The data comes from a variety of sources – the US National Registry of Sudden Death in Athletes, as well as news sources and other records. That said, there may be some risk of undercounting here.

By and large, we’re not talking about ironman competitions here. Nearly 50% of the deaths occurred in short-distance triathlons. And the majority of deaths, 67%, occurred in the swimming portion of the race – which is usually the first part of the triathlon. Most of those who survived cardiac arrest had a cardiac arrest during the land-based segments of the races. The implication is clear – the ability to get rapid medical treatment to stricken individuals may be critical, making swimming, and particularly open water swimming, a unique danger in triathlon not seen in other endurance sports.

Overall, the authors calculate a death rate of 1.74 per 100,000 triathlon competitors. That may seem like a vanishingly small number, but realize that the rate of sudden death in otherwise healthy athletes is 0.5 per 100,000 individuals per year. Put another way, running a triathlon elevates the risk of sudden death by a factor of more than 1000.

I don't mean to be alarmist – overall the risks here are really small. Triathlon is a relatively safe activity. But the risks cluster in certain individuals, as you can see from this graph.