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Background: Extreme heat is increasing and is associated with higher mortality. Comprehensive assessments of the health burden of heat should, however, also consider morbidity outcomes.

Aim/scientific question: What is the association between heat and hospitalizations due to cardiovascular, cerebrovascular, and respiratory diseases in warm-temperate climates representative of Germany and Austria?

Material & Methods: We conducted a systematic literature review (PROSPERO CRD420251015695) in accordance with PRISMA, PRESS, and SWiM guidance. We searched Medline and Embase (Ovid) until January 2025. Two authors independently screened studies against a-priori inclusion criteria and extracted data; disagreements were resolved by a third author. We included observational studies from warm-temperate climate zones (Köppen–Geiger classification) on the association between heat and hospitalizations for cardiovascular, cerebrovascular, or respiratory diseases (ICD-10: I00–I99, J00–J99), with adjustment for seasonality, long-term trends, and lagged effects. We performed a structured narrative synthesis with vote counting by direction of effect. A meta-analysis was planned a-priori.

Results: Forty-one studies from ten countries met the inclusion criteria; most were time-series analyses (n = 38). A meta-analysis was not conducted due to substantial methodological and effect heterogeneity (Cochran’s Q; I2>80%). Heat-related increases in hospitalization risk relative to a reference temperature were reported in 7/11 studies for cardiovascular (I00–I99), in 5/5 studies for cerebrovascular (I60–I69), and in 5/7 studies for respiratory diseases (J00–J99), respectively. Age-stratified analyses showed similar patterns with additional elevated risk among older adults. All six pediatric asthma studies (J43–J46) reported increased heat-related hospitalization risk.

Discussion: Despite considerable methodological heterogeneity, the evidence suggests that heat is associated with increased hospitalizations for cardiovascular and respiratory diseases, particularly with consistent signals for cerebrovascular diseases, asthma, and COPD. Older adults and children with asthma appear most at risk; sex-specific findings are inconsistent.

Take Home Message: These findings suggest that decision-makers in Germany and Austria should incorporate hospitalizations into comprehensive heat-related health burden assessments and prioritize the development of heat adaptation measures for high-risk groups.

Competing interests: The authors declare no conflicts of interest. This work was partially funded by the European Union's Horizon Europe research and innovation program under Grant Agreement No. 101057131, Climate Action To Advance HeaLthY Societies in Europe (CATALYSE).