However they caution that in obese individuals and in women who use oral contraceptives, the risk of DVT after long-haul flights may indeed be high. The link between air travel and venous thrombosis has existed for some time, but it has never been comprehensively studied until now.

According to the National Institutes of Health, deep venous thrombosis is a condition in which a blood clot forms in a vein that is deep inside the body, mainly in the lower legs and thighs. The problem with DVT is that this clot may break off and travel via the blood to vital organs like the brain, lungs and the heart, causing irreparable damage to that organ.

The risk factors for developing DVT include prolonged immobility as in a flight,  fractures, childbirth within the last 6 months, and the use of medications such as estrogen and birth control pills, the NIH adds.

However the new study by Frits Rosendaal of Leiden University Medical Center and colleagues finds that the risk of DVT posed by long-haul flights is not as much as previously thought. The study involved 8,800 people who worked for international companies and hence had to travel frequently.

The researchers followed the travel diaries of these employees between 1 January 2000 and 31 December 2005 and documented the incidence of thromboses that occurred during the period.

They report in the latest issue of PLoS Medicine that:
* 53 cases of DVT occurred during the study period.
* 22 of these occurred within eight weeks of a long-haul flight
* Using this data, the researchers calculated that the risk of DVT was one event every 4,656 long-haul flights.
* 3.2 per 1,000 people exposed to long haul flights were at risk of developing DVT as compared to 1.0 per 1,000 for those who did not travel as frequently.
* The risk of DVT was highest in the first two weeks after travel

"People who make several flights in a short time frame, people who make very long flights, women who use oral contraceptives, people who are overweight and people who are either short or very tall are at increased risk," said lead researcher Dr. Frits R. Rosendaal, of the department of clinical epidemiology and hematology at Leiden University Medical Center.

The researchers added that there was no reason why people traveling on long-haul flights should take anti-clotting medications like aspirin. "The only thing that makes sense is to move your feet," Rosendaal said.

Some of the signs of DVT, according to the NIH, include:
    * Leg pain in one leg
    * Leg tenderness in one leg
    * Swelling (edema) of one leg
    * Increased warmth in one leg
    * Changes in skin color (redness) in one leg

The best way to avoid DVT is to prevent it. Some proposed measures include losing weight, stopping smoking and moving your legs often during long plane trips.