Background Excessive heat is a leading weather-related cause of fatalities in the USA. Vulnerable populations can face greater exposure to health risks during extreme heat events. The aim of this study is to examine the effects of excessive heat and community-level social vulnerability on morbidity in San Antonio, Texas, in 2018.

Methods Heat Index (HI) data are from the National Oceanic and Atmospheric Administration. Social vulnerability is measured using the Centres for Disease Control and Prevention’s Social Vulnerability Index (SVI). Morbidity is measured as the number of emergency medical service (EMS) incidents. Sixty-one zip codes were analysed for the 153 constrained calendar days from 1 May to 30 September 2018. Negative binomial regression analysis with the time-stratified case-crossover design was conducted to predict the effects of HI and SVI on the rate of EMS incidents.

Results HI is significantly and positively associated with the rate of EMS incidents. Social vulnerability has a statistically significant association with EMS incidents, with higher levels of community-level social vulnerability associated with higher rates of EMS incidents. The effect of the HI on the rate of EMS incidents is significantly and positively moderated by the SVI.

Conclusions Social vulnerability and excessive heat increase the rate of EMS incidents. As the number of excessive heat days increases and San Antonio continues to have extreme disparities by location, there will be an effect on health systems, including EMSs.