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WEDNESDAY, Dec. 10 (HealthDayNews) -- Confirming previous research, new studies find air travel slightly increases the risk of potentially deadly blood clots.

Advanced age, oral contraceptive use and being overweight also heighten the risk of clot problems during flight.

But overall, the risk is mild, the researchers say, adding the findings do "not justify social alarm."

The findings are included in a trio of studies in this week's Archives of Internal Medicine.

In one study from the University of Milano in Italy, researchers evaluated 210 patients with venous thromboembolism, or VTE -- the formation of blood clots in the veins -- and compared them with 210 healthy individuals. During the month before the study, 15 percent of those with clots had flown, compared to 8 percent of the control subjects.

The researchers also found oral contraceptive use increased the risk of blood clot problems by 14-fold, and that those with genetic or other risk factors for VTE were 16 times more likely to get clots.

While those numbers may sound high, the researchers conclude that air travel is a "mild risk factor" for VTE.

In another study, researchers from Germany evaluated 964 passengers who had returned from flights of eight hours or more, comparing them to 1,213 subjects who didn't travel by air.

Long-haul flights doubled the risk for isolated calf muscle VTE, the researchers say. And that means an increased risk for deep vein thrombosis, or DVT. (DVT and pulmonary embolism, a blood clot in the lungs, together have been termed "economy-class syndrome" when they occur during or soon after air travel.)

In a third study, Spanish researchers reviewed cases of blood clots in the lung occurring to passengers arriving at Madrid-Baraja Airport. They found 16 cases of pulmonary embolisms over a six-year span, and all who were affected had been on flights of more than six hours.

For years, researchers have debated the link between air travel and blood clots. In 2001, the World Health Organization said the link is likely to be small and mainly affects passengers who have additional risk factors, such as a history of blood clot problems. Sitting in cramped quarters for lengthy periods -- such as a long flight -- is thought to be partly to blame.

"More research is necessary in this field," says Dr. Thomas Schwarz, of University Hospital of Dresden, Germany, who led the German study. "For passengers at risk (previous clots, elevated body mass index, older age), stretching exercises are important. Also, the intake of non-alcoholic beverages (to stay hydrated)."

"Videotapes on how to practice stretching exercises should be mandatory on all airlines on long-haul flights," he adds.

The value of other measures, such as wearing compression stockings, is unclear and unproven, he says. But Schwarz also says he sees no reason not to wear the stockings.

In an accompanying editorial in the journal, editor Dr. James Dalen advises those at increased risk of clot problems -- such as the elderly -- to avoid sitting for long periods during a flight by getting up and walking. And he cites studies that suggest below-the-knee compression stockings can help.

Dalen cites another study that found passengers at risk of clot problems who got a pre-flight dose of heparin -- used in the treatment of blood clots -- did not suffer clots during air travel.

More information

For information on blood clot problems, visit the British Heart Foundation and the National Library of Medicine.

(SOURCES: Thomas Schwarz, department of vascular medicine, University Hospital of Dresden, Germany; Dec. 8, 2003, Archives of Internal Medicine )

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