The mountains have never been more accessible. Trekkers walk to Everest Base Camp, tourists fly into high Andean cities, and pilgrims travel to shrines perched thousands of metres above sea level. Yet altitude remains one of the most underestimated hazards in travel medicine. Altitude illness does not care how fit or experienced you are, and in its severe forms it can kill within hours. Understanding how the body responds to thin air, and how to ascend safely, is essential for anyone heading above roughly 2,500 metres.
Why altitude makes us ill
As you climb, the air pressure falls and each breath delivers less oxygen to the body, even though the percentage of oxygen in the air stays the same. The body responds by breathing faster and producing more red blood cells, a process called acclimatisation that takes days to complete. Altitude illness occurs when you ascend faster than your body can adapt. The crucial point, often missed, is that susceptibility has little to do with physical fitness. Strong, healthy athletes are just as likely to be affected as anyone else, and sometimes more so because they push themselves harder and faster.
The single most important factor is the rate of ascent, particularly the altitude at which you sleep. Climbing high during the day is less of a problem than sleeping high. This is why the golden rule of altitude is built around sleeping elevation, not the highest point reached.
The three forms of altitude illness
Altitude illness exists on a spectrum, from a common and usually mild form to two rare but life-threatening conditions. Recognising where you are on that spectrum can save your life or that of a companion.
Acute mountain sickness, or AMS, is the mild and most common form. It feels like a hangover: headache combined with some mix of nausea, loss of appetite, fatigue, dizziness, and poor sleep. It typically appears within six to twelve hours of arriving at altitude. AMS is unpleasant but not dangerous in itself, provided you heed its warning and do not climb higher.
High-altitude cerebral oedema, or HACE, is a severe progression where fluid affects the brain. The hallmark sign is a change in the ability to think and move normally. The person may become confused, unusually clumsy, unable to walk a straight line, drowsy, or behave strangely. HACE is a medical emergency.
High-altitude pulmonary oedema, or HAPE, involves fluid building up in the lungs. Warning signs include breathlessness at rest, a persistent cough that may produce frothy or pink sputum, extreme fatigue, and a feeling of suffocation, often worse at night. HAPE can develop quickly and is the most common cause of altitude-related death.
Prevention through sensible ascent
The overwhelming majority of altitude illness is preventable, and prevention is far better than treatment. The foundation is gradual ascent, giving the body time to acclimatise. Widely used guidance for travel above 3,000 metres includes the following principles:
- Once above 3,000 metres, increase your sleeping altitude by no more than about 300 to 500 metres per night.
- Build in a rest day, without further ascent, every three to four days or roughly every 1,000 metres of gain.
- Follow the maxim “climb high, sleep low,” meaning you can ascend higher during the day as long as you return to a lower elevation to sleep.
- Stay well hydrated and avoid alcohol and sleeping tablets, especially in the first days, since both can mask or worsen symptoms.
- Plan itineraries that avoid flying or driving directly to a high sleeping altitude; where that is unavoidable, take extra care and consider preventive medication.
A particular trap is the high-altitude airport. Flying straight into a city at 3,500 metres or higher gives the body no chance to adapt, and a relaxed first day with light activity and plenty of fluids is wise. Some destinations almost guarantee mild symptoms on arrival simply because of how travellers get there.
The role of medication
Acetazolamide is the best-established drug for preventing and treating AMS. It works by speeding up acclimatisation rather than masking symptoms, and a clinician may recommend it when a rapid ascent is unavoidable or when you have a history of altitude illness. It is not a licence to climb faster; it is an aid to sensible ascent. Common, harmless side effects include tingling in the fingers and a changed taste of fizzy drinks. For severe illness, dexamethasone is used to treat HACE and nifedipine for HAPE, but these are emergency measures, not substitutes for descent, and should be used under medical guidance where possible.
Treatment: the golden rule of descent
If altitude illness develops, three principles govern the response. First, do not ascend further while you have symptoms. Many tragedies occur because someone with mild AMS pushed on to the next camp instead of waiting. Second, if symptoms are mild, rest, hydrate, and treat the headache; only continue upward once you feel well. Third, and most importantly, if symptoms are severe or worsening, descend immediately.
Descent is the definitive treatment for HACE and HAPE, and even a drop of 500 to 1,000 metres can produce dramatic improvement. Never leave someone with severe symptoms alone, and never let them descend alone, because their judgement and coordination may be impaired. Do not wait for morning; HACE and HAPE can be fatal within hours, and darkness is not a reason to delay a life-saving descent if it can be done safely.
Planning for the heights
Before a high-altitude trip, discuss your itinerary with a travel clinician, particularly if you have heart or lung disease, are pregnant, or have had altitude problems before. Build acclimatisation days into your schedule rather than treating them as optional luxuries, and resist the pressure of fixed tour timetables that ascend too fast. Carry a plan for what to do if a member of your group falls ill, and make sure everyone knows the warning signs.
The mountains reward those who respect them. By ascending slowly, listening to your body, and treating any severe symptoms as a cue to go down rather than up, you give yourself the best chance of enjoying the extraordinary landscapes of high altitude safely and returning home with nothing worse than good memories and aching legs.