You're three hours into your 14er climb, making good time, when the headache starts. At first it's mild—easily ignored as you push toward the summit. An hour later you're nauseous, dizzy, and your head pounds with every heartbeat. You've hit altitude sickness, and the only cure is going down. Understanding altitude sickness before it strikes can mean the difference between a successful summit and a dangerous epic.
What Is Altitude Sickness?
Altitude sickness, medically termed acute mountain sickness (AMS), occurs when you ascend to high elevation faster than your body can adapt to reduced oxygen levels. At sea level, each breath delivers a certain amount of oxygen to your bloodstream. At 10,000 feet, that same breath delivers 30% less oxygen. At 14,000 feet, you're getting 40% less oxygen per breath.
Your body can adapt to these conditions through acclimatization—physiological changes that improve oxygen delivery and utilization. But acclimatization takes time, typically days to weeks depending on altitude. When you ascend faster than your body can adapt, altitude sickness results.
Anyone can get altitude sickness regardless of fitness level, age, or experience. Olympic athletes get it. Elite mountaineers get it. The fittest person in your climbing party might be the one who suffers while others feel fine. Physical conditioning helps you hike faster and farther, but it doesn't prevent altitude sickness.
The Three Forms of Altitude Sickness
Acute Mountain Sickness (AMS) - Mild to Moderate
AMS is the most common and least dangerous form. Symptoms typically appear 6-24 hours after reaching higher elevation and include headache (the hallmark symptom), nausea or vomiting, fatigue beyond what exertion explains, dizziness or lightheadedness, difficulty sleeping, and loss of appetite.
Mild AMS feels like a bad hangover. You're functional but uncomfortable. Moderate AMS significantly impairs your ability to climb and enjoy the experience. Most cases of AMS resolve with rest, hydration, and time at altitude without ascending further. Severe AMS requires descent.
High Altitude Pulmonary Edema (HAPE) - Life-Threatening
HAPE occurs when fluid accumulates in the lungs, preventing proper oxygen exchange. It typically develops 2-4 days after reaching high altitude, though it can appear sooner with rapid ascent.
HAPE symptoms include: severe shortness of breath even at rest, persistent cough (sometimes producing pink or frothy sputum), extreme weakness and fatigue, chest tightness or congestion, rapid heart rate and breathing rate even when resting, and blue or gray lips and fingernails indicating oxygen deprivation.
HAPE is a medical emergency. Without immediate descent and treatment, it can kill within hours. The gurgling sound of fluid in lungs (audible with a stethoscope or sometimes by ear) confirms HAPE. If you suspect HAPE in yourself or a companion, descend immediately—do not wait for symptoms to worsen or morning to arrive.
High Altitude Cerebral Edema (HACE) - Life-Threatening
HACE results from brain swelling due to altitude. It's the least common form of altitude sickness but the most dangerous. HACE typically occurs above 12,000 feet after several days at altitude, though rapid ascents can trigger it sooner.
HACE symptoms include: severe headache unrelieved by medication, confusion and impaired judgment (victims may insist they're fine when clearly not), loss of coordination (ataxia—inability to walk straight line), hallucinations or altered consciousness, and progression to coma and death if untreated.
The "tandem walk test" can identify HACE early: have the person walk heel-to-toe in a straight line. If they can't do this simple task, suspect HACE and descend immediately. HACE is a medical emergency requiring immediate descent and emergency services.
Why Altitude Sickness Happens
At sea level, atmospheric pressure pushes oxygen into your bloodstream efficiently. As you climb, air pressure decreases. At 14,000 feet, atmospheric pressure is only 60% of sea-level pressure, meaning less oxygen gets pushed into your blood with each breath.
Your body compensates through several mechanisms during acclimatization: breathing rate and depth increase to pull in more air, heart rate increases to circulate oxygen-carrying blood faster, red blood cell production increases over days/weeks to carry more oxygen, and blood plasma volume decreases to concentrate red blood cells.
These adaptations take time. Ascend too quickly and the gap between oxygen demand and oxygen delivery creates altitude sickness. Why some people acclimatize easily while others suffer remains partially mysterious—genetics, previous altitude exposure, and individual physiology all play roles that science doesn't fully understand.
Elevation Thresholds
Different elevations present different risks:
Below 8,000 feet: Altitude sickness is rare. Most people feel no effects.
8,000-12,000 feet: Mild AMS can occur, especially with rapid ascent. Symptoms are typically manageable. Colorado's 14er trailheads and summit regions fall in this range.
12,000-18,000 feet: AMS becomes common. HAPE and HACE become possible, especially with poor acclimatization. Mount Rainier and other Pacific Northwest peaks reach this zone.
18,000-26,000 feet: Extreme altitude. Nearly everyone experiences some altitude sickness symptoms. Acclimatization is essential. Denali and international peaks like Aconcagua.
Above 26,000 feet: The "death zone" where human bodies cannot acclimatize long-term. Everest and the highest Himalayan peaks. Your body literally begins dying above this altitude.
Prevention Strategies
Gradual ascent: The most effective prevention. The mountaineering rule is "climb high, sleep low"—you can day hike to higher elevations but should sleep at lower elevations where your body recovers better. For multi-day climbs, limit your sleeping elevation gain to 1,000-1,500 feet per day once above 10,000 feet.
Acclimatization days: Build rest days into your climbing schedule. Spend 2-3 nights at moderate elevation (8,000-10,000 feet) before attempting summits above 14,000 feet. For Rainier or other big peaks, multiple weeks of gradual altitude exposure ideal.
Hydration: Dehydration worsens altitude sickness. Drink significantly more water than you think you need—urine should run pale yellow or clear. At altitude, you lose more water through breathing in dry air and increased respiration rate.
Avoid alcohol and sleeping pills: Both suppress breathing and worsen acclimatization. Skip the celebratory beer at base camp until after you're back at lower elevation.
Don't overtax yourself: Climbing slowly and steadily allows better acclimatization than racing uphill. "Pole pole" (slowly slowly in Swahili) is the mantra on Kilimanjaro for good reason—it works.
Consider medication: Acetazolamide (Diamox) helps prevent altitude sickness by speeding acclimatization. Typically 125-250mg twice daily starting 24 hours before ascent. Discuss with your doctor—it requires prescription and has side effects including increased urination and tingling fingers.
Recognizing Symptoms Early
Catching altitude sickness in its early, mild stages allows you to take action before it becomes serious. Monitor yourself and climbing partners for these warning signs.
Headache: The first and most common symptom. Not all high-altitude headaches are AMS—dehydration, sun exposure, and muscle tension also cause headaches. But assume headache at altitude is AMS until proven otherwise.
Unusual fatigue: Altitude makes everyone tired, but if you're significantly more exhausted than your companions with similar fitness, suspect AMS.
Nausea or appetite loss: Feeling queasy or having no interest in food at altitude suggests developing AMS.
Difficulty sleeping: Periodic breathing (Cheyne-Stokes breathing) at altitude is normal—your breathing stops briefly, then restarts. But severe insomnia combined with other symptoms indicates AMS.
The Lake Louise AMS scoring system helps quantify symptoms. Rate each symptom 0-3 (none/mild/moderate/severe): headache, gastrointestinal symptoms, fatigue, dizziness, difficulty sleeping. Scores of 3-5 indicate mild AMS, 6+ indicates moderate to severe AMS requiring action.
Treatment: What to Do When Symptoms Strike
Stop ascending: The first rule is don't go higher with symptoms. Many cases of mild AMS resolve with a rest day at current elevation. Pushing higher with symptoms invites progression to severe AMS or worse.
Hydrate and rest: Drink water, eat if you can, rest. Mild symptoms often improve after 6-12 hours at the same elevation as your body catches up with acclimatization.
Medication for symptom relief: Ibuprofen or acetaminophen can relieve headache and make you more comfortable while you rest and acclimatize. They don't cure AMS but reduce symptoms.
Descend if symptoms worsen or don't improve: If symptoms don't improve after 24 hours of rest, or if they worsen at all, descend. Even 1,000-2,000 feet of descent often provides dramatic relief.
Emergency descent for severe symptoms: HAPE and HACE require immediate descent regardless of time of day or weather. Descent is the only reliable treatment. In emergencies, descend to the lowest elevation feasible—the farther down, the better the chances of recovery.
Supplemental oxygen: If available (guide services sometimes carry oxygen), supplemental oxygen treats altitude sickness. But it's a temporary measure—descent is still necessary.
Portable altitude chamber (Gamow bag): A fabric chamber that uses a pump to increase pressure, simulating descent. Useful in emergencies when weather prevents actual descent, but descent remains the definitive treatment.
When to Call for Help
Altitude sickness can progress from mild to life-threatening rapidly. Know when you need emergency assistance:
Call for rescue if:
- Symptoms of HAPE or HACE are present
- Person cannot walk unassisted
- Mental confusion or altered consciousness occurs
- Breathing becomes severely labored even at rest
- Person cannot descend under their own power
In the United States, call 911. In national parks, rangers have specific emergency protocols. Many mountains have search and rescue teams experienced in altitude illness evacuations.
Don't hesitate to call for help. Altitude sickness kills, and outcomes improve dramatically with early intervention. The embarrassment of a "false alarm" beats the tragedy of waiting too long.
Who Is Most at Risk?
Certain factors increase altitude sickness susceptibility:
Living at low elevation: Sea-level residents are more vulnerable than people who live in Denver (5,280 feet) or other moderate-altitude cities.
Rapid ascent: Flying or driving from sea level to 10,000+ feet and immediately climbing increases risk dramatically compared to gradual approaches.
Previous altitude sickness: If you've had AMS before, you're more likely to get it again. Some people are simply more susceptible.
Youth: Children and young adults under 25 show slightly higher AMS rates, possibly due to rushing ahead without listening to their bodies.
Pre-existing conditions: Heart or lung conditions can worsen at altitude. Consult your doctor before high-altitude travel if you have cardiovascular or pulmonary issues.
Factors that DON'T increase risk: Being female (myth), being out of shape (fitness doesn't prevent AMS, though it helps with the climb itself), or being older (elderly climbers often do better than young ones due to conservative pacing).
Common Myths and Misconceptions
Myth: "I'm fit so I won't get altitude sickness." False. Fitness helps you hike faster and farther but doesn't prevent AMS. Ironically, very fit people sometimes push too hard too fast, worsening symptoms.
Myth: "Drinking more water prevents altitude sickness." Partially true. Hydration helps but doesn't guarantee prevention. You can be perfectly hydrated and still develop AMS if you ascend too quickly.
Myth: "If I've climbed to this altitude before without problems, I'm immune." False. Previous success doesn't guarantee future immunity. You can get altitude sickness on a mountain you've climbed before without issues.
Myth: "Altitude sickness only happens above 14,000 feet." False. AMS can occur as low as 8,000 feet, especially with rapid ascent. Most Colorado 14er trailheads sit above 8,000 feet.
Myth: "Toughing it out shows strength." Dangerous mindset. Ignoring symptoms and pushing higher is how people die. Smart mountaineers listen to their bodies and descend when necessary.
Long-Term Acclimatization for Extended Trips
Multi-week expeditions to high mountains require systematic acclimatization schedules. Climbers attempting peaks like Denali, Aconcagua, or Himalayan mountains spend 2-4 weeks gradually ascending with built-in rest days.
A typical Denali climb, for example, involves flying to base camp at 7,200 feet, spending several days there, moving to camps at 9,500 feet, 11,000 feet, 14,000 feet, and 17,000 feet with rest days built in. Summit day from 17,000-foot high camp involves climbing to 20,310 feet and descending the same day—"climb high, sleep low" in action.
This gradual approach allows red blood cell production to increase, blood chemistry to adjust, and breathing patterns to optimize. It's why Everest expeditions spend 6-8 weeks on the mountain despite the actual climbing taking far less time.
Special Considerations for Colorado 14ers
Colorado's 14ers present unique altitude challenges. Most trailheads sit at 9,000-10,000 feet—already at an elevation where some people feel symptoms. The summit push adds another 4,000-5,000 feet, reaching the zone where AMS becomes common.
For sea-level visitors: Arrive in Colorado 1-2 days early. Sleep in Denver or at moderate elevation. Do easy day hikes before attempting a 14er. Don't fly into Denver on Friday and attempt a 14er Saturday—you're asking for altitude sickness.
For Denver residents: You have built-in acclimatization living at 5,280 feet, but you're not immune. Take your time on summit day, stay hydrated, and recognize symptoms if they develop.
Start times matter: Beginning at 4-6 AM gives you more time to descend if altitude sickness strikes. Starting at 8-9 AM risks being high on the mountain when symptoms worsen in afternoon.
Return to Altitude After Descent
If altitude sickness forces you down, when can you try again? It depends on severity.
Mild AMS: After descending and feeling better, you can attempt the same peak 1-2 days later using a more gradual approach or better acclimatization.
Severe AMS, HAPE, or HACE: Wait at least 1-2 weeks before returning to high altitude. Your body needs recovery time. Some doctors recommend longer waiting periods or medical clearance before attempting similar elevations again.
Learn from the experience. What went wrong? Did you ascend too fast? Skip acclimatization days? Push through early symptoms? Adjust your approach for the next attempt.
The Bottom Line
Altitude sickness is preventable and treatable, but only if you respect it. The keys to staying safe are gradual ascent and acclimatization when possible, recognizing early symptoms and taking action, never ascending with worsening symptoms, and descending immediately if severe symptoms develop.
Mountains don't care about summit fever, tight schedules, or ego. They'll be there next year. You won't be if you ignore altitude sickness symptoms and push higher. The climbers who enjoy long, successful mountaineering careers are those who know when to turn around.
Listen to your body, respect altitude, and live to climb another day.
Track your high-altitude climbs and monitor your altitude tolerance over time at TheSummitLog.com