Most lay people know that Mount Everest is the highest in the world. What is less well known is that to reach the summit it is necessary to go up and down it about five times.
The air is so thin above Base Camp that the body needs time to acclimatise. That means that summiteers must ascend and descend repeatedly, going a little further each time to Camps Four, Three and Two, before they are ready for their final summit bid from Camp One, the highest on the mountain.
By the time they have reached the summit, most climbers will have ascended a distance almost twice the height of the mountain.
This is where Appa's experience and heritage give him a key advantage over Western climbers. As a Sherpa born and raised in the Himalayas, he has both the right genetic make-up and environmental background to cope with the rigours of the climb at altitudes where human beings can survive for only an extremely limited time.
The biggest hazard is the lack of oxygen. Climbers take tanks on their backs, but the weight limits the amount that can be carried. Low blood-oxygen causes nausea, headaches, fatigue and shortness of breath – the early signs of altitude sickness. It can lead to dizziness and confusion, the cause of countless deaths on the mountain.
To compensate the body ups the amount of oxygen-carrying haemoglobin in the blood, causing it to become more viscous. This is part of the process of acclimatisation.
In turn, the heart is forced to work harder to pump the blood round the body. In an unfit man that could lead to a heart attack. Appa's exceptional level of fitness is likely to protect him, even though he is 48. But, like every attempt at Everest, Appa's 19th will still be high risk.