Assess associations between exertional heat stroke (EHS) and sex, age, prior performance and environmental conditions, and report on resources needed for EHS cases at the Boston Marathon.
We analyzed participant characteristics, environmental data, and EHS medical encounters during the 2015-2019 Boston Marathon races.
Among 136,161 starters, there was an incidence of 3.7 EHS cases per 10,000 starters (95% CI, 2.8-4.9), representing 0.5% of all medical encounters. There were significant associations between sex and age (p<0.0001), sex and start wave (p<0.0001), and age group and start wave (p<0.0001). Sex was not significantly associated with increased EHS incidence, however, age younger than 30 years and assignment to the first two start waves were. All cases occurred at races with average wet bulb globe temperatures (WBGT) of 17°C-20°C. There was a linear correlation between EHS incidence and greater increases in WBGT from start to peak (R2 = 0.7688). The majority of cases (37, 72.5%) were race finishers; non-finishers all presented after mile 18. Most were triaged 3-4 hours after starting, and all were treated with ice water immersion. Treatment times were prolonged (mean 78.1 minutes (SD 47.5 minutes, range 15-190 minutes)), 29.4% (15 cases) developed post-treatment hypothermia and 35.3% (18 cases) were given intravenous fluids. Most (31 cases, 64.6%) were discharged directly, though 16 cases (33.3%) required hospital transport. There were no fatalities.
Younger and faster runners are at higher risk for EHS at the Boston Marathon. Greater increases in heat stress from start to peak during a marathon may exacerbate risk. EHS encounters comprise a small percentage of race-day medical encounters but require extensive resources and warrant risk mitigation efforts.

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