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Hypoxia is a major concern during both techniques; there is one documented fatality associated with a high altitude jump. Oxygen deprivation causes hypoxia, and its symptoms include dizziness, giddiness, a tingling sensation, euphoria, blurred or tunnel vision, lack of muscle coordination, and slow reaction time. To compensate for the body's craving for oxygen, the heart and breathing rate increases.
For your safety, SENE regularly uses a pulse oximeter to see if your body has enough oxygen at high altitudes throughout your Kilimanjaro climb.
There is also a general trend of smaller body sizes and lower species richness at high altitudes, likely due to lower oxygen partial pressures.  These factors may decrease productivity in high altitude habitats, meaning there will be less energy available for consumption, growth, and activity.