Altitude sickness is caused by your body’s reaction to reduced oxygen at higher elevations. As you climb, atmospheric pressure drops, which means each breath you take contains fewer oxygen molecules than your body is used to. This oxygen shortage, called hypobaric hypoxia, triggers a cascade of responses in your brain, lungs, and blood that produce the headaches, nausea, and fatigue most people associate with going too high, too fast.
Why There’s Less Oxygen at Altitude
The percentage of oxygen in the air stays the same at every elevation: about 21%. What changes is air pressure. At sea level, atmospheric pressure is 760 mmHg, pushing oxygen molecules tightly together so each breath delivers a dense dose. As you gain elevation, that pressure drops, and the oxygen molecules spread further apart. The result is that the partial pressure of oxygen, the actual force driving oxygen from your lungs into your blood, falls significantly. At 3,500 meters (about 11,500 feet), the effective oxygen pressure is roughly 40% lower than at sea level.
Your body detects this oxygen shortfall almost immediately. Within hours of arriving at high altitude, your breathing rate increases, your heart pumps faster, and your blood begins concentrating its red blood cells to carry more oxygen per unit of volume. These compensatory responses are collectively called acclimatization, and they work well if given enough time. Altitude sickness happens when you ascend faster than your body can adjust.
What Happens Inside Your Body
The faster, deeper breathing triggered by low oxygen has an important side effect: you exhale more carbon dioxide than normal. Carbon dioxide is acidic, so losing too much of it makes your blood more alkaline, a state called respiratory alkalosis. This shift disrupts normal cellular function and is one reason you feel so lousy at altitude. Your kidneys eventually compensate by excreting bicarbonate (a base) to rebalance blood pH, but that process takes one to three days.
Meanwhile, reduced oxygen reaching your brain causes blood vessels there to dilate, increasing blood flow in an attempt to deliver more oxygen. This vasodilation raises pressure inside the skull and is the primary driver of altitude headaches. Inflammatory signals, including molecules that increase capillary permeability, compound the problem by allowing small amounts of fluid to leak from blood vessels into surrounding tissue.
The Altitude Where Risk Begins
Most people can tolerate elevations up to about 2,500 meters (8,200 feet) without significant problems. Risk increases sharply above that. The Wilderness Medical Society recommends avoiding ascent to a sleeping altitude of 2,750 meters (9,000 feet) in a single day. Once above 3,000 meters (9,850 feet), sleeping altitude should increase by no more than 500 meters (1,650 feet) per night, with an extra acclimatization day built in for every 1,000 meters (3,300 feet) of gain.
Sleeping altitude matters more than the highest point you reach during the day. Low oxygen affects you most during sleep, so a day trip to a high summit followed by descent to sleep at a lower elevation is far less stressful on the body than camping at the summit overnight.
How Altitude Sickness Feels
The mildest and most common form is acute mountain sickness (AMS). It typically develops 6 to 12 hours after arrival at a new altitude and revolves around four core symptoms: headache, nausea or loss of appetite, fatigue, and dizziness. Headache is the hallmark. Under the Lake Louise scoring system used to diagnose AMS, you need a headache plus enough severity across the other symptoms to reach a combined score of at least 3 out of 12. Mild AMS (scores of 3 to 5) feels like a bad hangover. Moderate AMS (6 to 9) can force you to stop ascending. Severe AMS (10 to 12) may require evacuation to lower ground.
When It Becomes Dangerous
If AMS goes unrecognized or ignored, it can progress to two life-threatening conditions: high-altitude cerebral edema (HACE) and high-altitude pulmonary edema (HAPE).
HACE occurs when the blood-brain barrier breaks down under sustained hypoxia. Inflammatory mediators, including nitric oxide, free radicals, and vascular growth factors, cause brain capillaries to leak fluid into surrounding tissue. The resulting brain swelling raises intracranial pressure, leading to confusion, loss of coordination, and eventually coma. One theory, known as the “tight-fit” hypothesis, suggests that people whose skulls have less room for cerebrospinal fluid experience more dramatic pressure spikes and more severe symptoms. HACE is rare below 4,000 meters but can develop within a day or two of arrival at extreme altitude.
HAPE involves fluid accumulating in the lungs. Oxygen-starved blood vessels in the lungs constrict unevenly, creating areas of very high pressure that force fluid through capillary walls into the air sacs. The result is a wet cough, breathlessness at rest, and a characteristic crackling sound when breathing. HAPE can develop independently of AMS and is the most common cause of death from altitude illness.
Why Some People Are More Vulnerable
Physical fitness does not protect you. Marathon runners and couch potatoes are equally susceptible. The single strongest predictor of altitude sickness is how fast you ascend relative to your personal acclimatization rate, and that rate is largely genetic.
Research on populations that have lived at high altitude for thousands of years, including Tibetan, Andean, and Ethiopian highlanders, has identified dozens of genes involved in oxygen sensing and red blood cell production. One gene, EPAS1, helps regulate how aggressively the body produces red blood cells in response to low oxygen. Another, SENP1, plays a role in controlling red blood cell overproduction, a condition that can thicken the blood dangerously at altitude. Tibetans carry variants of these genes that keep their red blood cell counts moderate even at extreme elevations, a trait most lowlanders lack.
Beyond genetics, a few practical factors raise your risk. A history of altitude sickness is the best non-genetic predictor: if you’ve had it before, you’re likely to get it again at the same elevation. Living at or near sea level means your baseline acclimatization is zero. Alcohol and sleeping medications can suppress breathing during sleep, worsening the oxygen drop that already peaks overnight. And age offers a slight, counterintuitive benefit: people over 50 tend to get AMS less frequently than younger adults, possibly because age-related changes in fluid dynamics reduce brain swelling.
How Ascent Rate Drives the Problem
The root cause of nearly every case of altitude sickness is ascending too quickly. Your body needs time to ramp up breathing, adjust blood chemistry, and begin producing extra red blood cells. Rushing that timeline overwhelms compensatory systems. This is why altitude sickness is common on popular trekking routes where schedules are fixed: the mountain doesn’t care about your itinerary.
The “climb high, sleep low” principle captures the most effective prevention strategy. You can hike to a higher elevation during the day, giving your body a stimulus to acclimatize, then descend to sleep where oxygen pressure is slightly greater. This approach lets you gain altitude without forcing your body to cope with maximum hypoxia during the hours when it’s most vulnerable.
How Prevention Works
Gradual ascent is the best prevention, but when a slow schedule isn’t possible, a medication called acetazolamide (brand name Diamox) can help. It works by forcing the kidneys to excrete bicarbonate, making the blood slightly more acidic. The body interprets this acidity the same way it interprets high carbon dioxide levels, so it responds by breathing faster and deeper, even during sleep. The result is higher blood oxygen levels and a head start on the pH adjustment that would otherwise take days. Typical preventive doses range from 250 to 500 mg daily, started a day before ascent.
Hydration and avoiding alcohol help at the margins, but neither is a substitute for controlled ascent. The most important thing you can do is listen to your body: if you develop a headache and any other symptom of AMS, stop ascending. If symptoms worsen despite rest, descend. Altitude sickness is almost always reversible once you lose enough elevation, but continuing upward with symptoms is the single most dangerous decision you can make at altitude.