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For several weeks, Mogens Jensen acclimatised to the thin air on the mountain from a number of different camps at different levels. Even at the main Base Camp, the oxygen is only about half that at sea level. At the summit, it is about one-third. Mogens’ body needs time to adjust.

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“It is a frustrating but necessary process that takes time,” says Mogens. “I have to do little, resting and letting my body slowly 'absorb the altitude'. This is an absolutely necessary part of the climb.”

Before the attempt on the summit, acclimatisation saw Mogens go up some way, then descend. A few days later, climbing a little higher than before, perhaps then descending all the way or part of the way. Again, going even higher, then perhaps right back to Base Camp. The process depends on what his body tells him – individuals differ in their reaction to thin air – and what the weather is doing.

The human body is not able to acclimatise above 8,000m and the amount of time spent beyond this height - known among mountaineers as 'the death zone' - is reduced to a minimum.

To tank or not to tank
Mogens hoped to become the first asthmatic to climb Mount Everest without the use of additional oxygen. Among mountaineers, the use of bottled oxygen to climb Everest has been a topic of sometimes heated debate.

George Mallory said that using bottled oxygen was unsportsmanlike, though he conceded that it was necessary for the summit. Tenzing and Hillary used it for the first successful summit in 1953, setting the trend for the next 25 years.

In 1978, Italian climber Rheinhold Messner and another mountaineer became the first to summit without bottled oxygen. Messner repeated this in 1980 on his own.

The debate is around the effect that altitude has on the brain and clear-thinking abilities - vital if a mountaineer is to survive. Messner has been accused of recklessness and of damaging his own faculties through the Everest and other climbs without additional oxygen.

Others argue that bottled oxygen allows unqualified climbers to attempt the climb. Bottled oxygen, they say, gives climbers a false sense of security. There has been a proposal to ban bottled oxygen from the mountain to help restrict the increasing number of discarded tanks in the area.