Higher temperatures, greater demand for emergency medical services: a case-crossover study in a large urban system

Extreme heat is an increasing public health concern that elevates acute health risks and places additional pressure on emergency medical services (EMS). However, evidence on diagnosis-specific EMS demand during extreme heat remains limited and inconsistent, particularly for identifying vulnerable population subgroups and informing population health preparedness. This study assessed the short-term effects of extreme heat on EMS demand in Wuhan, China, and explored demographic vulnerabilities across heat-sensitive emergency conditions. We obtained 43,366 ambulance dispatch records from the citywide EMS system in Wuhan, China, between 1 January 2018 and 30 November 2019. A case-crossover study was used to assess the short-term association between extreme heat and EMS demand. Six diagnostic categories (heart disease, stroke, hypertension, syncope, dyspnea, and convulsion) were grouped into chronic-disease-related emergencies (CREs) or symptom-related emergencies (SREs). Conditional logistic regression with distributed lag nonlinear models was applied to estimate the associations between temperature and EMS demand, with stratified analyses by sex and age. Associations were reported as odds ratios (ORs) with 95% confidence intervals (CIs). Extreme heat was associated with increased EMS demand for hypertension (OR = 1.83, 95% CI: 1.33–2.52), syncope (1.55, 1.39–1.74), stroke (1.37, 1.09–1.71), and convulsion (1.33, 1.03–1.70), with a weaker positive association for dyspnea (1.26, 1.01–1.56). The estimate for heart disease (1.18, 0.96–1.46) was positive, although the association was not statistically significant. Stratified analyses suggested generally higher point estimates for CREs among women and participants aged ≥ 65 years, whereas higher estimates for SREs were more often observed among men and participants aged 15–64 years. Formal interaction tests further identified significant sex differences for syncope and convulsion and an age difference for convulsion. This study provides population-based evidence that extreme heat is associated with diagnosis-specific and subgroup-specific patterns of EMS demand. Citywide EMS dispatch data may serve as a useful health information source for heat-related population health monitoring and emergency preparedness. These findings support targeted heat-health prevention strategies, including early warning, protection of vulnerable groups, and adaptive allocation of prehospital emergency resources during extreme heat events.