What Is Altitude Sickness? Less Oxygen in Every Breath

Atmospheric pressure falls roughly exponentially with altitude, reaching about half its sea level value at around five and a half kilometres and about a third at the summit of the highest mountains. Because the percentage of oxygen stays at close to twenty-one, the partial pressure of oxygen falls in proportion to total pressure, and it is partial pressure that drives oxygen across the membranes of the lungs into the blood. The immediate physiological response is to breathe faster and more deeply, which raises oxygen intake and simultaneously blows off carbon dioxide, making the blood more alkaline, which itself suppresses the drive to breathe and is part of why the adjustment takes days rather than minutes. Heart rate rises. Over subsequent days the kidneys excrete bicarbonate to correct the alkalinity, allowing breathing to increase further, and the body begins producing more red blood cells, a process taking weeks and driven by a signalling system whose discovery earned a Nobel prize in 2019.

Altitude illness is conventionally divided into three conditions of increasing severity:

• •Acute mountain sickness, common above about twenty-five hundred metres, presenting as headache with nausea, fatigue, dizziness and poor sleep, which resolves with rest at the same altitude or with descent
• •High altitude cerebral oedema, a progression in which fluid accumulates in the brain, producing confusion, loss of coordination and eventually unconsciousness, which is life threatening and requires immediate descent
• •High altitude pulmonary oedema, in which fluid accumulates in the lungs, producing breathlessness at rest, cough and reduced exercise capacity, which can develop rapidly and is the most common cause of death from altitude illness
• •Loss of coordination, tested by asking someone to walk heel to toe in a straight line, is the practical field sign that distinguishes serious illness from ordinary discomfort
• •Severity correlates with rate of ascent, sleeping altitude and individual susceptibility, and notably not with physical fitness, which gives fit people false confidence
• •Descent is the definitive treatment for all three, and supplementary oxygen and portable pressure bags buy time rather than substituting for it

Prevention is almost entirely a matter of ascent profile. The standard guidance above three thousand metres is to increase sleeping altitude by no more than about three to five hundred metres per night with a rest day every few days, and the useful principle is to climb high and sleep low, since daytime excursions to greater height assist acclimatisation while the night at lower altitude allows recovery. Acetazolamide accelerates acclimatisation by causing bicarbonate excretion, effectively pre-empting the kidney's adjustment, and is used prophylactically where a rapid ascent is unavoidable. Dexamethasone treats symptoms without accelerating acclimatisation and is carried as an emergency drug. Hydration matters and is frequently overstated, since dehydration produces similar symptoms and does not cause the underlying condition. Alcohol and sedatives suppress breathing and are avoided. The recurring cause of serious incidents is a fixed itinerary combined with money already spent, which produces pressure to continue when the correct action is to stop or descend.

Several million people live permanently above three thousand metres, and three populations have been studied closely because they solved the problem differently, which is a notable case of convergent adaptation reaching separate answers. Tibetans, resident on the plateau for many thousands of years, show relatively normal haemoglobin concentrations alongside higher breathing rates and higher blood flow, and carry variants of a gene involved in oxygen sensing that appear to have been inherited from Denisovans, an archaic human population, which is one of the clearest examples of adaptive introgression known. Andean populations instead show substantially elevated haemoglobin and larger chest capacity, which carries a cost in blood viscosity and a condition called chronic mountain sickness that Tibetan populations experience far less. Ethiopian highlanders show yet another pattern, maintaining normal haemoglobin and oxygen saturation by mechanisms not yet fully identified. Visitors acclimatise partially over weeks and never reach the same position, which is why expeditions plan around acclimatisation schedules rather than around fitness.

Air keeps the same oxygen percentage at altitude while pressure falls, so each breath delivers fewer molecules. The body responds by breathing faster, which disturbs blood chemistry and takes days to correct, and by making more red cells over weeks. Acute mountain sickness is common above twenty-five hundred metres, and fluid accumulating in the brain or lungs is life threatening, with descent the definitive treatment. Ascent rate rather than fitness governs risk, and Tibetan, Andean and Ethiopian populations adapted differently.