July 6 (HealthDay News) -- A review of past research confirms what many experts have long believed: People who travel bear a significantly higher risk for developing potentially life-threatening blood clots in their legs.

That risk, the analysis finds, is up to three times higher for travelers when compared to non-travelers. What's more, the risk rises the longer the trip -- 18 percent higher risk for every two hours of travel, and 26 percent higher risk for every two hours of airplane travel.

"For the first time, we've identified conclusively that long distance travel is associated with a higher risk of getting a deep vein clot, and that the risk is a sizeable," said study co-author Dr. Dariush Mozaffarian who works in the division of cardiology at Harvard Medical School in Boston. "Previously, this had been assumed to be a plausible risk. But, pooling all of the worldwide data together, we were able to identify this risk clearly."

Mozaffarian and his colleagues report their findings in the July 7 issue of the Annals of Internal Medicine.

Known as deep vein thrombosis (DVT) or venous thromboembolism (VTE), this serious condition can sometimes lead to a pulmonary embolism, where a clot that forms in the leg migrates to the lungs and can cause death.

Given that by 2010 an estimated 2.5 billion passengers across the globe will be traveling by air alone, the researchers noted that any connection between DVT risk and travel could have an impact on a wide swath of the population.

So, to definitively nail down the connection between travel and DVT, the review authors conducted an in-depth analysis of 14 previous travel-related DVT studies, chosen from more than 1,500 studies.

In total, the chosen studies covered 4,055 cases of DVT, in which patients had traveled by a variety of transportation modes. All the studies had also included non-traveling patients for comparative purposes.

The research team observed that half the studies had concluded that there was an association between DVT and travel, while the other half had found no significant correlation.

But when they conducted their own statistical breakdown, Mozaffarian and his associates concluded that, taken together, the studies indicated that travelers actually bear "a substantially higher risk" -- two to nearly three times higher -- for developing DVT than non-travelers.

"What is key is if you're traveling and you begin to feel unwell when you arrive, or even over the following week or two, and you have some symptoms for DVT is to go get evaluated right away," Mozaffarian noted.

According to the National Heart, Lung, and Blood Institute, DVT symptoms include swelling of the leg or along a leg vein; leg tenderness and/or pain (sometimes only when standing or moving); abnormal warmth in an affected leg, and leg redness or discoloration. Signs of a pulmonary embolism include shortness of breath or pain when breathing, as well as the coughing up of blood.

Meanwhile, Mozaffarian said more research is needed to develop reliable ways to prevent the onset of problems in the first place.

"We actually don't have strong evidence for any preventive treatment," he noted. "There is some plausible reasoning that would suggest that walking around during long travel and hydrating might reduce this risk, but it's not confirmed. It seems reasonable to do those things, but we can't really say they are effective."

Nevertheless, Dr. Samuel Z. Goldhaber, director of the Venous Thromboembolism Research Group in the cardiovascular division of Brigham and Women's Hospital in Boston, said he puts stock in specific short-term and long-term steps any traveler can take to reduce risk.

"People need to know that much of the travel-related DVT can be prevented with common sense and a heart-healthy lifestyle," he said. "First of all, when traveling obviously stay well-hydrated. That means on a plane, after you pass security, you should purchase extra water and drink enough of it on the flight so that you have to go to the bathroom to urinate. And if you're traveling by car you should probably take a break every two hours, rather than trying to set a world record in terms of how fast you can get from point A to point B."

"Beyond that, it's also a fact that in general many of the things that increase pulmonary embolism and deep vein thrombosis risk are exactly the same as those that increase risk for a heart attack," Goldhaber added. "Things such as cigarette smoking, poorly controlled high blood pressure, high cholesterol, diabetes, being overweight, being immobile and not getting enough exercise. So, if you modify these risk factors you can kill two birds with one stone."

SOURCES: Dariush Mozaffarian, M.D., Dr.PH, division of cardiology, Harvard Medical School, Boston; Samuel Z. Goldhaber, M.D., professor, medicine, Harvard Medical School, Boston, and director, Venous Thromboembolism Research Group, cardiovascular division, Brigham and Women's Hospital, Boston; July 7, 2009, Annals of Internal Medicine