Altitude hypoxia: How lack of oxygen changes your body and mind

The air seems clean. Too clean.

A pulmonologist, on a sightseeing trip, smiles as she gets off the bus somewhere in the Andes, more than 4,000 meters above sea level. She's come for the photos, for the ruins, for that romantic notion of touching the sky. She doesn't yet know that something inside her has already begun to fail.

At first, it's just a slight pressure in her head. A dizziness she attributes to the journey. Then, her heart begins to beat with a strange violence, as if it were racing without moving. Her hands tremble. Her breathing becomes short, shallow, useless. She inhales, but the air is no longer enough. She looks around, and everything is the same. The guides are talking. Other tourists are laughing.

The landscape is brutally beautiful. But inside, something begins to go wrong.

His brain, starved for oxygen, begins to lose precision. Balance is disrupted, concentration dissolves, and an anxiety without emotional cause sets in. It's high-altitude hypoxia, the mechanism behind altitude sickness, the Andean soroche.

His blood no longer carries enough oxygen. His nervous system activates primitive alarms. What his mind interprets as panic is actually a silent suffocation.

She is not exhausted. She is breathing air that no longer supports what is happening inside her body.

What is high-altitude hypoxia?

As she tries to recover, her body has already entered a zone that physiology knows all too well. At 4,000 meters, the air contains far fewer oxygen molecules than at sea level. Her lungs continue to function, but each breath delivers less fuel. The hemoglobin no longer carries the same amount of oxygen as before.

The first organ to notice is the brain. Neurons, deprived of oxygen, reduce their ATP production and begin to lose precision. This disrupts their equilibrium, slows their thinking, and makes reality seem distorted. It's a nervous system running out of energy.

To try to compensate, your body activates an emergency protocol. Chemoreceptors in the carotid arteries and brainstem detect the drop in oxygen and trigger the sympathetic nervous system: the heart beats faster, breathing quickens, and blood pressure rises. It's a desperate maneuver to maintain oxygen flow to the brain.

But if the exposure continues, that strategy is no longer enough. Prolonged hypoxia can disrupt the blood-brain barrier, allowing fluid to leak into brain tissue and leading to profound confusion or loss of consciousness. As she stands facing the mountains, her body struggles to sustain the most fragile machine in existence: a human brain functioning without the oxygen it needs to remain itself.

Symptoms of altitude sickness (soroche): how it really starts

The body doesn't warn you with drama. It warns you with subtle cracks in perception. First, a dull pressure appears behind the eyes, as if the head were slightly swollen. Then a strange fatigue, different from normal tiredness: the muscles still respond, but the brain lags behind. She has trouble focusing her gaze. She struggles to organize simple thoughts. It's the first language of altitude sickness.

Then come the symptoms that most tourists recognize: persistent headaches, nausea that comes and goes, loss of appetite, and an uncomfortable feeling of not being able to fully fill their lungs. Walking 100 meters feels like running. Their pulse races for no reason. Sleep is fragmented, with abrupt awakenings because the brain, deprived of oxygen, no longer regulates breathing properly.

Mild altitude sickness usually manifests as headache, fatigue, nausea, and shortness of breath only with exertion. But there are red flags that are non-negotiable: confusion, clumsiness while walking (ataxia), difficulty breathing even at rest, persistent cough, or pink foam at the lips. These symptoms indicate that the brain or lungs are already accumulating fluid. At that point, the mountain ceases to be a physical challenge and becomes a medical emergency.

What to do if you have altitude sickness: how much time do you have to act?

She knows exactly what to do. She remembers it from university as if she were reading a slide projected onto her own skull: what most changes the prognosis isn't enduring the altitude, it's descending. At high altitude, it's not the lungs that fail; it's the air that fails. Every meter of descent increases the partial pressure of oxygen and tangibly improves what her blood can transport to the brain.

In a hospital, the alternative would be different: supplemental oxygen, a portable hyperbaric chamber, or even certain medications that reduce pressure in the lungs or brain. But there, in the middle of the mountains, none of that is available. For his body, there is only one realistic option: to seek denser air by descending.

Meanwhile, her body tries to compensate on its own. Chemoreceptors detect the drop in oxygen and trigger the sympathetic nervous system: the heart races, tachycardia develops, and blood pressure may rise. This isn't bravery or anxiety; it's an emergency mechanism to force more oxygen into the bloodstream. If she remains elevated, her body continues to suffer.

The real question is time. If the symptoms are mild and stabilize with rest, the body can sometimes acclimatize. But if there's shortness of breath at rest, clumsiness, confusion, a worsening headache, or a feeling of rapid deterioration, the window of opportunity narrows. In those cases, acting quickly matters more than any sightseeing plans: getting off the plane, calling for help, and, if available, using supplemental oxygen during the descent can make all the difference.

Is it possible to die from hypoxia in the mountains?

High-altitude cerebral edema can progress silently until it causes loss of consciousness. Pulmonary edema can fill the alveoli with fluid and cause oxygen exchange to collapse. She's seen X-rays like that. She's signed death certificates with that cause written in cold blood.

Yes, you can die from hypoxia. Not from exhaustion, nor from weakness, but because the cells stop receiving what they need to produce energy. While the Andean landscape remains untouched before her eyes, she understands that she's not just having a bad tourist experience: she's entering a medical emergency. And for the first time since getting off the bus, her mind becomes brutally clear. She doesn't need more photos. She doesn't need any more altitude. She needs to get down now.

Recovery and acclimatization: when is it safe to climb again

She begins to notice the change almost immediately as the car starts its descent. As it loses altitude, the oxygen pressure in the air increases, so there is a higher concentration of oxygen with each breath, and with each breath, her hemoglobin levels are replenished a little more.

The brain fog dissipates first, then the dizziness, followed by that seemingly unexplained feeling of suffocation. That's the recovery pattern. The brain is quick to respond to the return of oxygen, and many of the functions that appeared lost return within minutes or hours if there has been no structural damage.

He also knows when he can ascend again, and it's not when he feels a little better, but when his physiology has truly readjusted. Sleeping one or two nights at a lower altitude allows for increased ventilation, the kidneys to correct their acid-base balance, and the blood to begin transporting oxygen more efficiently. Only when he can walk and breathe without symptoms, even with light exertion, does he feel that his body has stopped struggling to survive.

Why does it affect some people more than others?

Not everyone reacts the same way to altitude sickness or hypoxia. Some people's lung ventilation increases rapidly when oxygen levels drop, while others respond slowly and inefficiently. The affinity of hemoglobin for oxygen, capillary density, the sensitivity of carotid chemoreceptors, and the brain's tolerance to oxygen deprivation vary from person to person. This is why two people can be breathing the same Andean air and only one develop severe altitude sickness.

It also doesn't depend as much on physical fitness as is commonly believed. An athlete may have a powerful heart and large lungs, but if their respiratory response to altitude is poor, they will also suffer from hypoxia. In contrast, a sedentary person can acclimatize better if their body effectively activates its adaptation mechanisms.

And yes, there can be differences between men and women, although they are not decisive. Evidence suggests that hormones influence ventilation, red blood cell production, and how oxygen is distributed in tissues, which can modify how each organism responds to altitude. But in practice, these variations are just one more piece of the puzzle: in the mountains, it's not sex or muscle mass that matters, but how each individual's body manages the lack of oxygen.

What is altitude sickness usually like for most tourists who suffer from it?

For most tourists, altitude sickness doesn't manifest as a dramatic emergency, but rather as a progressive and quite recognizable syndrome. It typically appears within the first 6 to 24 hours after arriving in a high-altitude city or area, with symptoms such as persistent headaches, extreme fatigue, nausea, loss of appetite, and a strange feeling of "not being able to fully breathe." It's not that the lungs are damaged; it's that the air contains less oxygen, and the body hasn't yet adjusted its physiology to make better use of it.

In most cases, the body adapts on its own if the person rests, sleeps at that altitude, and avoids strenuous activity for a day or two. The body increases ventilation, the kidneys adjust the acid-base balance, and the blood begins to transport oxygen more efficiently. That's why many people feel terrible the first day and surprisingly better by the second or third. It's uncomfortable, sometimes very unpleasant, but it's usually temporary and reversible.

The important thing to understand is that this discomfort doesn't mean weakness or poor physical condition. Even young, healthy people can experience it. It's simply the response of a human brain facing an environment it wasn't designed for. Most people acclimatize and continue their journey. But a small fraction don't… and that's when the mountain ceases to be a tourist destination and begins to be a medical one.

Does altitude sickness or soroche always imply hypoxia and vice versa?

No, altitude sickness and hypoxia are not the same thing, and one doesn't always cause the other. Altitude sickness is a syndrome that occurs when a person ascends rapidly to high altitude and their body doesn't adapt to the decrease in ambient oxygen. In that context, yes, the cause is almost always hypoxia. But hypoxia itself isn't exclusive to the mountains. It's simply a situation in which the tissues don't receive enough oxygen, and that can happen for many different reasons.

For example, a person with pneumonia, severe asthma, COVID-19, pulmonary fibrosis, or pulmonary edema can experience hypoxia even at sea level because oxygen is unable to pass effectively from the lungs into the blood. This also occurs in severe anemia, where there is oxygen in the air but not enough red blood cells to transport it. It can also happen in heart failure, where the blood is not pumped with sufficient force. Even carbon monoxide poisoning can cause hypoxia, even in healthy lungs, because hemoglobin becomes trapped and cannot carry oxygen to the brain.

However, in the mountains, a different scenario occurs: the lungs and blood function properly, but the air carries little oxygen. This is hypobaric hypoxia at high altitude. When this hypoxia causes neurological and general symptoms, we call it altitude sickness or soroche. So, all cases of soroche involve hypoxia… but a great deal of hypoxia occurs every day in hospitals, cities, and homes, far from any mountains. And that's what makes it so dangerous: it doesn't always come with a beautiful view.

Frequently Asked Questions

What is hypoxia and why does it occur at high altitude?

Hypoxia is a condition in which the body's tissues receive less oxygen than they need to maintain their metabolism. At high altitudes, this doesn't occur because the lungs are damaged, but because the oxygen pressure in the air is lower. This means that, even when breathing clean air, the blood cannot carry enough oxygen into the hemoglobin, and the brain begins to function with an energy deficit.

What is the difference between hypoxia and altitude sickness?

Hypoxia is the physiological phenomenon of a lack of oxygen in the tissues. Altitude sickness, or soroche, is the clinical syndrome that appears when this hypoxia causes neurological, digestive, and general symptoms. In other words, a person can experience hypoxia without being at high altitude, but soroche is a specific form of hypoxia caused by altitude.

At what altitude can hypoxia occur?

Oxygen pressure begins to drop significantly above 2,000 meters, but the clinical risk increases most significantly above 2,500–3,000 meters. The rate of ascent is as important as altitude: ascending rapidly without acclimatization greatly increases the likelihood of developing symptomatic hypoxia, even at moderate altitudes.

How can I tell if I have hypoxia or just tiredness?

Tiredness is relieved by rest. Hypoxia is not. If persistent headaches, dizziness, confusion, clumsiness, palpitations, or difficulty breathing even at rest occur, it's not normal fatigue: it's a sign of oxygen deprivation.

Is it possible to die from hypoxia in the mountains?

Yes. In some cases, hypoxia triggers high-altitude cerebral edema or high-altitude pulmonary edema, two conditions in which fluid accumulates in the brain or lungs. Both can progress within a few hours to loss of consciousness, respiratory failure, and death if the descent is not rapid.

How is high-altitude hypoxia treated?

The treatment that most significantly alters the prognosis is descending to a lower altitude, because it immediately increases the amount of available oxygen. When available, supplemental oxygen, portable hyperbaric chambers, and some medications can stabilize the patient, but they do not replace the physiological effect of returning to an environment with more oxygen-rich air.

Scientific references

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• Centers for Disease Control and Prevention. (2024). High-altitude travel and altitude illness. In CDC Yellow Book 2024: Health Information for International Travel. Oxford University Press.
• Hackett, P. H., & Roach, R. C. (2001). High-altitude illness. New England Journal of Medicine, 345(2), 107–114.
• Luks, A. M., Auerbach, P. S., Freer, L., Grissom, C. K., Keyes, L. E., McIntosh, S. E., Rodway, G. W., Schoene, R. B., Zafren, K., & Hackett, P. H. (2019). Wilderness Medical Society clinical practice guidelines for the prevention and treatment of acute altitude illness: 2019 update. Wilderness & Environmental Medicine, 30(4S), S3–S18.
• Roach, R. C., Hackett, P. H., Oelz, O., Bärtsch, P., Luks, A. M., MacInnis, M. J., & Baillie, J. K. (2018). The 2018 Lake Louise Acute Mountain Sickness Score. High Altitude Medicine & Biology, 19(1), 4–6.
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