses to coma. Seizures can also occur.
Some of these signs, particularly apathy and irritability, are subtle enough that traveling companions may notice them before the affected person does. If someone at altitude starts acting unlike themselves, seems confused, or can’t walk straight, treat it as an emergency.
Who Is More Likely To Get Sick
The strongest risk factors are straightforward: going high, going fast, and having gotten altitude sickness before. People who live near sea level and ascend rapidly to sleeping elevations above 8,000 feet are the classic candidates. A prior episode of altitude sickness is one of the best predictors of another one.
Age plays a surprising role. Risk is highest in younger adults and decreases with middle age, then rises again after about 70 as the heart and lungs lose some of their ability to compensate for lower oxygen. Women develop AMS at higher rates than men in several large studies (one found rates of 89% in women versus 69% in men at the same altitude). HAPE, however, occurs more often in men.
Pre-existing lung conditions like chronic bronchitis, emphysema, or COPD increase the risk, as does obesity. Active respiratory infections, even a common cold, substantially raise the chances of developing HAPE specifically, because the lungs are already compromised. People with unstable heart conditions are generally advised against physical activity at high altitude altogether.
Telling Altitude Sickness From Dehydration or Exhaustion
This is one of the trickiest parts of being at altitude, because dehydration and simple exhaustion share several symptoms with AMS: headache, fatigue, and nausea all overlap. A few things help you sort it out.
Timing and context are the most useful clues. Altitude sickness develops specifically after a gain in elevation, usually within the first 6 to 24 hours at a new height. Dehydration headaches tend to respond quickly to fluids, while AMS headaches don’t fully resolve with water alone. Exhaustion improves with rest; AMS often doesn’t, and it frequently worsens overnight. If you’ve been drinking enough water, you’ve rested, and you still feel lousy at altitude, AMS is the likely explanation. The safest approach is to assume any new illness above 8,000 feet is altitude-related until proven otherwise.
What To Do When Symptoms Appear
Mild AMS is manageable. Stop ascending, rest at your current elevation, and give your body time to adjust. Most people feel significantly better within 12 to 24 hours if they don’t go higher. Over-the-counter pain relievers can help with the headache. If symptoms don’t improve or they worsen, descending even 1,000 to 2,000 feet often brings noticeable relief.
For HAPE and HACE, descent is the primary treatment and should happen as soon as possible. Both conditions can deteriorate quickly, and waiting to see if things improve on their own is dangerous. Even a modest drop in elevation, 1,000 feet or so, can be enough to stabilize someone while you arrange further help. The key principle is simple: don’t go higher with symptoms of altitude sickness, and go lower if symptoms are getting worse or involve your lungs or brain.