A major new medical study will examine the causes of deep vein thrombosis, and whether cabin class really matters.

After years of conflicting and inconclusive findings by leading researchers, the World Health Organization (WHO) has launched the largest-ever study of the connection between deep vein thrombosisand air travel. DVT, or the formation of blood clots in the legs, is a serious medical condition thatleads to diminished circulation. Cases of passengers developing DVT following long-haul flights havebeen documented since the 1950's—earning DVT the nickname "economy-class syndrome"—but doctors are only beginning to understand the link between air travel and blood clots.

The Geneva-based WHO plans to track a large sample of airline passengers and explore such factors asimmobility, dehydration, medical history, length of flight, and seat location. It will also investigatewhether cabin pressure and reduced oxygen intake play a role.

Symptoms, which occur in only half of DVT cases, include swelling, localized pain, tenderness, andtightness and heaviness in the legs. When DVT is caught early—as it was with former vice president Dan Quayle, who developed the condition in 1994 after extensive flying—it's treatable with anti-coagulants. But a clot that dislodges and travels to the lungs (a pulmonary embolism) can cause shortness of breath and coughing up of blood; DVT can also be fatal if a clot moves to the lungs or the brain.

In October 2000, Emma Christofferson, a healthy 28-year-old woman, collapsed and died after a 20-hourflight in coach from Australia to London, bringing worldwide attention to the possibility offlight-induced DVT. Since then, several studies have suggested that DVT can strike first-classpassengers too: in a 2001 study by Dr. Noritake Hata of Japan's Nippon Medical School Chiba-Hokusohhospital, 5 of the 12 passengers who developed DVT after long-distance flights into Tokyo's NaritaAirport had been seated in the more spacious business-class cabin. In an analysis of 67 British caseslast year, the U.K.-based Aviation Health Institute found that 12.5 percent occurred in business-classtravelers.

But your flight doesn't have to be 20 hours long to put you at risk. A 2001 study of air travelerspublished in the New England Journal of Medicine found that the incidence of DVT resulting inpulmonary embolism was "markedly higher" among passengers who'd flown for as little as six hours. Of the 56 passengers who developed a pulmonary embolism, 42 had remained in their seats throughout theflight.

Airlines are eager to hear WHO's conclusions, as the establishment of a link between DVT and airtravel could mean more lawsuits. In December, Britain's High Court ruled in a class-action suit thatpassengers who develop blood clots cannot sue airlines, but an Australian court ruled against Qantasand British Airways, stating that DVT could be classified as an accident. Many carriers have begunproviding more information to passengers on their Web sites and on in-flight tapes; they emphasize thatDVT is also associated with car and bus travel. "The term traveler's thrombosis is more accurate," British Airways notes. American Airlines informs passengers that they could develop symptoms of DVT "while seated at home or in the office, or when watching a movie." That's true in theory, but there is less opportunity to get up and walk around on a plane.

Preliminary findings from the WHO study will be released in two or three years. In the meantime, Dr.Bruce Evatt, chief of the Centers for Disease Control's Hematologic Diseases Branch, has this advice:"Make sure you wiggle those toes and get up and move around every hour—regardless of where your seat is."

How to Safeguard Against DVT

Limit caffeine and alcohol intake before and during your flight, and drink plenty of water. • Leave as much room as possible for your legs under the seat in front of you. • During the flight, walk around the cabin whenever you can. Compression kneesocks may help prevent swelling in feet and lower legs. • Check with your flight attendant for exercise tips (British Airways shows Pilates videos). • Consult your doctor before flying if you are pregnant or have recently given birth; use birth control or other hormone therapies; or have had a stroke or recent surgery. For more information, log on towww.spotlighthealth.com.