A spatiotemporal Bayesian hierarchical model of heat-related mortality in Catalonia, Spain (2012--2022): The role of environmental and socioeconomic modifiers

Background: Extreme heat is a major public health risk, yet its relationship with mortality may be confounded or modified by air pollution and social determinants. Objectives: We aimed to quantify the effects of extreme maximum temperatures and heatwaves on daily mortality in Catalonia (2012--2022), and to assess the modifying and confounding roles of air pollutants and socioeconomic factors. Methods: We conducted a time--series ecological study across 379 basic health areas (ABS) during summer months. Mortality data from the Spanish National Statistics Institute were linked with meteorological and air pollution data. A hierarchical Bayesian spatiotemporal model, incorporating structured and unstructured random effects, was used to account for spatial and temporal dependencies, as well as observed socioeconomic confounders. Results: In total, 730,634 deaths occurred, with 216,989 in summer. Extreme heat alone was not independently associated with mortality, as its effect was fully confounded by high ozone levels and partly by socioeconomic indicators. Ozone concentrations ($\ge 120 μg/m^3$) significantly increased mortality risk, especially among individuals aged $\ge 85$ years. Greater income inequality and higher proportions of older residents also amplified vulnerability. Conclusion: Mortality risks from extreme heat in Catalonia were strongly influenced by ozone levels and social determinants. Adaptation strategies should address both compound environmental exposures together with socioeconomic vulnerability to better protect older and disadvantaged populations.