Regional variations in emergency care vulnerability to summer heat

Extreme heat is intensifying under climate change, yet evidence on regional and temporal variation in heat-related morbidity remains limited. Here we analyzed over 100 million emergency department (ED) visits across five countries and territories from 2000 to 2019 using a time-stratified case-crossover design to quantify associations between summer temperature and acute healthcare demand. Here we show that higher summer temperatures are associated with increased risks of ED visits across all study regions. At the 95th percentile of local daily maximum temperature, cumulative excess odds ratios are highest in Australia (42.1%, 95% confidence interval 38.1-46.2) and Taiwan (25.7%, 16.7-35.3), followed by Brazil (18.0%, 17.6-18.4), New Zealand (16.6%, 13.5-19.7), and Canada (6.1%, 4.1–8.1). Over time, heat-related risks declined in Australia but increased in Brazil. These findings reveal substantial regional and temporal heterogeneity in vulnerability and underscore the need for locally tailored heat-health adaptation strategies under a warming climate. Heat exposure is associated with increased emergency department visits across multiple territories, with distinct vulnerability patterns by region and cause. These findings inform targeted adaptation strategies and healthcare resource allocation under increasingly severe heat events.