Researchers on Monday reported that traveling long distances by plane, bus, car or train nearly triple the risk of developing blood clots in leg veins and lungs.

The research published online today in the Annals of Internal Medicine said for every two-hour increase in travel time, the risk of developing a clot, known as venous thromboembolism, increased 18 %. Research showed in the case of air travelers, the risk rose 26 % for every two-hour increase in time spent on the plane.

The researchers wrote, "Worldwide, 2.5 billion passengers will travel by air alone in 2010, which underscores the large global population at risk for this serious condition."

The review was the largest and most comprehensive of its kind and 14 previous studies were analyzed including 4,055 cases. Under the review lead author Dr. Divay Chandra a fellow in the division of pulmonary, allergy and critical care medicine at the University of Pittsburgh Medical Center, and colleagues at Harvard University in Boston looked specifically at venous thromboembolism, the development of a blood clot in a vein, usually in the legs.

The researchers looked at the rate of blood clots in travelers and compared them to those of non-travelers. They reported the risk of developing a blood clot tripled for travelers and the risk was higher for those who traveled by plane compared with those who took ground transportation, but the increase wasn't statistically significant.

They said the absolute risk is one case in every 4,600 airline trips and noted that some studies have shown no risk of blood clots but said the way those studies were done could be questioned.

"Our findings demonstrate for the first time a clear association between travel and VTE," the researchers wrote in their report.

The exact reason why traveling may cause blood clots is being examined by experts but some suspect that lack of mobility may discourage blood flow and enable clots to form.

"People who travel, particularly if they travel long distances, they should be aware of these symptoms," said Chandra. "They should not be ignored."

The researchers reported that women who are pregnant or take birth control pills and the obese have an especially high risk.

"The findings of this report suggest that, at least among generally healthy individuals, even a three-fold increase in relative risk is unlikely to produce a sufficiently high absolute risk to justify higher-risk interventions, such as oral anticoagulation during travel," they wrote.

Chandra said all travelers should drink water and try to walk occasionally to try to prevent the clots. In the case of high risk people for blood clots, should talk to doctors and may need to take blood thinners before making a long trip, Chandra said.