These finding are clearly the tip of the iceberg

Dr Jack Ansell

Clot risk

The phenomenon is dubbed "economy class syndrome" because it is thought to be caused by inactivity and a lack of room on flights.

Studies since 1954 have noticed a link between air travel and clots.

Doctors can test for clots

The French team found that although no clots were seen for people travelling less than 1,550 miles, clots did appear in one out of every 2.5m passengers who flew between 3,100 and 4,500 miles.

And the rate increased to one in 376,000 for passengers travelling 4,500 miles to 6,215 miles and one in 210,000 for people flying over 6,215 miles.

Over the seven year study, 56 people were found to develop the serious lung clots.

But there are warnings that the problem could have been ever greater.

Because the study only examined people who were affected within an hour of their plane landing, the risks could in fact be higher.

People who suffered fatal clots while in flight and those who developed symptoms after leaving the airport were not included in the study.

Minimising risks

Researchers say that although moving around during a flight has not been shown to reduce the risk of clots passengers should still do some minor exercise such as walking or leg exercise during their journey.

They suggest avoiding alcohol during the flights, wearing loose clothing and elastic stockings and avoiding sitting with crossed legs.

In an editorial Dr Jack Ansell, of Boston University Medical Center said: "These finding are clearly the tip of the iceberg."

He said that although drugs like aspirin reduce the clot risk that "it may be premature and even dangerous" to recommend anti-clotting drugs to air passengers.