Urbanization’s dual edge: mitigating yet magnifying mortality risks from urban heat islands amidst global warming

Urbanization’s dual effects on temperature-related mortality are modulated by climatic backgrounds and urban characteristics. However, China lacks targeted quantitative assessments covering large-scale, diverse types of cities with different climatic backgrounds. We evaluated the temperature-related mortality due to urbanization across five major urban agglomerations of China, utilizing high-resolution regional climate and health risk assessment models. The results show that urbanization has an overall protective effect on mortality risk, with a median 7.8% (interquartile range (IQR): −10.4 to −4.0) reduction in mortality risk (12.3 fewer daily deaths), primarily due to reduced cold-related mortality. However, urbanization increases mortality risk by 9.6% (IQR: 0.5–49.1) in summer (4.0 extra daily deaths), particularly during nights with high minimum temperatures. During heatwaves, the increase in urbanization-related mortality risk was also substantial. The effects vary by climate zone and are influenced by geographic, demographic, and meteorological factors. The findings highlight the substantial impact of climatic and urban characteristics on mortality risks associated with urbanization. Quantitative assessment of these factors and tailored public health strategies for diverse contexts are therefore imperative.