Flying at high speed and high altitude in a pressurised container across time zones produces a cocktail of negative effects that can make the journey uncomfortable for passengers.
While jet aircraft are designed to operate efficiently at high altitudes, the human body is not. Humans are land animals, evolved to exist comfortably close to sea level at a maximum speed of little more than 15 miles per hour. Anything else is a foreign, and potentially lethal, environment.
So what exactly does flying do to your body? And what can you do to avoid the worst?
Dehydration is the most common problem because our bodies are used to humidity in the air of about 40 to 70 percent but in an aircraft cabin it falls to about 20 percent. It is unlikely that your body will become seriously dehydrated but it is common to suffer from complaints such as dry eyes or dry throat and nose.
Dr Yousef Abou Allaban, managing director of the American Centre of Psychiatry and Neurology which has been working with Etihad Airways to improve sleeping conditions on board its planes, has a simple solution. “Drink lots of water, avoid caffeine,” he says.
The lower pressure in the cabin also has an impact on the body including a reduced amount of oxygen being absorbed by your blood, known as hypoxia. One effect of this is to leave you listless and perhaps dizzy or faint. Lower pressure can also cause pain or discomfort in your ears.
Confinement is arguably the biggest issue for most travellers and the most controversial on health grounds. Apart from the sheer discomfort of minimal legroom – cramps, neck cricks and so on – long periods of immobility lead to a higher risk of deep vein thrombosis (DVT), and the risks may be greater at lower pressure.
DVT occurs when blood in the lower leg flows sluggishly and eventually clots, causing pain and swelling in the short term and a risk of sudden death from clots reaching the heart or lungs.
Doctors recommend that you wear comfortable, non-restricting clothes and get up and walk around the cabin at least once every hour.
Gas in the stomach or intestine expands as an aircraft climbs and in some people, it can lead to abdominal pains so before flying, avoid food and drink that can cause a build-up of. Try eating peppermint capsules to help absorb gases.
Large changes in altitude can also cause toothache, when tiny pockets of gas become trapped in deep fillings, or collected in areas of decay.
Dr Khaldoun Mozahem, neurology consultant at the Centre, also points out why the effects of flying at 35,000ft can lead to anxiety: “The atmosphere in an airplane is not the same as on the ground so first you have dehydration and the change in pressure. Even though they (airlines) try to fix it, it is not exactly the same on the earth and anxiety can set in about being in a different setting.”