When we take a plane trip our body takes a journey that affects our cardiovascular system, our sensory perceptions and possibly even our immune system.
In the space of just a few minutes, your body is shot into an environment whose conditions resemble those found at altitudes of 2.5km above sea level. It's as if you've suddenly scaled Mt Kosciuszko.
The air pressure drops to around 750 hectopascals – a quarter lower than at sea level. For perspective, air pressure at ground level doesn't drop below 850 hectopascals, even in the eye of the severest cyclone or hurricane. And even then this process takes at least 24 hours.
If that wasn't enough, the airflow around your body is sometimes an icy 5°C when it leaves the air-conditioning vents. This semi-frost is designed to counter the heat radiated by your fellow passengers (as much as a 100-watt light bulb per person) and the equipment – from the boiling hot galley to the entertainment system.
The air you breathe onboard a plane is usually taken from the atmosphere via a 'tap' in the jet engines. It heats up from -50°C to +200°C within seconds, before dropping back to zero.
Meanwhile, the air humidity falls to around 10 percent or lower. Normal room humidity, in comparison, is around 60 percent – and even in the bone-dry Sahara it's still around 20 percent.
So conditions are indeed extreme when you're on board an aircraft. But what effect do they have on us?
"It is possible to design a plane whose on-board conditions match those on the ground," says the president of the German Society of Aerospace Medicine, Professor Jochen Hinkelbein from the University Hospital of Cologne.
But this would require an aircraft's aluminium skin to be much thicker, and thus weigh several tonnes more, in order to withstand the higher air pressure inside. Airlines would also need hundreds of extra kilograms of water per flight to keep the air humidified.
"The on-board conditions are a compromise between wellbeing on the one hand, and profitability and eco-friendly practices on the other.
"Long-haul flights do not pose a problem for healthy people – which is why relatively little research has been conducted in this field," Professor Hinkelbein adds. "Only now that flying has become a mass phenomenon, with a growing number of older, less healthy people travelling, is a greater interest being shown."
Our body is similar to a pressure hull. On the ground, the external air pushes against it at about the same intensity as it pushes back from inside.
In the air, this balance suddenly falls out of sync – the pressure in the digestive system is higher than the lowered air pressure. The gases in the stomach expand, causing bloating and flatulence, which try to find a way to escape.
Pain in the ear or behind the forehead, felt particularly by those suffering from colds, is another consequence of differing pressure between your surroundings and your head during take-off and landing.
Without people, the cabin air humidity would only be two percent, but the combined breathing and sweat of the crew and passengers increase this to 15 percent.
During a flight, our body becomes parched – we can lose up to 1.5 litres of fluid for every three hours of flying.
Up to 37 percent of the skin's moisture evaporates, which can be seen in the callouses on our fingers we get while flying, while contact lens wearers often suffer from eye irritations.
Tomato juice with salt and pepper is more popular on board than any other drink, including beer.
The reason? Our ability to taste and smell decreases when flying, so our senses need a larger quantity of a substance in order to register it.
Salt and sugar taste up to 30 percent less salty/sweet. Chefs who cater for airlines season their meals accordingly.
The higher above the earth's surface you are, the less the atmosphere protects you from cosmic radiation, though protection is higher over the equator compared to the Poles.
An eight-hour flight increases the 2100 microsieverts of radiation naturally absorbed by humans from their surroundings by around five per cent – 20 hours on a plane is roughly equivalent to having a head x-ray.
While this is not considered dangerous, crew exposure to radiation is monitored.
Fainting is the most common medical emergency that occurs on board.
Susceptible people are also at risk of high blood pressure, cardiac arrhythmia and blood clots. This is due to a lack of oxygen in the blood and a lack of exercise during the flight.
Without muscular strength, leg veins are unable to properly transport blood back to the heart.
Initial studies on the impact of oxygen deficiencies on the immune system have found that some proteins change the way they work in these cases.
Thus, the notorious 'aeroplane cold' could not just be due to the air-conditioning system, but also to the body's altered immune response to germs.
The photoreceptors in our eyes need lots of oxygen – and they start showing signs of deficiency from altitudes of just 1500 metres. This reduces our ability to see sharply in dimmed light by 10 to 15 percent.
Studies have found that 15 percent of men and six percent of women cry more when watching in-flight entertainment compared to on the ground.
According to researchers, this is due to the heightened emotional situation – such as being away from home or excitement about the trip – but possibly also to the slight lack of oxygen in the body.
The fact that cabin pressure is 25 percent lower than on the ground makes it harder for our lungs to exchange gases. This causes our blood's oxygen content to drop by six to 25 percent, which can make us feel sleepy.
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