What are the most common problems?

People complain of dry eyes, skin and mouth – and dehydration. Others of catching colds. The biggest worry is a blood clot in the leg (a deep vein thrombosis), which occurs in only one in 6,000 people after a flight of four hours, and one in 1,000 after longer or frequent flights.

What causes dehydration?

It is said that flying dries out the mucous membranes. This is because the relative humidity of cabin air is only 20%, compared to 40-70% in air-conditioned buildings. But there is no evidence that flying causes dehydration – the amount of water lost is about a glass's worth.

What causes other problems?

Air pressure in the cabin has an oxygen level equivalent to flying around 5,000ft or 8,000ft. Many people live at this altitude. It is only a problem for people with heart or lung disease. The Aviation Health Unit says that air is recirculated every two to three minutes (compared with three times less often in air-conditioned buildings) and that it is filtered as effectively as air in an operating theatre. However, you can catch infections from other passengers. Some viruses may be spread by people not washing their hands. Blood clots in the legs are caused by legs being crammed into a small space (there's an increased risk in tall people) – any journey that squashes your legs will do it.

What can be done?

Drink soft drinks (not tea or coffee). Move around the cabin so the blood in your legs gets a chance to move too. Use Vaseline for your lips if they get dry. Suck sweets to avoid a dry mouth. Wash your hands during the flight.

When should I see my doctor?

If you have a swollen, painful leg or shortness of breath, or chest pain after a flight. If you think you have a chest infection or you have a fever.