Abstract
Heatwaves are increasing in frequency and intensity, yet their impacts on hospitalizations for mental and behavioural disorders remain insufficiently quantified across countries. Here we show, using a time-stratified case-crossover analysis of 2,618,307 warm-season hospitalization records from 852 locations in Brazil, Canada, Chile and New Zealand from 2000 to 2019, that sustained extreme heat was associated with increased hospitalization risk. Heatwaves were primarily defined as periods with daily mean temperature above the location-specific 97.5th percentile for at least 4 consecutive days. Under this definition, the relative risk was 1.033 (95% confidence interval, 1.007–1.059) on the same day and 1.056 (1.011–1.103) cumulatively from the same day through the next 8 days. Associations were stronger among older adults and residents of low-population-density areas. These findings indicate that prolonged extreme heat can acutely increase mental health-related hospital demand and support targeted preparedness during severe heatwaves.