A City Under Heat: Assessing urban heat island impacts on mortality risk from non-communicable diseases in metropolitan zones of Mexico, 2003-2019

Urban heat island (UHI) has been recognized as a serious health threat in urban cities worldwide, which can contribute to thermal discomfort, and a range of morbidity and mortality outcomes. Using mortality records for five International Classification of Diseases chapters (4, 5, 6, 9, and 10) from 2003–2019 across metropolitan regions in Mexico, and summer and winter daytime/nighttime UHI intensity, we assessed the association between UHI intensity and non-communicable disease mortality, stratified by climatic zone, sex, and age group. Results show that UHI intensity varied across climatic contexts, reaching up to 6 °C in temperate regions, where the strongest associations with mortality were also detected. Nighttime UHI during summer emerged as a critical driver of risk, with significant associations observed even at 1 °C increments. Vulnerability extended beyond the elderly, with consistent associations among children, adolescents, and women of reproductive age, highlighting overlooked groups at risk. These findings reveal that UHI effects are heterogeneous across seasons, regions, and demographics, and underscore the urgency of integrating UHI mitigation into climate adaptation and public health strategies to prevent the amplification of health inequities under future warming.