When you picture mountain climbers scaling Mount Everest, what probably comes to mind are teams of climbers with Sherpa guides leading them to the summit, equipped with oxygen masks, supplies and tents. And in most cases you'd be right, as 97 per cent of climbers use oxygen to ascend to Everest's summit at 8,850 metres above sea level. The thin air at high altitudes makes most people breathless at 3,500 metres, and the vast majority of climbers use oxygen past 7,000 metres.