Symptoms of altitude illness can begin to occur at 8000ft (2400m) or lower still, but serious altitude illness is rare below 10,000 ft (3000 m). Symptoms occur due to our body not adapting well to having less oxygen at high altitudes. At 18,000 ft (5500m), there is 1/2 the oxygen available as at sea level and it is 1/3rd on top of Mount Everest. Body tries to adapt to lower amounts of oxygen in the air mainly by increasing the rate and depth of breathing so you breathe faster and deeper. There is also an increase in heart rate. Both of these mechanisms try to bring more oxygen to the body. Increase in the number of red cells occurs after staying at high altitude for 2 weeks or longer and is important for persons who live at high altitude. There is wide individual susceptibility to altitude which seems to be genetically determined.
What happens to the body in altitude illness? Lack of oxygen causes fluid leakage and accumulation in between cells in the brain and/or the lungs. Symptoms can be mild or severe. Mild symptoms of acute mountain sickness or AMS are headache, loss of appetite, nausea, fatigue, lack of sleep and dizziness. These symptoms can resolve once someone is acclimatized e.g. by spending one or two extra nights at the same altitude or symptoms may worsen needing someone to descend to lower altitudes.
AMS can progress to High Altitude Cerebral Edema (HACE) and symptoms of HACE are mental confusion, difficulty with balance and co-ordination. Fluid accumulation in the lungs can lead to High Altitude Pulmonary Edema (HAPE). HAPE results in shortness of breath at rest, extreme fatigue, cough – dry in the beginning, and later productive of frothy or blood-tinged sputum. HAPE and HACE are severe symptoms, often co-exist in severe cases and can be rapidly fatal if untreated. Treatment is DESCEND, DESCEND and DESCEND.