Emergency Department Presentations During Dry and Humid Heatwaves: A Case‐Crossover Study in the Northern Territory, Australia

Abstract In a rapidly warming climate, heatwaves pose an increasing threat to human health. However, there is limited knowledge of heatwave impacts on health outcomes, or the role of humidity in the tropical and arid climates of Australia's Northern Territory (NT). Using a space‐time‐stratified case‐crossover design and conditional Poisson regression models, we analysed the association between heatwaves and emergency department (ED) presentations from 2001 to 2023, across all six NT public hospitals. Heatwaves were identified using the Excess Heat Factor (EHF) method, with both temperature‐only (heatwavesT) and temperature‐plus‐humidity, heat‐index (heatwavesTH) metrics. We undertook sub‐group analyses by sociodemographic characteristics and principal diagnosis. All‐cause ED presentations increased by 4. 4% (RR = 1. 044, 95%CI 1. 018–1. 071) for severe/extreme and 1. 6% (RR = 1. 016, 95%CI 1. 002–1. 030) for low‐intensity heatwavesT. For heatwavesTH, presentations increased by 6. 1% (RR = 1. 061, 95%CI 1. 025–1. 098, severe/extreme) and 0. 9% (RR = 1. 009, 95%CI 0. 995–1. 024, low‐intensity). Subpopulation increases for severe/extreme heatwavesT occurred for ages 19–49 years (RR = 1. 052; 95%CI 1. 018–1. 087), visitors (RR = 1. 162, 95%CI 1. 038–1. 301) and skin conditions (RR = 1. 116, 95%CI 1. 048–1. 189). Specific to severe/extreme heatwavesTH, presentations increased for Aboriginal peoples (RR = 1. 059, 95%CI 1. 006–1. 114), ages 50–64 years (RR = 1. 141, 95%CI 1. 059–1. 230) and cardiovascular conditions (RR = 1. 111, 95%CI 1. 015–1. 216). Comparing heatwave indexes, 57. 1% of heatwaveT days were not captured by heatwaveTH, and conversely 49. 6% of heatwavesTH days were not captured by heatwaveT. These findings call for dual heatwave warning systems in the NT, incorporating both EHF temperature and heat‐index, and further humidity‐inclusive studies in varied climates. Preventative interventions should target high‐risk populations, prioritizing resources for severe and extreme heatwaves.