Your body starts feeling the effects of high altitude above roughly 8,000 feet (2,400 meters), where the air contains meaningfully less oxygen than at sea level. At 10,000 feet, each breath delivers only about 69% of the oxygen pressure you’d get at the coast, and your blood oxygen saturation can drop from a normal 95-99% to as low as 88%. The good news: with the right pacing, hydration, and awareness, most healthy people can adapt comfortably to elevations well above that threshold.
Why Altitude Affects Your Body
The air at high elevation contains the same percentage of oxygen as sea-level air (about 21%), but the atmospheric pressure pushing that oxygen into your lungs is significantly lower. At sea level, a healthy young person maintains arterial oxygen levels around 90 to 95 mm Hg. At 9,200 feet, that drops to about 60 mm Hg. By 20,000 feet, it falls to roughly 35 mm Hg.
Your body responds to this oxygen deficit immediately. Your heart rate increases, you breathe faster and deeper, and your kidneys start adjusting your blood chemistry over the following days. This adjustment process is called acclimatization, and it’s remarkably effective when you give it enough time. Rush the process, and you risk altitude sickness.
The Most Important Rule: Ascend Slowly
Gradual ascent is the single most effective way to prevent altitude illness. Once you’re sleeping above 10,000 feet, try to increase your sleeping elevation by no more than 1,000 to 1,500 feet per night. For every 3,000 feet of elevation gain, plan a rest day where you sleep at the same altitude as the night before.
If you’re flying directly to a high-altitude city (like Cusco at 11,200 feet or La Paz at nearly 12,000 feet), expect your body to need two to three days of light activity before you feel normal. Avoid strenuous exercise on your first day. Walk slowly, take breaks, and don’t interpret your sluggishness as poor fitness. Even elite athletes feel altitude on arrival.
Recognizing Altitude Sickness
The standard diagnostic tool used in mountain medicine scores four symptoms: headache, dizziness, gastrointestinal distress (nausea, vomiting, or loss of appetite), and fatigue. Altitude sickness is diagnosed when you have a headache plus at least one other symptom. Most cases are mild and resolve on their own if you stop ascending and rest at your current elevation.
A mild case feels like a hangover: throbbing headache, low energy, slight nausea. A moderate case adds persistent vomiting, significant dizziness, and difficulty sleeping. Both respond well to staying put, drinking fluids, and taking over-the-counter pain relief for the headache. If symptoms don’t improve within a day, or if they worsen, descend at least 1,000 to 2,000 feet.
Warning Signs That Require Immediate Descent
Two severe conditions can develop from untreated altitude sickness, and both are medical emergencies. High-altitude pulmonary edema (HAPE) occurs when fluid leaks into the lungs. Early signs include unusual breathlessness during activity that progresses to breathlessness at rest, a cough that may produce pink or frothy mucus, a racing heart rate, fast breathing, and a bluish tint to your lips or fingernails. High-altitude cerebral edema (HACE) involves swelling in the brain and shows up as confusion, loss of coordination (the person walks as if drunk), severe headache unresponsive to medication, and altered behavior.
If you or someone in your group develops these symptoms, descend immediately. Even a drop of 1,000 to 2,000 feet can be lifesaving. The Wilderness Medical Society recommends portable hyperbaric chambers (inflatable bags that simulate lower altitude) for situations where descent is delayed or impossible, though these require someone trained to operate them and should never replace getting to lower ground when that’s an option.
Hydration and Nutrition
You lose water faster at altitude through increased breathing and lower humidity. Aim to drink enough that your urine stays light yellow. There’s no magic number, but most people need noticeably more fluid than they would at home, typically an extra liter or two per day beyond their normal intake.
Eat carbohydrate-rich meals. Your body metabolizes carbohydrates more efficiently than fats or proteins when oxygen is limited. You may lose your appetite for the first day or two, which is normal. Eat anyway, even if it’s just crackers or bread. Going without food compounds fatigue and makes acclimatization harder.
Why Alcohol Is Riskier at Altitude
Alcohol and altitude are a genuinely bad combination, especially during sleep. A study simulating airplane cabin pressure (equivalent to roughly 8,000 feet) found that drinking alcohol before sleep dropped blood oxygen saturation to an average of just over 85%, compared to about 88% at the same altitude without alcohol. Time spent below the clinically healthy threshold of 90% oxygen saturation jumped from 173 minutes without alcohol to 201 minutes with it.
Alcohol also disrupted sleep quality significantly. The deepest, most restorative stage of sleep was cut nearly in half (46.5 minutes versus 84 minutes with alcohol at sea level). At real-world altitudes of 10,000 feet or higher, these effects would be even more pronounced. If you want to drink, wait until you’ve had at least two full days to acclimatize, and keep it minimal.
Medication Options
A prescription medication called acetazolamide is the most widely used preventive drug for altitude sickness. It works by changing the acidity of your blood, which stimulates deeper breathing and speeds acclimatization. You typically start taking it the day before your ascent. Common side effects include tingling in your fingers and toes, increased urination, and a metallic taste with carbonated drinks. Talk to your doctor before your trip if you’re heading above 10,000 feet, especially if you’ve had altitude sickness before or your itinerary involves rapid ascent.
A steroid called dexamethasone is reserved primarily for treating altitude sickness rather than preventing it. It’s very effective at reducing symptoms but does not help your body acclimatize, meaning symptoms can return once you stop taking it. It’s most important as an emergency treatment for severe cases, particularly cerebral edema.
Monitoring With a Pulse Oximeter
A fingertip pulse oximeter is a cheap, lightweight tool worth carrying. At sea level, a healthy reading is 95-99%. At 10,000 feet, expect readings of 88-91% during your first day or two, gradually improving as you acclimatize. There’s no universally agreed-upon “danger number,” but readings consistently below 80% at rest, or a significant drop from your baseline at that altitude, warrant attention. Pulse oximeters are most useful for tracking trends: if your numbers are declining over hours rather than improving, that’s a signal to stop ascending.
Sun Protection Matters More Than You Think
UV radiation increases by 10-12% for every 1,000 meters (roughly 3,300 feet) of elevation gain. At 12,000 feet, you’re getting about 40% more UV exposure than at the beach. Add snow or glacier reflection, and the effective dose can double. Wear SPF 50 sunscreen and reapply frequently, especially on your nose, ears, and the underside of your chin (snow reflects UV upward). Wrap-around sunglasses or glacier glasses are essential. Snow blindness, a painful sunburn of the cornea, can happen in as little as an hour of unprotected exposure on snow at high altitude.
Sleep and Rest at Elevation
Sleep is one of the hardest things at altitude, even for experienced climbers. You may notice periodic breathing at night: cycles where your breathing speeds up, slows down, and briefly pauses before restarting. This is a normal neurological response to low oxygen and doesn’t mean something is wrong. It does disrupt sleep quality, though, and contributes to the fatigue most people feel during their first few days.
Sleep with your head and chest slightly elevated if possible, as this helps your breathing mechanics. Acetazolamide can reduce periodic breathing and improve sleep quality at altitude. Avoid sleeping pills, which can suppress your respiratory drive and worsen oxygen levels during sleep.
Planning Your Trip
Build buffer days into your itinerary. If you’re trekking, schedule your rest days proactively rather than waiting until someone feels sick. If you’re visiting a high-altitude city, plan your lightest activities for the first two days. Physical fitness does not protect against altitude sickness. Fit people tend to push harder and ascend faster, which actually increases their risk.
Carry more water than you think you need, bring sun protection for every exposed surface, pack layers for temperature swings (altitude means cold nights even in summer), and know your descent options at every point on your route. The ability to go down quickly is the most important safety net you have.