Invisible Heat, Visible Mortality: Quantifying the Delayed Cardiometabolic Impact of Urban Heat Waves in Southern Europe

Nikos Papadopoulos¹, Eleni Kosta², Andreas Georgiou³

Abstract

Background: Extreme heat events are increasing in frequency and intensity across Southern Europe. While acute heat-related mortality is recognized, delayed cardiometabolic consequences remain underexplored. This study quantified short- and medium-term mortality and hospitalization risks associated with urban heat waves.

Methods: We conducted a population-based time-series study across five major Greek cities between 2010 and 2024. Heat waves were defined as ≥3 consecutive days above the 95th percentile of historical temperature distribution. Distributed lag non-linear models assessed associations between temperature exposure and cardiometabolic mortality and hospital admissions over 21 lag days. Models adjusted for air pollution, humidity, seasonal trends, and long-term temporal variation.

Results: A total of 14,732 cardiometabolic deaths were recorded during the study period. Heat waves were associated with a 17.8 percent increase in cardiometabolic mortality (relative risk 1.18; 95 percent confidence interval 1.12 to 1.24; p < 0.001) and a 12.4 percent increase in hospital admissions (relative risk 1.12; 95 percent confidence interval 1.07 to 1.18; p < 0.001). Peak risk occurred at lag days 2 to 4, but elevated risk persisted up to 14 days. Individuals aged ≥75 years exhibited the highest vulnerability (relative risk 1.29; 95 percent confidence interval 1.19 to 1.41). Incorporating heat exposure metrics improved predictive accuracy of mortality models (AUC increase from 0.74 to 0.83).

Conclusion: Urban heat waves exert substantial delayed cardiometabolic impacts beyond immediate exposure periods. These findings underscore the urgency of adaptive public health strategies, early warning systems, and targeted protection of vulnerable populations in the context of accelerating climate change.

Background: Extreme heat events are increasing in frequency and intensity across Southern Europe. While acute heat-related mortality is recognized, delayed cardiometabolic consequences remain underexplored. This study quantified short- and medium-term mortality and hospitalization risks associated with urban heat waves.

Methods: We conducted a population-based time-series study across five major Greek cities between 2010 and 2024. Heat waves were defined as ≥3 consecutive days above the 95th percentile of historical temperature distribution. Distributed lag non-linear models assessed associations between temperature exposure and cardiometabolic mortality and hospital admissions over 21 lag days. Models adjusted for air pollution, humidity, seasonal trends, and long-term temporal variation.

Results: A total of 14,732 cardiometabolic deaths were recorded during the study period. Heat waves were associated with a 17.8 percent increase in cardiometabolic mortality (relative risk 1.18; 95 percent confidence interval 1.12 to 1.24; p < 0.001) and a 12.4 percent increase in hospital admissions (relative risk 1.12; 95 percent confidence interval 1.07 to 1.18; p < 0.001). Peak risk occurred at lag days 2 to 4, but elevated risk persisted up to 14 days. Individuals aged ≥75 years exhibited the highest vulnerability (relative risk 1.29; 95 percent confidence interval 1.19 to 1.41). Incorporating heat exposure metrics improved predictive accuracy of mortality models (AUC increase from 0.74 to 0.83).

Conclusion: Urban heat waves exert substantial delayed cardiometabolic impacts beyond immediate exposure periods. These findings underscore the urgency of adaptive public health strategies, early warning systems, and targeted protection of vulnerable populations in the context of accelerating climate change.