Altitude Sickness: What Happens to Your Body at High Altitude and How to Stay Safe
Every year, millions of people travel to high-altitude destinations — ski resorts in Colorado, ancient ruins in Peru, trekking routes in Nepal — without knowing what's about to happen to their bodies. Some feel fine. Others are hit with a pounding headache before they've even unpacked. A small number end up in serious medical trouble.
The air at high altitude isn't "thinner" in the way people often imagine. The percentage of oxygen in the atmosphere stays constant at about 21% regardless of elevation. What changes is the atmospheric pressure — and with it, the amount of oxygen your lungs can actually extract with each breath. At 10,000 feet, you're getting roughly 30% less oxygen per breath than you would at sea level. Your body notices immediately.
Understanding what your body goes through at altitude — both the changes you'll feel and the ones happening silently — is the best preparation for a safe, enjoyable trip.
What Happens to Your Body at High Altitude
Physiological changes begin at around 8,000 feet and become more pronounced with every additional gain in elevation. Some of them you'll feel right away. Others are happening beneath the surface whether you notice them or not.
The changes you'll notice
Faster, deeper breathing: Your body's first response to reduced oxygen is to breathe more frequently and more deeply. This happens automatically, often before you're aware of it.
Increased heart rate: The heart compensates for lower oxygen delivery by pumping faster. Physical exertion that felt easy at sea level can feel surprisingly taxing.
Reduced exercise tolerance: Even routine activity — walking up stairs, carrying a bag — can leave you noticeably winded. This is normal and expected.
Disrupted sleep: Many people experience a pattern called Cheyne-Stokes respiration at altitude: periodic breathing where the breath naturally pauses, triggering a sudden gasp. It's unsettling but common, and it tends to improve as the body acclimatizes.
Increased urination: The kidneys adjust blood chemistry to compensate for the faster breathing, and frequent urination is a byproduct. It's actually a sign the body is adapting.
The silent changes
Blood thickening: Over days to weeks at altitude, the body produces more red blood cells to carry oxygen more efficiently. This is useful for acclimatization but increases the risk of blood clots during the transition period.
Fluid shifts: Fluid moves into tissues, including the brain and lungs. In most people this is mild and temporary. In some, it progresses into the more serious forms of altitude illness.
Reduced cognitive performance: Even without obvious symptoms, decision-making, reaction time, and memory can subtly decline at high altitude. This is one reason altitude sickness can be particularly dangerous: the impairment can affect your ability to recognize how impaired you are.
Pulmonary vasoconstriction: Blood vessels in the lungs constrict in response to low oxygen, raising pulmonary blood pressure. For most people this is mild. In susceptible individuals it can progress to high altitude pulmonary edema (HAPE).
What is Altitude Sickness?
Altitude sickness — formally known as acute mountain sickness (AMS) — occurs when the body can't acclimatize fast enough to compensate for the reduced oxygen availability at elevation. It's not a sign of weakness or poor fitness. In fact, physical fitness is a surprisingly poor predictor of who will develop AMS. Genetics and individual physiology play a much larger role.
Symptoms can begin at elevations as low as 6,500 to 8,000 feet, though risk increases significantly above 10,000 feet. Cities like Cusco, Peru (11,150 feet) and La Paz, Bolivia (11,940 feet) sit well within the danger zone — and travelers often arrive by plane, with no time for gradual acclimatization.
High altitude sickness presents in three forms:
Acute mountain sickness (AMS): The most common form; uncomfortable but not immediately dangerous
High altitude cerebral edema (HACE): A rare but life-threatening progression involving fluid accumulation in the brain
High altitude pulmonary edema (HAPE): Fluid accumulation in the lungs; the most common cause of death from altitude illness
Key Point: Why Don’t People Get Altitude Sickness on Airplanes?
Airplane cabins are pressurized to the equivalent of about 6,000–8,000 feet, which is right at the lower threshold where altitude effects begin. Mild physiological changes may still occur on planes: you're getting slightly less oxygen per breath, which is why long flights can leave you feeling sluggish or dehydrated faster than usual.
The reason most people don't get altitude sickness, though, comes down to a few things: The cabin pressure, while reduced, stays within a range the body handles fairly well without needing to acclimatize. True altitude sickness typically kicks in above 8,000 feet of sleeping elevation — meaning sustained exposure. A flight keeps you at that equivalent altitude for hours, but you're sedentary, not exerting yourself, which dramatically reduces oxygen demand. Physical activity at altitude is what tends to push the body into symptomatic territory.
People also vary widely in their sensitivity, and the mild hypoxia of a pressurized cabin simply isn't enough of a physiological stressor for most healthy adults to cross into AMS symptoms. People with heart or lung conditions are the exception — they can genuinely feel the effects of cabin pressure more acutely.
Altitude Sickness Symptoms
Acute mountain sickness (AMS)
The hallmark symptom of AMS is a headache — typically dull, persistent, and located across the forehead or behind the eyes. It's often accompanied by fatigue, nausea, loss of appetite, dizziness, and difficulty sleeping. Symptoms usually appear within six to 12 hours of arrival at altitude and tend to be worse in the morning. Mild AMS is common, manageable, and resolves on its own with rest and acclimatization in most cases.
High altitude cerebral edema (HACE)
HACE represents a serious progression of AMS in which fluid accumulates in the brain. Warning signs include a severe headache that doesn't respond to pain medication, confusion or unusual behavior, loss of coordination (a stumbling, unsteady gait), and in advanced cases, altered consciousness. HACE is a medical emergency. Descent must begin immediately — waiting to see if symptoms improve is not an option.
High altitude pulmonary edema (HAPE)
High altitude pulmonary edema develops when fluid accumulates in the lungs, impairing oxygen exchange. It can develop rapidly and is the leading cause of altitude-related death. Symptoms include breathlessness at rest (not just during exertion), a persistent dry cough that may progress to producing pink or frothy fluid, extreme fatigue, and in severe cases, a blue tinge to the lips or fingernails indicating dangerously low oxygen levels. HAPE requires immediate descent and emergency medical care.
Important: Seek Medical Attention
If you or someone in your group develops symptoms of HACE or HAPE, descend immediately. Do not wait until morning. Do not sleep on it. These conditions can deteriorate rapidly.
How to Prevent Altitude Sickness
Ascend gradually
This is the single most effective prevention strategy. Above 8,000 feet, the widely followed guideline is to increase sleeping elevation by no more than 1,000 feet per day, with a rest day built in for every 3,000 feet gained. The mountaineering principle of "climb high, sleep low" — ascending to a higher point during the day and returning to a lower camp to sleep — is grounded in solid physiological evidence. If your itinerary involves flying directly to a high-altitude city, building in one to two low-activity days upon arrival gives your body time to begin adjusting.
Hydrate well
Dehydration worsens altitude sickness symptoms and can make them harder to distinguish from AMS. The drier air at altitude increases fluid loss through breathing, and the increased urination of early acclimatization compounds this. Drink more water than you normally would — even if you don't feel particularly thirsty.
Avoid alcohol and sedatives
Both alcohol and sleep medications suppress the respiratory drive — the body's automatic signal to breathe faster when oxygen is low. At altitude, where that drive is already working hard to compensate, suppressing it worsens overnight oxygen levels and significantly increases AMS risk. Save the celebratory drinks until you've acclimatized.
Altitude sickness medication
Acetazolamide (commonly known by the brand name Diamox) is the most widely used prescription medication for altitude sickness prevention. It works by stimulating faster breathing, which speeds up acclimatization. It's typically started one to two days before ascending to altitude and continued for the first few days at elevation. Common side effects include tingling in the hands and feet and more frequent urination.
Acetazolamide requires a prescription, so it's worth discussing with a healthcare provider before your trip — especially for ascents above 10,000 feet, trips with rapid elevation gain (such as flying into a high-altitude city), or if you've experienced AMS before. A provider can assess whether it's appropriate for your situation and medical history.
Managing Altitude Sickness If it Occurs
If you have symptoms of AMS, do not ascend further. Going higher when the body is already struggling to acclimatize will almost always make things worse.
Mild AMS
Rest at your current altitude, drink plenty of water, and use over-the-counter pain relief like ibuprofen or acetaminophen for headache. Avoid exertion. Most cases of mild AMS resolve within 24 to 48 hours as the body catches up on acclimatization. If symptoms are stable or improving, it's generally safe to continue ascending after a full recovery.
Worsening symptoms: Descend
If symptoms worsen rather than improve — or if any signs of HACE or HAPE develop — descend immediately. Even a drop of 1,000 to 2,000 feet can produce rapid and significant relief. Descent is the most reliable treatment for all forms of altitude illness, and it should take priority over everything else.
Medications for treatment
Acetazolamide can be used to treat AMS as well as prevent it. Dexamethasone, a corticosteroid, is used in more serious cases of AMS and HACE to reduce brain swelling. Nifedipine, a calcium channel blocker, is used to treat HAPE by reducing pulmonary blood pressure. All of these require medical guidance and should not be self-administered without prior consultation.
Supplemental oxygen and hyperbaric chambers
Supplemental oxygen is useful for managing HAPE and can provide meaningful relief while descent is being arranged. In remote high-altitude settings where descent isn't immediately possible, portable hyperbaric chambers — known as Gamow bags — can simulate a lower altitude environment and buy critical time. These are most commonly found in expedition contexts and established mountain rescue operations.
Prepare Before You Go
If you're planning a trip to high altitude — especially one involving rapid elevation gain, an ascent above 10,000 feet, or a history of altitude sickness — it's worth speaking with a healthcare provider before you travel. A provider can evaluate your individual risk, advise on a safe ascent plan, and discuss whether preventive medication is appropriate for your trip.
This is also relevant for anyone with pre-existing heart or lung conditions, which can be significantly affected by the physiological demands of high altitude. A short consultation before departure is a small investment compared to managing a medical situation thousands of feet above sea level.
How LifeMD Can Help
With LifeMD, you can connect with a licensed healthcare provider from the comfort of your home within an hour. If appropriate, your provider may prescribe medication and send it directly to your local pharmacy — so you're prepared before your trip begins.
For even more flexibility, a LifeMD+ membership gives you 24/7 access to care, same-day prescription refills, exclusive wellness perks, and easy access to lab testing — all designed to make managing your health simple and stress-free.
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