Altitude Sickness
What is Altitude Sickness?
Altitude sickness (Acute Mountain Sickness, AMS) is a physiological illness triggered by reduced oxygen availability at high elevation, typically affecting travellers who ascend above 2,500 metres too rapidly for their bodies to acclimatise. As altitude increases, atmospheric pressure drops and the partial pressure of oxygen in each breath decreases — the body must produce more red blood cells and make other adaptive changes to maintain oxygenation, a process that takes days to weeks.
AMS symptoms begin with headache, nausea, fatigue, dizziness, and poor sleep, and in serious cases can progress to High Altitude Pulmonary Edema (HAPE — fluid in the lungs) or High Altitude Cerebral Edema (HACE — brain swelling), both of which are life-threatening emergencies requiring immediate descent. Affected regions include the Bolivian and Peruvian Andes, the Tibetan Plateau, Ethiopian highlands, and any mountainous trekking region above 3,000 metres. The pharmaceutical prophylactic Acetazolamide (Diamox) is commonly prescribed to accelerate acclimatisation.
Example of Altitude Sickness
James flew directly from London to Cusco, Peru — at 3,399 metres. He arrived at 11am feeling marginally dizzy. By 4pm he had a headache requiring paracetamol every four hours. That evening he could not eat dinner. By midnight he had a splitting head and was genuinely nauseous. He had ignored the acclimatisation advice in every guidebook he'd read. He spent two days in his room, drinking coca tea and chewing coca leaves, and recovered by day three. He now always arrives at altitude via a lower intermediate stop first.