Travelling on planes significantly increases the chances of catching a cold.

A study by scientists in the United States has found one in five passengers becomes ill after flying.

And the figure may be even higher for people who take long-haul flights.

It may be that just being on a plane, packed with a lot of people is the primary factor in transmission of colds

Professor John Balmes, UCSF

The scientists believe a wide variety of factors, including travelling with a large number of people, may be to blame.

Professor John Balmes and colleagues at the University of California at San Francisco (UCSF) examined 1,000 passengers.

They found that one in five passengers reported developing a cold within one week of flying. Women reported more colds than men.

Fresh air

The scientists were originally trying to identify whether the type of ventilation used on planes increased the risks of catching cold.

However, their study showed that there was little difference between passengers who travelled on planes which recycled air and those on planes with a fresh air supply.

That led scientists to suggest other factors may be to blame for high infection rates.

Professor Balmes said: "I think there's a fairly universal feeling that flying increases your risk of getting colds and other infections, but air circulation may not be the key issue.

"Filters used on planes with recirculated air are designed to filter out infectious particles and they may be doing a good job.

"It may be that just being on a plane, packed with a lot of people is the primary factor in transmission of colds, or maybe travelling is the issue - changing time zones and losing sleep have been documented to increase rates of viral infections.

"My best advice is to wash up after shaking hands and avoid touching your nose."

The study looked at passengers travelling between San Francisco and Denver in early 1999 on a two hour flight.

The findings suggest that travelling on a long haul flight may be even less healthy.

Professor Balmes said: "It could be that long flights would result in more exposure and more colds."