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How altitude affects the human body

Author Dr Ian Perry

In most modern aircraft flying long haul routes, the cabin pressure is maintained at around 5-6000ft (1500m) when the aircraft is flying at 37-41,000ft (10,000m). It’s something we give little thought to as we jet off but how does altitude affect the bodies of the 7 billion people who live at or around sea level. Of these 7 billion, 140 million live up mountains, in high places such as Quito, the capital of Ecuador which sits at 9,350ft (2,850m) high with a population of 2.67m people. Mexico City sits at 7,350ft (2,240m) with a larger population of 8.84 million.

All these inhabitants are acclimatised to living at such heights but when those who live at sea level visit one of these higher cities, we could initially find we feel breathless, faint, get a headache, become confused, make the wrong decisions, feel physically weak, have an accident or have disturbed bodily functions. But what causes this?

In simple terms, there’s less pressure of oxygen (O2) in the air the higher you go. Acclimatisation to this decreased oxygenation of the blood takes on average about 3-4 weeks. Mountain climbers take longer to acclimatise, with only a few managing to climb Mt Everest at 29,000ft (8,850m) without extra O2. This gradual preparation takes time; if not done carefully it can result in mountain sickness, requiring urgent medical treatment and O2.

The amount/concentration of O2 in normal air is 20.9%. The atmospheric pressure is the weight exerted by the mass of air all around us, and up to 50-60,000ft (15,250m). It is measured as one atmosphere at sea level. This atmospheric pressure gets less the higher up you go, so there is less pressure to drive the O2 gas through your lung membranes giving you the oxygenation that you require. Above 10-11,000 ft (3,000m), everyone has to breathe extra supplemental O2. At 18,000ft (5,500m), pressure has to be applied to any breathing equipment used by a pilot/mountaineer, to force the O2 into the blood stream in your lungs. In mountaineering above 18,000ft (5-5,000m) the altitude is known as the “Death Zone” as without supplemental O2 and the extra pressure, the majority of us not acclimatised, would obviously die. The early balloonists experienced these problems as they ventured higher and higher into the upper sky. Some died from oxygen lack.

As the human body goes higher, and gets less oxygen, the effects above 10,000ft (3,000m) can be very variable from person to person. You become “anoxic”, short of O2. The first sign can be a loss of colours in your vision.  The red cells in your blood carry the oxygen, on becoming deficient, the tissues throughout the body begin to suffer. The human eye is a very sensitive organ depending on oxygen to stimulate the light receptors at the back of the eye. These light receptors distinguish what colours you see. If there is a lack of oxygen, the light receptors cannot interpret colours, so a loss of colour vision can be an early sign of oxygen lack.  You can begin to feel aggressive, or sleepy. Speech can become slurred as you begin to speak nonsense as your words become jumbled. Someone writing who starts to become anoxic will lose the ability to put words and letters together. As the brain tissues begin to lose oxygen all the normal functions begin to falter. You may not be aware of these subtle changes until it is too late. In flying training most pilots undergo some form of experience in oxygen lack and its early signs. It is done formally in military pilots, in a decompression chamber.

This training is invaluable and has taught many pilots what to look out for and how to correct it.  As an aircraft passenger, the only risk is if the aircraft was to decompress, that is, to lose the cabin pressure. One of the standard drills performed by the cabin staff before all flights is to demonstrate to the passengers what to do if the pressure system fails. It is always to put on the oxygen mask first as you cannot help others until you are breathing normally, pulling on the gas tube to turn it on, making sure it fits snugly. There are control dials and switches at the pilots end to ensure steady cabin pressure at around 5-6000ft (1500m) when the aircraft is flying at 37-41,000ft (10,000m).

However, this can still cause some people to feel a little tight in the chest, even a little breathless. It is like being up a mountain of the same height, but it wears off as the flight continues. You get acclimatised, but it can take time, so no one should over exert themselves during a flight. You should also not make any cognitive decisions on the day of travel as your cognition will have been impaired travelling.