The Heat That Kills Quietly: Urban Heat Islands and the Hidden Rise of Cardiovascular Mortality

Léa Fontaine¹, Louis Martin²

Abstract

Abstract

Urban heat islands (UHI) intensify temperature exposure in densely built environments, amplifying health risks during extreme heat events. This population-based environmental epidemiology study assessed the relationship between UHI exposure and cardiovascular mortality across 12 metropolitan regions between 2020 and 2024. A total of 148,320 deaths were analyzed alongside high-resolution temperature mapping and land-surface heat indices.

During heatwave periods, mortality increased by 13.9% (95% CI 11.8–16.2; p<0.001) in high-UHI zones compared with low-exposure areas. Night-time temperature elevation was a critical factor, with each +2°C increase associated with a 7.4% rise in cardiovascular deaths. Individuals aged ≥70 years showed the highest vulnerability (RR 1.21), while socioeconomically deprived neighborhoods experienced 1.8-fold higher mortality risk.

Hospital admissions for cardiac events increased by 19.6%, and stroke-related mortality rose by 14.2% during peak heat exposure. Urban areas with limited green space (<15% coverage) demonstrated significantly higher risk compared with greener districts. A predictive heat-health model combining temperature, humidity, and land-use variables achieved AUC 0.85, identifying high-risk mortality days with strong accuracy.

These findings highlight urban heat islands as a major environmental determinant of cardiovascular mortality. Expanding green infrastructure, improving urban design, and strengthening early warning systems are critical strategies for reducing heat-related health risks in cities.