At around 2500 metres above sea level most climbers start to suffer.
Progressively ascending, descending and recovering alleviates altitude sickness but the higher mountaineers tread, the harder that physical toll.
This year, the Himalayan climbing season got off to a good start, after a two-year hiatus, but in the space of three days the mountain claimed the lives of three climbers and a sherpa guide while two more mountaineers are missing.
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