High altitude starts at 8,000 feet (2,438 meters) above sea level. At this elevation, the air contains noticeably less oxygen, and your body begins making measurable adjustments to compensate. Above that threshold, altitude is further divided into “very high” and “extreme” categories, each carrying progressively greater physiological demands and health risks.
Altitude Categories and What They Mean
Altitude is classified into three tiers based on how severely the thinner air affects the human body:
- High altitude: 8,000 to 12,000 feet (2,438 to 3,658 meters)
- Very high altitude: 12,000 to 18,000 feet (3,658 to 5,486 meters)
- Extreme altitude: Above 18,000 feet (5,486 meters)
To put these numbers in perspective, Denver sits at about 5,280 feet and falls below the “high altitude” threshold, though some people still notice the thinner air there. La Paz, Bolivia, at 11,942 feet, sits squarely in the high altitude range. Quito, Ecuador, at 9,350 feet, is right in the middle of it. The cabin of a commercial airplane is pressurized to a maximum equivalent of 8,000 feet per federal regulations, which is why you sometimes feel slightly off on long flights.
Why High Altitude Affects Your Body
The percentage of oxygen in the air stays the same at any altitude: 21%. What changes is atmospheric pressure. As pressure drops, each breath delivers fewer oxygen molecules to your lungs. At 5,500 meters (about 18,000 feet), the available oxygen pressure is half of what it is at sea level. At the summit of Everest, it drops to just 30%.
Your body typically doesn’t ramp up its breathing rate until you reach around 3,000 meters (roughly 10,000 feet). Below that point, the reduction in oxygen is mild enough that your system compensates without dramatic changes. Above it, breathing increases exponentially as the oxygen deficit grows. Blood oxygen saturation, which normally ranges from 95% to 100% at sea level, begins declining between 2,000 and 3,000 meters. Values below 90% are considered abnormal at any altitude.
How Altitude Affects Physical Performance
Even fit, healthy people lose aerobic capacity at altitude. Research on endurance athletes found that maximum oxygen uptake (a key measure of aerobic fitness) drops in a straight line as elevation increases: about 6.3% per 1,000 meters of ascent. That means at 2,800 meters, a well-trained athlete has already lost roughly 17% of their peak aerobic capacity compared to sea level.
The performance impact is even steeper. Time to exhaustion during high-intensity exercise fell by 14.5% per 1,000 meters in the same study. If you’re planning a hike, a ski trip, or any strenuous activity at altitude, expect everything to feel significantly harder than it would at home, especially in the first few days.
Altitude Sickness: Symptoms and Severity
Acute mountain sickness (AMS) is the most common altitude-related illness. It typically strikes unacclimatized people who ascend above 2,500 meters (about 8,200 feet), though highly susceptible individuals can develop symptoms at lower elevations. The core symptoms are headache, nausea, fatigue, and dizziness. A formal scoring system called the Lake Louise scale rates each of these on a 0 to 3 scale. A score of 3 or higher, with headache present, qualifies as AMS.
Most cases of AMS are uncomfortable but manageable. Headache is the hallmark symptom and is required for diagnosis. Nausea can range from mild appetite loss to incapacitating vomiting. Fatigue and dizziness round out the picture. These symptoms usually appear within 6 to 12 hours of arriving at altitude and often resolve within a day or two if you stop ascending.
The more dangerous conditions are high-altitude pulmonary edema (fluid in the lungs) and high-altitude cerebral edema (brain swelling). Cerebral edema is rare below 3,500 meters and most often occurs above 4,000 meters, affecting roughly 0.5% to 1% of travelers at those heights. Between 13% and 20% of people who develop fluid in their lungs also develop brain swelling. Both conditions are medical emergencies that require immediate descent.
Safe Ascent and Acclimatization
The single most effective way to prevent altitude illness is controlling how quickly you go up. The Wilderness Medical Society recommends avoiding a sleeping altitude above 2,750 meters (9,000 feet) on the first day if you’re coming from low elevation. Once above 3,000 meters, your sleeping altitude should increase by no more than 500 meters (about 1,600 feet) per night. For every additional 1,000 meters gained, plan an extra rest day for acclimatization.
A practical rule used by climbers: “climb high, sleep low.” You can hike or climb above your target sleeping altitude during the day, as long as you descend to a lower camp for the night. This gives your body repeated exposure to lower oxygen levels while letting it recover during sleep. If you’re heading to a high-altitude city like Quito or spending a ski vacation above 9,000 feet, even a couple of days of gradual adjustment can make a significant difference in how you feel.
Prescription medication is available for people who can’t ascend gradually or who have a history of altitude sickness. Your doctor can discuss preventive options before a trip, particularly if you’ll be flying directly into a high-altitude destination with no opportunity for gradual acclimatization.