Too hot to race? Demonstrating heat stress modelling as an adaptation tool for endurance events

Heat stress is an increasing concern as climate change drives more frequent and intense extreme heat episodes. In particular, participants in outdoor endurance events face a high risk of heat stress related health issues due to prolonged exposure to heat and physical exertion. The Wet Bulb Globe Temperature (WBGT) is a widely used ISO-standard heat stress index that integrates air temperature, humidity, wind speed, and radiation (both shortwave and longwave) to assess heat stress. Meteorological heat stress metrics such as WBGT can be used to activate heat management plans and to guide or evaluate heat mitigation strategies. To this end, we present a modelling system that combines a numerical weather prediction (NWP) model with a micro-scale radiation model to simulate meter-scale heat stress. We conducted a mobile measurement campaign to collect spatio-temporal WBGT data along the 2024 Dodentocht, a 100-km ultra-endurance hiking event in Belgium. These observations were used to evaluate the modelling system, which demonstrated a reasonable approximation of the observed WBGT along the route and was able to capture some local shade-induced cooling effects. Finally, we used the modelling system as an adaptation tool to simulate various event-management scenarios, including alternative start times, routes, and walking speeds. For the 2024 Dodentocht, starting 3 hours earlier, at 18:00 local time (UTC + 2), would have reduced the cumulative heat-stress exposure, measured as degree hours above C WBGT, by 35%. The 2023 route exhibited 16.5% more cumulative degree hours above C WBGT than the 2024 route for the official 21:00 start time under identical meteorological conditions. Our findings demonstrate the potential of high-resolution heat stress modelling systems as reliable and actionable tools to support the development and implementation of effective heat management strategies for outdoor endurance events.