Hospital Evidence on the Health Impacts of Cumulative Heat

BackgroundHeatwaves are becoming more frequent, intense, and prolonged. High temperat-ures 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 sim-ilar changes in hospital admissions. Understanding the full health burden of heat requires assessing both outcomes under a unified exposure framework. MethodsWe 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. FindingsExposure to 14 days above 30{degrees}C increases the relative risk of in-hospital death by 17{middle dot}3% (95% CI 14{middle dot}8-19{middle dot}9%), with larger effects among individuals aged 75+. Among this group, the affected diagnostic groups during deadly stays include mental health disorders (111{middle dot}1%, 95% CI 44{middle dot}4-172{middle dot}3), nervous system disorders (37{middle dot}2%, 95% CI 9{middle dot}9-71{middle dot}2), and respiratory diseases (66{middle dot}4%, 95% CI 44{middle dot}4-91{middle dot}7). The effect for hospital admissions is modest at 1{middle dot}1% (95% CI 0{middle dot}5-1{middle dot}7%) but among individuals aged 75 years or older, increases are pronounced for specific diagnoses, including infectious diseases (16{middle dot}5%, 95% CI 24{middle dot}9-105{middle dot}3), endocrine disorders (66{middle dot}8%, 95% CI 49{middle dot}9-85{middle dot}7), and genitourinary diseases (8{middle dot}2%, 95% CI 4{middle dot}4-12{middle dot}2). InterpretationThese findings show that heatwaves lead to higher in-hospital mortality across diagnostic groups, with the increase in admissions being among the most vulnerable. This challenges the assumption of uniform increases in hospital demand and highlights the importance of targeted action plans. FundingThe project was funded from the Austrian Federal Ministry for Innovation, Mobility, and Infrastructure (BMIMI) through the program "Digital Solutions for People and Society," which is managed by the Austrian Research Promotion Agency (FFG). Research in ContextO_ST_ABSEvidence before this study.C_ST_ABSHigh ambient temperatures and extreme heat events have been linked to increased mortality and morbidity. Studies have reported an increase in deaths during heatwaves, particularly from cardiovascular and respiratory causes. There is some evidence linking heat to mental health crises, adverse pregnancy outcomes, and infectious diseases. There are fewer morbidity studies, which are often limited to specific diagnoses or urban centers and rarely assess hospital admissions and in-hospital deaths together. We searched PubMed for English-language studies published up to March 2025 using the terms "heatwaves,#x201D; "heat exposure,#x201D; "mortality,#x201D; "morbidity,#x201D; and "hospital admissions.#x201D; Additional relevant publications were identified through reference lists and other sources. We identified several multicity studies and one recent national analysis from Portugal. However, we found no nation-wide study that assessed morbidity and mortality across all major disease groups using a unified exposure framework. Added value of this study.Our study provides one of the most comprehensive assessments to date of the effects of cumulative heat exposure on in-hospital mortality and hospital admissions. By applying a unified exposure definition to all outcomes, demographic groups, and ICD-10 chapters, we can compare mortality and morbidity risks within the same framework. Our cumulative exposure metric considers intensity, duration, and lagged effects, which distinguishes it from the single-day thresholds used in most heat-health warning systems. This approach reveals sharp, consistent mortality risks contrasted with more selective morbidity effects and quantifies absolute excess cases to inform hospital capacity planning. Implications of all the available evidenceThe available evidence shows that heatwaves increase the risk of death, but the impact on morbidity is inconsistent and diagnosis-specific. Our findings demon-strate that the number of heat-related deaths increases more sharply and consistently than the number of hospital admissions, which suggests that many patients either die before reaching the hospital or deteri-orate rapidly once admitted. At the same time, we identified clear morbidity burdens in older adults with infectious, endocrine, mental, and genitourinary conditions. These findings indicate that health systems cannot rely on mortality data alone to prepare for heatwaves, but must also anticipate selective surges in admissions. Furthermore, warning systems should integrate cumulative exposure metrics. Together, this evidence provides a foundation for designing adaptation strategies that protect vulnerable groups and plan hospital capacity in a warming climate.