There is one thing I can never have too many of when I am on a long car journey with my children. Plastic bags. Two of my three young ones suffer acutely from what researchers in space and aeronautical medicine call "sense spatial disorientation" - travel sickness, in other words. On a recent six-hour car journey to Cornwall we managed to get through seven changes of clothing, and by the time we arrived they were sitting in the back naked. At least the four-year-old can now ask for a bag when he feels the urge to regurgitate the contents of his tummy, but the toddler explodes without warning, torrents of projectile vomit splattering the back of the driver's seat.

There was one moment of sheer panic when I had to share a bag between them as they simultaneously heaved. So far the baby is immune, but I suspect it is only a matter of time.

Summer holidays are made of such moments. Whether it be boat, car, train or plane, travelling is frequently a stomach churner, especially when it comes to the children. Apparently up to 90% of us suffer from travel sickness at some stage, a harmless but deeply unpleasant condition that does no favours for your car's upholstery. According to recent research by the Gallup Organisation, boats are the worst: 70% of the people interviewed had felt queasy on a boat, 60% in a car and 57% in a bus or coach.

Researchers believe that our sense of balance is to blame. It is maintained by a complex interaction within the nervous system, which involves four different processes: the inner ear which monitors the directions of motion; the eyes which tell you where you are in space and the directions of your motions; skin pressure receptors which tell you which bits of your body are touching the ground or a seat; and muscles and joint sensory receptors which tell which bits of your body are moving. The central nervous system endeavours to make sense of all this information.

The symptoms of travel sickness appear when the central nervous system receives conflicting information from its messengers. Basically, your body is being thrown around but your eyes do not detect the motion.

John Golding, a psychologist at the University of Westminster, who researches motion sickness for the Medical Research Council, says that animals get it too. In fact it occurs right down the evolutionary chain to frogs; even fish can get seasick if their aquarium is transported around. "It's likely to have some sort of evolutionary survival purpose," he says.

One theory is that feeling disorientated by motion sets off a type of toxin detector designed to protect us from consuming poisons. The sensations of feeling off balance and dizzy are similar to those we might experience when we've consumed something toxic (too much alcohol for example). "It's the exact calibration of the imbalance you feel when poisoned," says Golding.

Children are more susceptible than adults, especially between the ages of 8 to 13. Younger children are less likely to get it because their sense of balance has not yet stabilised. "After that the system of calibration probably gets tuned up, but there are individual differences," says Golding. Women are slightly more prone to it than men, and get extra queasy when pregnant or menstruating.

Oddly, you are unlikely to feel nauseous riding a horse, but take the sick bag if it's a camel. Our urge to throw up is affected by the vibrational frequency of the mode of transport. Fast, short jerky motions which have a high frequency are less distressing than long, swaying motions which move at a lower frequency. Going round bends and roundabouts lowers the frequency, which is why a long a winding road is almost sure to get the children going.

The good news is that we learn to adapt long-term to motion, which is why children tend to grow out of it eventually. We also adapt short term, getting our sea legs, as the sailors would say. In fact we can adapt so well that some people who have been at sea for an extended period feel sick when they are back on land.

How to beat travel sickness

Prevention
Always travel so your eyes will see the same motion that your body and inner ears feel. Look out of the window as much as possible.

Place children in a high car seat so that they can look out.

Sit by the window in a plane and go up on deck on a ship.

Don't read, but look at distant scenery. Avoid facing backward.

Make sure your car is well ventilated. Don't travel on an empty stomach. A high carbohydrate snack, such as a bowl of cereal, is recommended. Avoid food with strong odours and anything greasy.

Get the sick bag ready when... Your children look sleepy or apathetic. Look out for signs of pallor, headache, sweating and drooling. Older children will be able to complain that they feel nauseous.

Treatment

Drugs
Over-the-counter treatments contain hyocine and antihistamines, and offer good protection. They can, however, make you drowsy and aren't usually suitable for children under three. Best taken 20 minutes before travel.

Acupressure wrist bands
A popular non-drug alternative thought to work by stimulating acupuncture points in the wrists. Anecdotal evidence as to their effectiveness abounds, but there's little scientific proof. In one Italian study in 1990, a doctor in Milan distributed wristbands to the parents of children aged 2-15 known to suffer travel sickness. In 55 cases (88.7%) the results were excellent. In the remaining cases there were moderate improvements. At around £7.50 a pair they are a costly try-it-and-see. Sea Bands make them in child sizes. They need to be positioned accurately, which is bad news if your child is likely to fiddle with them.

Homeopathic remedies
Nelson Travella is a formulated remedy with ingredients including theridion, tabacum, petroleum, cocculus and nux vomica. Can be safely used for children of all ages. Little scientific evidence to support claims. Start to take two hours before travelling and then hourly. Available from Boots, £4.20.

Herbal treatments
Good evidence that ginger might work. Chew on pieces of crystallised ginger, or if your child does not like the taste give them ginger tablets to swallow. Available from health food stores.

Aromatherapy
Peppermint oil has been thought to settle the stomach. Sprinkle some on a hankie and sniff. Only for children over one.

Travelwell
A new audio tape which is meant to desensitise the vestibular system in the inner ear. Special frequencies and pulses are apparently concealed within the music, but it has received no firm scientific backing so far. Must be played on a personal stereo. Can be used for children. Available from Boots, £9.99. For details go to www.travelwell.co.uk.