Exertional Heat Stroke at the Boston Marathon: Demographics and the Environment.

Purpose: This study aimed to 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.

Methods: We analyzed participant characteristics, environmental data, and EHS medical encounters during the 2015-2019 Boston Marathon races.

Results: Among 136,161 starters, there was an incidence of 3.7 EHS cases per 10,000 starters (95% confidence interval, 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 yr and assignment to the first two start waves were. All cases occurred at races with average wet bulb globe temperatures of 17°C-20°C. There was a linear correlation between EHS incidence and greater increases in wet bulb globe temperature from start to peak (R2 = 0.7688). A majority of cases (37; 72.5%) were race finishers; nonfinishers all presented after mile 18. Most were triaged 3-4 h after starting, and all were treated with ice water immersion. Treatment times were prolonged (mean (SD), 78.1 (47.5) min; range, 15-190 min); 29.4% (15 cases) developed posttreatment hypothermia, and 35.3% (18 cases) were given intravenous fluids. Most (31 cases; 64.6%) were discharged directly, although 16 cases (33.3%) required hospital transport. There were no fatalities.

Conclusions: 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.