Extreme Heat and Ambulance Dispatches in Chicago

Background: Global climate change is contributing to increases in extreme weather events, including extreme heat days and heatwaves. Heat-related morbidity and mortality is expected to increase by 2050. However, the specific impact of extreme heat events on public health and healthcare systems is unknown. Objectives: To examine the association between extreme heat days and Emergency Medical Service (EMS) dispatches in Chicago; to characterize the association between temporal and weather factors and the EMS dispatch count; and to develop a predictive model that can estimate the number of dispatches in relation to weather patterns. We analyzed predictive model functionality within geographic regions of Chicago. Methods: We analyzed National Oceanic and Atmospheric Administration local climatological data and Chicago Fire Department EMS dispatch data for the periods May to September 2014-2018. The humidity index (humidex) was used as a measure of the perceived combined effect of heat and humidity. We used exploratory time series analysis to examine the relationship of time with the daily humidex and dispatches per day. We applied multiple linear regression to create a predictive model for the number of dispatches per day. The final model was applied to regional dispatch data to assess heterogeneity between geographic regions. Results: Humidex was significantly associated with the number of dispatches per day (β=3.15, p<0.0001), accounting for 33% of variability in number of dispatches. In addition to humidex, time factors (month, month2, year), dispatch count on prior days (1, 6, and 7 days prior), and percentage of daily dispatches from the critical age group (under 5 and over 65 years) are all significant predictors. Finally, the humidex’s effect on the number of dispatches per day was 81% greater on Chicago’s West Side than on the Far North Side, and was not accounted for by population size differences. Conclusions: Overall, a 10°F humidex increase was associated with 32 more dispatches per day. The final model predicts the true number of dispatches per day well (R2 = 0.52, root MSE = 61.37, mean dispatches per day = 1079.6). Sub-analyses revealed different associations by geographic region between humidex and number of dispatches.