A study led by the Minneapolis Heart Institute’s Dr. Kevin Harris has found that triathlons at least double the risk of sudden death when compared with marathons, due largely to the swimming portion of the event.

Statistics show that 15 out of 1 million athletes participating in swimming/biking/running events will die as a result of the competition. Nearly all those deaths occur during the swim portion, which often is the first leg of the race, because of the shock of cold water to the heart and the inability to stop and rest during that time. For the study, researchers collected information on 922,810 triathletes between January 2006 and September 2008.

The results were presented this spring at an American College of Cardiology conference.