Heat-related emergency medical service operations: the relevance of the heat definition

Heat is associated with an increase in Emergency Medical Service (EMS) operations. However, different heat definitions in research, including outside air temperature (T), Heat Index (HI) and Universal Thermal Climate Index (UTCI), impair the assessment of heat onthe frequency of EMS operations. Therefore, this study aims to compare (1) the relationship between different heat definitions and the frequency of days defined as heat events, (2) the percentage change in the number of EMS operations per heat definition, and (3) the goodness of model fit of each heat definition. We analyzed data from EMS operations and 106 weather stations in Bavaria, Germany (2018-2020), comparing 40 heat definitions with varying thresholds, percentiles (e.g. 90th percentile), and durations (≥ 1 or ≥ 2 days) based on T, HI, and UTCI. Negative binomial regression models were adjusted for confounders. All definitions indicate significant increased EMS operations during heat events, with effect sizes ranging from 8.3% (T_night≥20.0 °C; confidence interval: 4.2-12.5%) to 18.7% (HI_mean≥26.7 °C ≥ 2 days; confidence interval: 9.7-28.4%). Definitions with higher thresholds, percentiles, or longer durations have larger effects but worse model fits. Heat definitions based on HI or UTCI are not superior to temperature-based ones, and definitions using mean or maximum values yield similar frequencies, effects, and model fits. All heat definitions show increased EMS operations during heat events, with larger effects for higher thresholds or longer durations. Complex definitions offer no advantage over temperature-based ones, which we recommend for further research. Percentile-based definitions are preferable for better comparability.