Microclimate Variability In A Large City Marathon: Hong Kong Marathon 2025

PURPOSE: Sporting events implement safety policies to mitigate exertional heat illness (EHI), often relying on single-location environmental monitoring near the start/finish or medical stations. However, endurance races, including mass-participation marathons, span large distances (42 km) and use staggered start times, exposing runners to varying microclimates. This study investigated environmental variability across the 2025 Hong Kong Marathon. METHODS: Portable weather stations (Pulse 4G-LTE, XM Weather, Greece) were placed every 5 km along the 10 km, half marathon, and marathon routes. Data were streamed in real-time, sampled every 5 min, and analysed descriptively across the race (05:30-13:30). Variability was assessed using the coefficient of variation (CV%, SD/mean) for inter- and intra-site measurements. RESULTS: Mean ambient temperature (TA) was 13.3 ± 1.6 °C (range 10.0-20.8 °C), relative humidity (RH) 36.9 ± 4.3% (22-50%), Wet Bulb Globe Temperature (WBGT) 9.7 ± 3.7 °C (5.7-25.9 °C), solar irradiance (SI) 113 ± 197 W/m2 (0-798 W/m2), and wind speed 0.7 ± 0.8 m/s (0-3.9). Inter-site variability (CV%), across 11 locations was TA 6%, RH 7%, WBGT 19%, SI 95%, wind speed 78%. The highest intra-site variability was at the finish: TA 13% (range 11.6-17.2 °C), WBGT 40% (6.6-25.9 °C), SI 89% (0-798 W/m2). CONCLUSIONS: Post-sunrise (circa 07:00), significant environmental variability was observed along the course. WBGT and SI fluctuations indicate single-point monitoring inadequately captures microclimate diversity. Multi-site monitoring is recommended, especially in locations of varying shade, to improve EHI risk assessment. Advances in environmental monitoring now integrate runner biometrics with real-time weather data, enhancing EHI prediction based on precise conditions along the course.