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Understanding Altitude Sickness: What Happens to Your Body at High Elevations

Altitude sickness, also called acute mountain sickness (AMS), occurs when your body struggles to adjust to lower oxygen levels at higher elevations. As you climb higher, the air pressure decreases, which means there is less oxygen available in each breath you take. At sea level, the air contains about 21% oxygen. At 8,000 feet above sea level, that same air still contains 21% oxygen, but the air is thinner, so you're actually breathing in fewer oxygen molecules with each breath.

Your body normally maintains a careful balance of oxygen and carbon dioxide in your blood. When you reach higher elevations too quickly, your body doesn't have time to make the adjustments needed to maintain this balance. This triggers altitude sickness symptoms. The condition becomes more likely as you go higher. Most people start noticing effects around 5,000 to 8,000 feet, though some experience symptoms at lower elevations depending on their individual sensitivity.

The severity of altitude sickness varies from person to person. Factors that influence your risk include how quickly you ascend, your age, physical fitness level, previous altitude exposure, and individual genetic factors. Interestingly, being in good physical condition doesn't protect you from altitude sickness—athletes can get it just as easily as less active people. This is because the condition depends on oxygen availability, not fitness level.

Common symptoms include headache, nausea, fatigue, shortness of breath, and dizziness. Most people with mild altitude sickness feel better within 24 to 48 hours as their body acclimates. However, in rare cases, altitude sickness can progress to more serious conditions like high altitude cerebral edema (HACE) or high altitude pulmonary edema (HAPE), where fluid builds up in the brain or lungs. These serious forms are medical emergencies requiring immediate descent and medical treatment.

Practical Takeaway: Understanding how altitude affects your body helps you recognize early symptoms and take preventative action. Altitude sickness is not a sign of weakness—it's a normal physiological response that happens when your body hasn't had time to adjust to lower oxygen levels.

Common Altitude Sickness Prevention Methods and Gradual Acclimatization

The most effective way to prevent altitude sickness is gradual acclimatization, which means ascending slowly and allowing your body time to adjust. Experts generally recommend gaining no more than 1,000 to 1,500 feet per day once you reach 8,000 feet in elevation. This slow pace gives your body time to make physiological adjustments that help it use oxygen more efficiently.

The acclimatization process involves several changes in your body. Your breathing becomes deeper and more frequent, which brings more oxygen into your lungs. Your heart rate increases to pump oxygen-rich blood throughout your body more efficiently. Your kidneys adjust the pH balance of your blood to help your respiratory system work better at high altitudes. Your body also increases production of red blood cells, which carry oxygen throughout your system—though this process takes several weeks. These adaptations happen gradually as you spend time at higher elevations.

One practical acclimatization strategy is the "climb high, sleep low" method. This means ascending to a higher elevation during the day to expose your body to altitude, then returning to a lower elevation to sleep at night. This approach provides some altitude exposure while still allowing your body adequate oxygen during sleep. Many mountaineers use this method during their expeditions.

Other commonly discussed prevention approaches include staying hydrated, which helps your blood carry oxygen more effectively, and avoiding alcohol and sedatives, which can depress your breathing. Getting adequate sleep at your current altitude helps your body focus on acclimatization. Eating carbohydrate-rich foods may also help, as carbohydrates require less oxygen to metabolize than fats and proteins do.

Some people explore medications that may help with altitude sickness prevention. Acetazolamide, commonly known by the brand name Diamox, is a prescription medication that can reduce symptoms and help with acclimatization. However, medications are most effective when combined with gradual ascent and proper hydration. A doctor can discuss whether medication might be appropriate for your specific situation based on your health history and altitude plans.

Practical Takeaway: Gradual acclimatization remains the most reliable prevention method. If you're planning a trip to high altitude, building in extra time for slow ascent is your best strategy for avoiding altitude sickness.

Recognizing Altitude Sickness Symptoms Early

Recognizing altitude sickness symptoms early allows you to take action before the condition becomes serious. Mild altitude sickness typically begins 6 to 12 hours after arriving at a high elevation. The most common symptom is a headache, which occurs in roughly 50% of people who ascend quickly to 8,000 feet or higher. This headache may feel similar to a tension headache or hangover.

Along with headache, people often experience nausea, which may or may not include vomiting. Fatigue and weakness are also typical—you might feel unusually tired even if you've been resting. Many people describe a sensation of shortness of breath, even when sitting still. Dizziness or lightheadedness is common, as is difficulty sleeping. Some people lose their appetite or experience a dry mouth and throat.

The Lake Louise Scoring System is a medical tool used to assess altitude sickness severity. It evaluates headache presence and severity, plus the presence of nausea, fatigue, and dizziness. Using this framework can help you objectively track how you're feeling. A score of 3 or more typically indicates acute mountain sickness, while scores below 3 suggest mild symptoms that may resolve with rest and hydration.

It's important to distinguish altitude sickness from other conditions. Dehydration at high altitude can cause similar symptoms, so addressing hydration may resolve some discomfort. Lack of sleep can also mimic altitude sickness symptoms. However, if you develop symptoms 6 to 12 hours after arrival at high elevation, altitude sickness is the likely cause.

Warning signs of more serious altitude illness include severe headache that doesn't respond to pain relievers, persistent vomiting, difficulty walking or loss of coordination, confusion or difficulty concentrating, and shortness of breath while resting. These symptoms require medical attention and possible descent to lower elevation. Immediate descent is necessary if someone develops these serious symptoms.

Practical Takeaway: Pay attention to how you feel in the first 24 hours after arriving at high elevation. Mild symptoms often improve with rest, hydration, and time, but worsening symptoms require descent and medical evaluation.

Practical Strategies for Your High-Altitude Trip

Planning is essential for altitude sickness prevention. If you're traveling to high elevation, research the altitude of your destination and surrounding areas. Websites and maps show elevation clearly. If you're planning mountaineering or backcountry travel, review elevation profiles of your planned route. This information helps you plan your ascent rate.

When possible, plan your itinerary to include rest days at intermediate elevations. For example, if you're traveling to a city at 10,000 feet, arriving a day or two earlier and spending time at 7,000 to 8,000 feet first gives your body a head start on acclimatization. Airlines and travel websites show flight itineraries and connection points, helping you plan intermediate stops.

Pack appropriate supplies for your trip. Bring plenty of water and electrolyte beverages—aim to drink about 3 to 4 liters daily at high altitude, more if you're hiking or active. Pack snacks rich in carbohydrates, such as crackers, granola, nuts, and dried fruit. These foods are easier to eat when you have no appetite but need calories for acclimatization. Bring over-the-counter pain relievers for headaches, plus any medications your doctor has recommended.

Once you arrive at high elevation, plan low-intensity activities for your first day. Avoid strenuous exercise, which increases oxygen demand and can worsen symptoms. Instead, rest, spend time outdoors at your arrival elevation to begin acclimatization, and eat light, easy-to-digest meals. Many people sleep poorly their first night at altitude due to periodic breathing, a temporary pattern where breathing stops briefly then resumes—this is normal and typically improves after a few days.