WEDNESDAY, Dec. 3, 2014 (HealthDay News) -- Longer surgery times may increase a patient's risk of dangerous blood clots, a new study suggests.

Blood clots are associated with more than 500,000 hospitalizations and 100,000 deaths a year, according to background information in the study.

Researchers analyzed data from more than 1.4 million people who had surgery under general anesthesia at 315 U.S. hospitals between 2005 and 2011.

Of those patients, 0.96 percent developed a blood clot after surgery, 0.71 percent developed a deep vein thrombosis (a blood clot in the veins deep in a limb, usually the legs), and 0.33 percent developed a pulmonary embolism, a blood clot that travels to the lungs.

Patients who had the longest operations were 1.27 times more likely to develop a blood clot than those who had average-length surgeries. The risk was lowest among patients who had the shortest surgeries, the researchers reported online Dec. 3 in the journal JAMA Surgery.

It's widely believed that there is a link between longer surgery and increased risk of blood clots and death, but there has been no actual proof of a connection, the researchers noted.

"Given the observational design of our study, it is not possible to definitively conclude that the observed relationship between surgical duration and [blood clot] incidence reflects a strict cause-and-effect relationship," Dr. John Kim, of the Northwestern University Feinberg School of Medicine, and colleagues wrote.

Still, these findings lend support to the belief that longer operations increase the risk of blood clots and could help doctors make more informed medical and surgical decisions to reduce that risk, they concluded.