WASHINGTON - DUTCH researchers said on Monday they had finally nailed down the absolute risk of getting a blood clot while sitting in a cramped airline seat - one in 4,656.

While flying has been linked with blood clots for more than 50 years, no one had actually calculated what the true risk was - although airlines now offer in-flight exercise plans to reduce the likelihood.

Dr Frits Rosendaal of Leiden University Medical Center and colleagues surveyed 8,755 employees of international companies that collected data on their travel, including whether these travellers developed deep-vein thromboses or other types of blood clot.

They found that the risk of a thrombosis increased as people made more flights within a short time - especially long-haul flights.

People aged under 30, women who used birth control pills and people who were particularly short, tall or overweight had a special risk.

The rate of blood clots stayed high for two weeks after a flight, they reported in the Public Library of Science journal PLoS Medicine, available online at

http://medicine.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pmed.0040290

-- REUTERS