"More common" is a relative term. An average death rate is 1.5 deaths per 100,000 participants. One recent study reviewed all sanctioned triathlons in 2006-08, nearly a million participants. There were 14 deaths: 13 drownings and 1 fatal bike crash. 6 out of 13 drownings occurred during short-distance (<750 m) swims. One might think that there would be more deaths in longer distance races and further down the course, but apparently not: susceptible people are most likely to have a heart attack shortly into the race.
In marathons, the odds of a heart attack is 0.5 to 1 cases per 100,000 marathons. There's a pattern: people over 45 have heart attacks because they have ischaemic heart disease; they stand a good chance of being resuscitated because there are usually volunteers with defibrillators all over marathon courses. (Of course, this isn't of much help if you have a heart attack in a triathlon during the swim portion.) People under 45 have heart attacks because they have hypertrophic cardiomyopathy (a congenital condition) and defibrillators aren't of much use for them.
To put these numbers in perspective, your odds of dying during a single triathlon are about as good as your odds of being murdered in the next 3 years if you live in an average American city.
Last edited by hamster; 06-12-13 at 11:45 AM.