WELLINGTON -- Long-distance air travelers' risk of blood clots is much higher than currently believed, according to the head of a major study into the condition, DPA reported on Tuesday.

Doctor Rodney Hughes of Green Lane Hospital in Auckland, New Zealand, said that one study indicated that up to 10 percent of high-risk travelers -- Those over 50, overweight or with health problems -- risked developing deep-vein thrombosis (DVT), or blood clots that can form in the legs during extended sitting as on long flights. Such clots can move to the lungs and cause death, Wellington's ***Dominion*** newspaper reported.

Hughes heads a six-month study of more than 1,000 volunteers flying long distances from New Zealand that is set to start this week, the paper said. The study is the largest in the world focussing on deep-vein thrombosis.

New Zealand's geographic isolation and high rate of international travel make it an ideal location to conduct the study, said Hughes.

Travelers will have a blood test for clots before they leave and on their return, followed by ultra-sound scans on the legs and possibly lungs if warranted.

Data will be gathered on the length of travelers' flights, seat positions and fluid intake.