Researchers believe they may know why so many people die during triathlons compared to other races, such as marathons. Approximately 1.5 people die out of every 100,000 who participate in a triathlon—which includes running, biking, and swimming portions, LiveScience reports.

That's two to three times the death rate for marathon participants. According to a study published Monday in BMJ Open Sport & Exercise Medicine, more than 72% of triathlon deaths looked at by researchers happened in the water during the swimming portion of the race.

And researchers believe the culprit is something called immersion pulmonary edema, or IPE. IPE occurs when a person quickly gets into water, especially cold water. That causes the body to take blood from the arms, legs, and other extremities and move it toward the core.

This changes the body's internal pressure and may force fluids from the blood into the lungs. This can cause people to experience confusion and difficulty breathing, sometimes to the point of death, according to a press release.

A number of triathletes who died for no discernible reason had significantly enlarged hearts, even to a greater degree than other athletes. Researchers say this condition—left ventricular hypertrophy—is a risk factor for IPE.

Researchers conclude future triathletes should be checked for the condition before participating in or training for a race. (A boy with cerebral palsy inspires in triathlon finish.)

This article originally appeared on Newser: Researchers Think They Know What's Killing Triathletes