When is it too hot to run a marathon?


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Ever since the sun-baked fiasco of the 2007 Chicago Marathon, there’s been renewed discussion about how to figure out when it’s unsafe to hold a major road race. In the February issue of Medicine & Science in Sports & Exercise, the medical director of the Twin Cities Marathon, University of Minnesota med school prof William O. Roberts, has an interesting analysis of this question — and he ends up with pretty conservative recommendations.

What’s interesting about his analysis is that he takes a population-level approach: instead of sticking a few representative subjects on a treadmill, he analyzes two large data sets. First, he looks at eight unexpectedly hot races that results in either mid-race cancellations or “mass casualty events” (where the number of patients overwhelms the medical resources available in a community), and calculates the “wet bulb globe temperature” (WBGT) at start time. He finds that WBGT above about 21 C (70 F) is an indicator of serious trouble for marathons at northern latitudes with participants who haven’t acclimatized to the unexpectedly hot weather.

Then he does a more specific analysis for the Twin Cities marathon, plotting the percentage of marathon starters who were either unable to finish or required medical attention as a function of WBGT. (The graph, along with the full text of the paper, is available here.) In this case, he finds that a start WBGT above 20.5 C (69 F) is trouble.

These conclusions contrast with American College of Sports Medicine guidelines allowing starts with WBGT up to 28-30 C (82-86 F) — guidelines based on tests of young military recruits. The key problems are (a) the average recreational marathon runner is not G.I. Joe, and (b) the participants in a northern (latitude greater than 40 degrees) marathon will be less prepared for a hot day than people who live in hot places.

This probably isn’t welcome news for race directors — no matter how hot it is, few participants are going to take kindly to a decision to cancel something they’ve spent months training for. But knowledge is good — and even if a race isn’t cancelled, this is the kind of information that runners themselves should take into account when they’re facing adverse weather conditions.

(A note on WBGT: it’s a scale that takes into account the effects of humidity and solar radiation, as well as air temperature, on humans. As a very rough rule of thumb, if humidity is above about 50%, WBGT will be higher than air temperature; if humidity is below 50%, WBGT will be less than air temperature.)

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