medRxiv · Dec 12, 2025
Hospital Evidence on the Health Impacts of Cumulative Heat
By Ledebur K, Brugger K, Schmidt AE et al.
Abstract Heatwaves are becoming more frequent, intense, and prolonged. High temperatures raise mortality and morbidity, particularly in older adults and those with pre-existing conditions. However, it remains unclear whether increases in deaths during hot periods are accompanied by similar changes in hospital admissions. Understanding the full health burden of heat requires assessing both outcomes under a unified exposure framework. We analyzed national hospital data from Austria (2007–2019) using Poisson generalized linear models to estimate the effects of heat on daily admissions and deaths. We defined exposure as the 14-day rolling sum of heat days across various temperature thresholds. Exposure to 14 days above 30°C increases the relative risk of in-hospital death by 22 · 3% (95% CI 16 · 4-28 · 5%), with larger effects among individuals aged 75+. The affected diagnostic groups during deadly stays include respiratory diseases (129 · 7%, 95% CI 97 · 0–167 · 8), bacterial and vector-borne infectious diseases (41 · 4%, 95% CI 20 · 2–66 · 3), and endocrine, nutritional and metabolic diseases (39 · 7%, 95% CI 8 · 3–80 · 2). The effect for all-cause hospital admissions is negative at -4 · 6% (95% CI -5 · 0 -4 · 01%). However, increases are observed for specific diagnoses, including infectious bacterial and vector-borne infectious diseases (14 · 6%, 95% CI 11 · 3–18 · 1), endocrine, nutritional and metabolic diseases (13 · 3%, 95% CI 9 · 7–17 · 0),and viral, fungal, and parasitic diseases (4 · 6%, 95% CI 1 · 9–7 · 5). These findings demonstrate that heat waves are associated with higher in-hospital mortality rates across diagnostic groups, with the most vulnerable experiencing the greatest increase in admissions. These results challenge the assumption of uniform increases in hospital demand and underscore the need for targeted action plans.