Altitude Sickness: Prevention, Symptoms, and Treatment
Altitude sickness is one of the most underestimated hazards in outdoor recreation. It affects fit and unfit people equally, cannot be predicted by sea-level fitness, and kills people who ignore the warning signs. The good news is that altitude sickness is almost entirely preventable with proper acclimatization, and the early symptoms are unmistakable if you know what to look for. This guide covers the physiology, prevention, and treatment of altitude-related illness.
How Altitude Affects Your Body
At sea level, the atmosphere pushes oxygen into your blood efficiently. At 10,000 feet, atmospheric pressure is about 70 percent of sea level, and each breath delivers 30 percent less oxygen. At 14,000 feet, you get about 60 percent of sea-level oxygen. Your body compensates by breathing faster, increasing heart rate, and producing more red blood cells — but this adaptation takes days to weeks.
Altitude sickness occurs when you ascend faster than your body can acclimatize. The threshold for most people is 8,000-10,000 feet, though some experience symptoms as low as 6,500 feet. Fitness does not protect you. Genetic variation in oxygen processing determines susceptibility, and the only reliable predictor is your personal history at altitude.
Acute Mountain Sickness (AMS)
AMS is the mildest form of altitude sickness and the most common. Symptoms include headache (the hallmark symptom), nausea, fatigue, dizziness, loss of appetite, and difficulty sleeping. Symptoms typically begin 6-24 hours after arrival at altitude and feel similar to a hangover.
AMS is not dangerous by itself, but it is a warning. If you continue ascending with AMS symptoms, you risk progression to the life-threatening forms: High Altitude Pulmonary Edema (HAPE, fluid in the lungs) and High Altitude Cerebral Edema (HACE, swelling of the brain). Both can be fatal within hours if not treated with immediate descent.
Prevention: The Golden Rules of Acclimatization
The primary rule: above 10,000 feet, increase your sleeping elevation by no more than 1,000-1,500 feet per day. Take a rest day (no elevation gain) every 3,000 feet of ascent. You can hike higher during the day as long as you return to sleep at the appropriate elevation — "climb high, sleep low."
Hydrate aggressively (3-4 liters per day at altitude). Avoid alcohol for the first 48 hours at a new altitude. Eat carbohydrate-rich meals (your body prefers carbs for fuel in low- oxygen environments). Avoid sleeping pills, which suppress breathing and worsen oxygen levels during sleep.
Medication: Acetazolamide (Diamox)
Acetazolamide (Diamox) is a prescription medication that speeds acclimatization by increasing breathing rate and improving blood oxygenation. It is used preventively (125-250 mg twice daily, starting 24 hours before ascent) and for treatment of mild AMS. It is not a substitute for proper acclimatization but a useful supplement.
Side effects include tingling in fingers and toes, altered taste of carbonated beverages, and increased urination. These are normal and not dangerous. Acetazolamide is a sulfa drug, so people with sulfa allergies should not use it. Dexamethasone is an alternative for sulfa-allergic individuals and is the emergency treatment for HACE. Consult a physician before any altitude trip above 14,000 feet.
When to Descend: Non-Negotiable
Descend immediately if: AMS symptoms worsen despite rest and medication at the same elevation. Any signs of HAPE (persistent cough, breathlessness at rest, gurgling sounds when breathing, blue lips or fingernails). Any signs of HACE (confusion, staggering gait, inability to walk heel-to-toe, altered consciousness).
Descent of just 1,000-3,000 feet often produces dramatic improvement. Do not wait until morning. Do not wait for the group. Do not assume symptoms will improve with sleep. Altitude sickness deaths almost always involve someone who recognized symptoms but decided to wait. The mountain will be there tomorrow. Descend now and come back when you are well.
Frequently Asked Questions
At what elevation does altitude sickness start?
Most people become susceptible above 8,000-10,000 feet, though some experience symptoms as low as 6,500 feet. The threshold varies by individual and cannot be predicted by fitness level. If you have a history of altitude sickness, you are more susceptible in the future.
Does physical fitness prevent altitude sickness?
No. Altitude sickness is determined by genetic variation in oxygen processing and acclimatization rate, not cardiovascular fitness. Elite athletes get altitude sickness at the same rate as sedentary individuals. Fitness helps you perform better at altitude, but it does not prevent AMS.
How fast can I ascend safely?
Above 10,000 feet, increase sleeping elevation by no more than 1,000-1,500 feet per day with a rest day every 3,000 feet. Below 10,000 feet, most people can ascend freely. These are guidelines — if you develop symptoms at any rate of ascent, stop and acclimatize before going higher.
Should I take Diamox preventively?
Diamox is recommended for rapid ascent profiles (unavoidable due to logistics), for people with a history of AMS, and for trips above 14,000 feet. Start 125-250 mg twice daily 24 hours before ascent. It is prescription-only — consult a physician. It is not a substitute for proper acclimatization.