Air gets thinner the higher you go. On the summit of Kilimanjaro, a lungful contains half the oxygen you would have if you inhaled on sea-level. When the human body is given enough time to adjust, it simply produces more red blood cells to transport more oxygen. However, without weeks to acclimatize, almost everyone climbing Kili will experience the effects of high altitude, known as altitude (or mountain) sickness. These include shortness of breath, lightheadedness, headaches, insomnia, nausea and, naturally enough, exhaustion.

The symptoms appear towards the end of the second or third day. Normal altitude sickness isn’t much to worry about, although vomiting should be treated seriously with rehydration as this is essential at high altitude.
Acute mountain sickness (AMS) is the chronic form of altitude sickness and is much more serious. Symptoms of AMS include most of the above, plus any or all of the following: severe headaches, shortness of breath at rest, flu-like symptoms, persistent dry cough, blood-tinged saliva or urine, unsteadiness or drowsiness, lack of mental clarity or hallucinations, and finally, chest congestion. In theses cases, descend immediately to a lower altitude. Be aware that mental fuzziness may convince the victim that he or she if fit to continue. They are not. A porter will usually accompany the victim, so the whole party won’t have to turn back. Ignoring the symptoms of AMS can be fatal: complications like pulmonary oedema and cerebral oedema claim the lives of about a dozen climbers each year. Predicting who will get sick is impossible: AMS affects young and old alike, fit and not so fit, so don’t deny the signs if you start feeling them, and geed your guide’s advice.

Some drugs are claimed to eliminate such problems. However, opinion is sharply divided over their pros and cons, so consult a doctor before taking anything.

Given the confusion over medication, prevention is a better approach. Let your body acclimatize naturally by taking an extra day or two when climbing the mountain (at least six days, whichever route you’re taking, ideally seven); stay hydrated, climb slowly and if you ascend a lot in one day, camp at a lower altitude, if possible. Lastly, don’t go higher than the tree line (2700m) if you’re suffering from fever, nosebleed, cold or influenza, sore throat or a respiratory infection.