As altitude increases, air pressure drops, temperatures fall, oxygen becomes harder to absorb, and your body launches a cascade of adjustments to compensate. These changes start gradually but become dramatic above 8,000 feet, affecting everything from how water boils to how well you sleep. Here’s what actually changes and why it matters.
The Air Gets Thinner and Colder
Air pressure decreases steadily with elevation because there’s simply less atmosphere stacked above you. At sea level, atmospheric pressure sits around 29.9 inches of mercury. By 5,000 feet, it drops to about 24.9, and by 10,000 feet it’s down to roughly 20.6. That’s about a 31% reduction in the weight of the air pressing down on you.
The oxygen concentration stays the same at every altitude, roughly 21% of the air. But because the air itself is less dense, each breath delivers fewer oxygen molecules to your lungs. At around 4,600 feet, the oxygen pressure in your blood is already measurably lower than at sea level. By about 12,600 feet, it drops by roughly 27%.
Temperature falls at a predictable rate: about 3.6°F for every 1,000 feet of elevation gain (6.5°C per 1,000 meters). A comfortable 70°F day at a trailhead can feel like 52°F at the summit if you’ve climbed 5,000 feet. Wind and reduced humidity at altitude make it feel even colder.
UV Exposure and Sunburn Risk Climb Fast
Ultraviolet radiation increases about 12% for every 1,000 meters (roughly 3,300 feet) of altitude gained. At 10,000 feet, you’re receiving about 36% more UV than at sea level. Thinner air filters out less radiation, and snow or exposed rock can reflect UV back at you from below. This is why sunburns happen surprisingly fast on ski slopes and mountain hikes, even on overcast days.
Water Boils at Lower Temperatures
Lower air pressure means water doesn’t need as much energy to transition from liquid to gas. At sea level, water boils at 212°F. At 5,000 feet, the boiling point drops to 203°F. At 10,000 feet, it’s down to about 193.6°F. This has real consequences for cooking: pasta takes longer, rice needs more water, and hard-boiled eggs require extra time. Baking is affected too, since leavening agents produce gas more easily in thinner air, causing cakes to rise too fast and then collapse.
Your Body’s Immediate Response
Within minutes of arriving at a higher elevation, your body detects the reduced oxygen in your blood and starts compensating. Your breathing rate increases, pulling in more air per minute to capture more oxygen. Your heart rate rises as well, pushing blood through your system faster to distribute what oxygen is available.
The relationship between these responses is more complex than it appears. Specialized sensors in your neck (the carotid bodies) detect low oxygen and actually trigger a slowing effect on the heart. But the increased breathing overrides this, producing a net increase in heart rate. The result is that you feel winded doing things that would be effortless at lower elevations, like walking uphill or climbing stairs.
Altitude Sickness and Who Gets It
Acute mountain sickness (AMS) is common. About 25% of visitors sleeping above 8,000 feet in Colorado experience it, and the rate approaches 50% when people fly directly to elevations above 11,150 feet. Even with a careful, gradual ascent, prevalence can reach 30% at higher elevations.
The symptoms feel like a bad hangover. Headache is the hallmark, usually joined by some combination of fatigue, nausea, dizziness, and loss of appetite. Most people notice symptoms within 6 to 12 hours of arriving at a new elevation. The condition is uncomfortable but not dangerous on its own, and it typically resolves within a day or two as the body adjusts.
Medical guidelines classify altitude into three zones that correlate with increasing risk:
- High altitude: 5,000 to 11,500 feet (1,500 to 3,500 meters)
- Very high altitude: 11,500 to 18,000 feet (3,500 to 5,500 meters)
- Extreme altitude: above 18,000 feet (above 5,500 meters)
Severe Altitude Illness
Two life-threatening conditions can develop when the body fails to cope with extreme oxygen deprivation. High-altitude pulmonary edema (HAPE) occurs when fluid leaks into the lungs, causing breathlessness, a persistent cough, and an inability to exert yourself. High-altitude cerebral edema (HACE) happens when fluid accumulates in the brain, driven by low oxygen triggering blood vessels to dilate and leak.
HACE is the more immediately dangerous of the two. It typically begins as worsening mountain sickness, then progresses to confusion, loss of coordination, and extreme fatigue so severe that a person may be unable to dress themselves or walk without stumbling. Ataxia, a staggering or unsteady gait, is the earliest and most telling sign. Without descent to lower altitude, HACE can progress to coma and death within 12 to 24 hours.
How Sleep Changes at Altitude
Sleep quality often suffers significantly at elevation, sometimes more than people expect. The reduced oxygen triggers an unstable breathing pattern during sleep called periodic breathing: cycles of deep, rapid breaths alternating with pauses where breathing stops entirely for several seconds. This happens because faster breathing at altitude blows off too much carbon dioxide, and when CO2 drops low enough, the brain temporarily shuts off the drive to breathe. Only when oxygen falls further does breathing restart, creating a repeating cycle.
The result is frequent awakenings, a sensation of gasping, and overall poor rest. Research on healthy mountaineers shows that sleep quality does improve with acclimatization as oxygen levels stabilize, though the periodic breathing pattern itself can persist. The sleep disruption appears to be caused more by low oxygen levels than by the irregular breathing itself.
How Your Body Acclimatizes Over Days and Weeks
Given time, the body makes deeper adaptations that go well beyond breathing faster. Low oxygen triggers your kidneys to release a hormone that stimulates red blood cell production in your bone marrow. More red blood cells means more oxygen-carrying capacity per unit of blood. This process ramps up within hours of altitude exposure, but it takes a few weeks for the proportion of red blood cells in your blood to reach a new, higher steady state. That elevated level holds for as long as you stay at altitude.
Your muscles also adapt. Blood vessels in oxygen-hungry tissues expand to improve blood flow, and over time, cells become more efficient at extracting oxygen from the blood they receive. These changes are why athletes sometimes train at altitude before competing at sea level, carrying their enhanced oxygen delivery back down with them.
The general approach for minimizing altitude sickness is to ascend gradually and give your body time to adjust at intermediate elevations before pushing higher. This is especially important above 8,000 feet, where sleeping elevation gains become the critical variable. Adding a rest day for every 3,000 to 4,000 feet of elevation gain gives the body time to catch up with the thinning air.