Spatial variations in wet-bulb globe temperature during the Bank of America Chicago Marathon

Prior work has recommended the use of Wet-Bulb Globe Temperature (WBGT) to inform operational management of marathons. However, there is limited and potentially conflicting results on the spatial extent at which WBGT should be measured to sufficiently capture racecourse variability. The purpose of this study is to assess how WBGT varies spatially and temporally across the Bank of America Chicago Marathon course, and if practically and statistically significant differences exist. WBGT was measured at 60-second intervals and averaged every five minutes at five different locations across the Chicago Marathon course in 2022, 2023, and 2024. Substantial temporal and spatial variability in WBGT and meteorological conditions was observed within and between marathon years. Statistically significant differences in WBGT, and other heat stress metrics, existed between the five weather stations in each of the three marathon years (W-ANOVA, p < 0.01), with maximum mean absolute differences during 07:30 − 11:00 of 2.6 °C, but with maximum differences of 9.3 °C at individual timesteps using a rolling intervals framework. Results were consistent between fixed periods of time and when considering only the periods in which runners were present at any given portion of the course. Collectively, the observed differences in WBGT and related meteorological conditions along the Chicago Marathon route were statistically and practically significant for operational management. The use of a multiple-station observational approach is warranted for most appropriately capturing racecourse variability and thus informing decisions related to participant health and safety.