Altitude Sickness: What It Actually Feels Like and How to Prevent It
The first time I got altitude sickness I was convinced I was just tired. We had driven up to Leadville, Colorado at 10,152 feet from Denver the previous day, I had slept reasonably well, eaten a normal breakfast, and felt okay for the first mile of trail. Then the headache arrived. Not the kind that pauses when you press your temples. The kind that sits behind your eyes and pulses with each heartbeat and does not care how much water you drink or how much ibuprofen you take.
By midday I had that headache, a persistent nausea that made every bite of trail food feel like a gamble, and a bone-deep fatigue that was completely out of proportion to the effort I was putting in. I had been climbing for two years at that point. I was fit, well-rested, and well-hydrated. None of it mattered.
Altitude sickness does not negotiate with your fitness level. It is a straightforward physiological response to reduced oxygen availability, and the sooner you understand exactly what is happening in your body, the better your decisions will be in the mountains.
The Physiology: What Is Actually Happening
At sea level, you breathe air containing about 21% oxygen at an atmospheric pressure of roughly 760 mmHg. At 10,000 feet, the oxygen percentage is still 21%, but the atmospheric pressure has dropped to around 523 mmHg. That pressure drop means each breath delivers significantly fewer oxygen molecules to your alveoli, which means less oxygen makes it into your bloodstream, which means less oxygen reaches your muscles, brain, and organs.
Your body responds through several mechanisms: faster, shallower breathing; increased heart rate; and over days to weeks, the production of additional red blood cells to carry more of whatever oxygen is available. That last adaptation is acclimatization, and the critical problem is that it takes time your schedule often does not account for. In the meantime, the short-term compensations create their own side effects, including the headache, fatigue, dizziness, and nausea that define Acute Mountain Sickness, or AMS.
AMS typically begins above 8,000 feet, though some people experience symptoms at lower elevations. It is not a character flaw. Elite endurance athletes get it. The severity depends on your individual physiology, your rate of ascent, and factors researchers still do not fully understand.
Recognizing AMS Before It Gets Serious
The Lake Louise Criteria, used by wilderness medicine providers to score AMS severity, requires a headache plus at least one of the following: fatigue or weakness disproportionate to your effort, loss of appetite or nausea, dizziness, or difficulty sleeping at altitude. If you have a headache and any of those additional symptoms above 8,000 feet, you likely have mild AMS.
Mild AMS is manageable: stop ascending, rest, hydrate, and let your body catch up. Most people resolve within 24 hours without descending.
What you cannot treat lightly is the escalation pattern. High Altitude Pulmonary Edema, or HAPE, involves fluid accumulating in the lungs. Early signs include a persistent dry cough at rest, reduced exercise tolerance, and unusual breathlessness on mild exertion. Late signs include a cough producing pink or frothy sputum and extreme fatigue. This is a medical emergency. High Altitude Cerebral Edema, or HACE, involves fluid accumulating in the brain and presents with severe headache unresponsive to medication, loss of coordination you can assess with a heel-to-toe line walk, and altered consciousness. Also a medical emergency.
If anyone in your group develops signs of HAPE or HACE, the treatment is immediate descent and, if available, supplemental oxygen or a portable hyperbaric chamber like a Gamow bag. Do not wait for morning. Do not see if they sleep it off. Descend.
Prevention: The Principles That Actually Work
The most effective prevention is unhurried ascent. The general rule above 8,000 feet is to gain no more than 1,000 feet of sleeping elevation per day, taking one full rest day for every 3,000 feet of total gain. The “climb high, sleep low” principle works within this framework: during acclimatization, you can push to higher elevations during the day and return to a lower sleeping elevation. You get the acclimatization stimulus without sacrificing recovery quality.
Hydration matters more than most people realize. Aim for three to four liters of water per day at altitude, and avoid alcohol for at least the first 24 hours at any new elevation. Alcohol suppresses the breathing drive and impairs your body’s ability to acclimatize efficiently.
Acetazolamide, brand name Diamox, is a prescription diuretic that stimulates faster, deeper breathing and genuinely speeds the acclimatization process. It is not a substitute for proper acclimatization, but it is useful for planned rapid ascents. Side effects include increased urination, tingling in the extremities, and occasional allergic reactions in people sensitive to sulfa drugs. Talk to your physician before any objective that takes you above 12,000 feet in the first day or two of your trip.
The cardinal rule has never changed and never will: when in doubt, go down. No summit is worth dying for. You can always come back. The mountain is patient in a way that a deteriorating climber at 15,000 feet simply is not.
