WASHINGTON (Reuters) - People who travel have nearly triple the normal risk of developing a dangerous blood clot, with a measurable increase for every two hours spent sitting in a car or wedged into an airline seat, researchers reported on Monday.

They said the risk is serious enough to merit research into better ways to keep travellers healthy, although not severe enough to justify giving airline passengers anti-clotting drugs.

Dr. Divay Chandra 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.

Blood clots can also cause strokes and heart attacks when they form in arteries but VTEs can cause local damage or travel to the lung and kill.

Chandra's team did what is called a meta-analysis, pooling the results of many different studies to see what they found collectively.

They found 14 studies involving 4,000 patients that met their criteria for quality.

"Our findings demonstrate for the first time a clear association between travel and VTE," they wrote in their report, published in the Annals of Internal Medicine.

Women who are pregnant or take birth control pills and the obese have an especially high risk, they found.

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

"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.

But ensuring that people drink extra fluids and get up and move every two hours or so is worthwhile, they said.

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