Travel broadens the mind but it can play havoc with the body. Sea sickness and car sickness are both variations of travel sickness (also known as motion sickness) which occurs when our sense of balance and equilibrium are disturbed.

We naturally sense our spatial orientation: which way we are facing and moving, whether we are turning or standing still. When at sea, with your boat pitching and rolling, or in a moving vehicle, you send confusing messages to your brain, making you feel sick.

Your eyes tell your brain you're not moving. At the same time another sensory organ, your inner ear, picks up the motion of the car or boat and tells your brain you are moving. Motion sickness starts when the brain receives conflicting messages. Dizziness and nausea are the common symptoms.

There are over-the-counter antihistamines and assorted old wives' tales available for those who suffer from motion sickness.

Car sickness is best avoided by focusing your eyes outside the vehicle - so that the visual signals to the brain confirm what the inner ear is saying about movement. For this reason car drivers are never car sick. In a coach or bus, try to sit close to the front. Drink plenty of fluid to avoid dehydration.

A few simple steps to relieve the symptoms of sea sickness:

• Remain on your feet;
• Go on deck, where the air is fresh, and stay at the stern (rear) of the ship, which moves less than the bow;
• Stare at the horizon, so the visual signals sent to your brain are correct;
• Eat small amounts of food such as biscuits, bread or apples, both to absorb the acid in the stomach and to line an empty stomach;
• Sip (don't gulp) water, fruit juice or other liquids - but not coffee or alcohol - to prevent dehydration.

Motion sickness can be affected by a certain frequency of movement. The frequency most likely to cause sickness is 0.2 Hertz, and is comparable to a boat rolling every five seconds. Strangely, if the boat moves more than this you are less likely to suffer sickness. Windsurfers, for example, rarely suffer.

If the worst comes to the worst on a long voyage, remember that practically everyone develops a resistance to motion sickness within three days.

Fortunately, not everyone is susceptible to motion sickness. Children under two years rarely suffer, despite travelling backwards in their car seat, but their susceptibility rapidly increases as they get older, peaking between 4 and 10 years, then gradually declines.

Deep-veined thrombosis (DVT)

The so-called "economy class disease" results in a blood clot (pulmonary embolism) forming in a vein, which then makes its way to the heart. The elderly and infirm are most at risk but all long-haul passengers should take note.

The causes are straightforward: high altitude lowers blood circulation, while cramped aeroplane conditions make exercise difficult.

So, too, are the best methods of prevention: passengers on long-haul flights are advised to exercise their lower legs and feet, and to walk up and down the aisles regularly to avoid clots developing. Taking an aspirin to thin the blood is also known to help.

Pressure on the ears

For many air passengers, the build-up of pressure on the ears is a great discomfort.

The pressure in the aircraft is reduced gradually to a safe level, rather than achieved in one go. So your ears pop as they respond to this gradual change. Planes fly at around 12,000 metres, an optimum height for fuel economy, where the air pressure is only around one fifth of that at sea level. If sea level pressure was maintained at that height, the pressure inside the plane would be so great in relation to that outside that the plane would explode.

The air pressure inside the plane is therefore maintained at the safest level, that of around 2,500 metres, while the plane is cruising. Even so, anyone with respiratory problems or who has been drinking alcohol while flying may still feel ill at this reduced pressure.

Take-off and landing is the hardest time for babies because of the extra pressure on their ears. Try to feed them at this point as the sucking will help relieve the pressure.

By the time you land the pressure on the outside should be the same as that on the inside and the pressure on either side of your ears should be equal. If not, pinch your nose gently and blow to increase the pressure in your head slightly, which will pop your ears.

Mountain Sickness

The change in pressure, and the reduced oxygen in the air, can also affect people on the ground that travel quickly to higher altitudes. The nervous system, lungs, muscles and heart can all be affected. Symptoms can include shortness of breath, headaches, dry coughing, general fatigue, dizziness and nausea. Some sufferers report difficulty in sleeping.

Prevention is the best cure. Acclimatise to increasing altitude by ascending gradually, stopping every 2,000 feet or so to rest and recover. If you cannot avoid the rapid increase in altitude (e.g. flying direct to La Paz, Bolivia or to Mexico City), avoid physical exertion, drink plenty of fluids and prepare to ride out the symptoms, which should disappear as your body adjusts. If symptoms persist you should obtain professional advice. 2