Chapter 2 · From sea level to the summit of Everest, 0–8,849 m
High on a mountain
Imagine you climb
Imagine you take a cable car up a mountain. The valley is warm, you breathe easily, and after a while you notice you are breathing a little faster. Nothing is wrong with the air: it is still about 21 per cent oxygen at every height. But the air is thinner, because it has less weight above it. Pressure falls with height. At about 5,500 metres it is already half of what it is at sea level, and every breath brings you half as many oxygen molecules. Look at the mountain in the picture and drag the altitude slider upwards in the sandbox.
A third of the oxygen
Mount Everest is 8,849 metres high, according to the 2020 survey by China and Nepal. On the summit the air pressure is only about one third of the pressure at sea level. Oxygen is still 21 per cent of the air, so its share of the pressure falls too. Worked example: at sea level oxygen gives 0.21 × 1 atm = 0.21 atm; on the summit it gives about 0.21 × 0.33 = 0.07 atm. That is like breathing air with only 7 per cent oxygen at the sea. Slide the altitude to the top and watch the oxygen bar shrink.
How the body adapts
The body notices the shortage and answers. First you breathe faster and your heart beats quicker. Over days and weeks it makes more red blood cells, which carry oxygen, and this slow change is called acclimatisation. If someone goes up too quickly, it has no time to adapt, and altitude sickness can appear above about 2,500 metres: a headache, nausea, dizziness, poor sleep. The general rule of mountain medicine is simple: go up slowly and give the body time. If the symptoms get worse, the cure is to go down, and serious symptoms need a doctor.
The death zone
Above roughly 8,000 metres climbers speak of the death zone. There the body can no longer adapt: it uses up more than it can get, it does not digest food well, and rest does not restore strength. Most people who climb the highest peaks therefore breathe bottled oxygen from a mask. A few did it without. In 1978 Reinhold Messner and Peter Habeler were the first to reach the summit of Everest with no extra oxygen, and many doctors had thought it impossible. They still needed years of training, and they moved very slowly.
Water that boils too early
Water boils when its vapour pressure equals the air pressure, so in thin air it boils sooner. At sea level it boils at 100 degrees Celsius. Every 300 metres or so of height lowers the boiling point by about 1 degree, and on the summit of Everest water boils at about 70 degrees. That is hot enough to scald but not hot enough to cook rice properly. Try the sandbox: at 2,000 metres the result is about 93 degrees, and tea made on a high pass tastes weaker.
Why an airliner is pressurised
A passenger jet cruises at around 11,000 metres, where the outside pressure is less than a quarter of the sea-level value and nobody could breathe it. So the cabin is pressurised to the equivalent of about 1,800 to 2,400 metres. That is thin air, like in a mountain village, yet comfortable for healthy people, and it explains why a sealed bag of crisps swells during a flight. The wall of the plane holds the difference in pressure, which is why cabin doors cannot be opened in the air.
In numbers
- ≈ 5,500 m
- where the air pressure is half of the sea-level value
- 8,849 m
- the height of Everest, 2020 survey
- ≈ 70 °C
- the boiling point of water on the summit