Synergistic health effects of temperature and air pollution: a continental-scale European study

Temperature extremes and air pollution are major environmental drivers of mortality. Although several studies have examined the joint health effects of heat and air pollution, the evidence remains largely confined to the summer season, and synergistic effects throughout the year are poorly understood. In particular, the combined effects of air pollution with cold temperatures, as well as how these interactions vary across population subgroups and over time, have received little attention. This study provides a comprehensive continental-scale assessment of the synergistic effects of temperature and air pollution on mortality across both warm and cold seasons in Europe during 2003-2019.

We analyzed daily temperature and mortality data from the EARLY-ADAPT project, covering 654 contiguous regions across 32 European countries and a population of 539 million people, combined with daily estimates of PM_2.5, PM₁₀, NO₂, and O₃ at 10 km spatial resolution. Region-specific analyses were conducted using over-dispersed Poisson regression models, followed by a multilevel random-effects meta-analysis. Joint associations were modelled using product terms between non-linear functions of temperature and linear functions of air pollutants. Relative risks and attributable numbers were estimated, with stratified analyses by sex, age group, cause of death, and time period.

Our findings provide robust evidence of substantial synergistic effects between temperature extremes and air pollution, with pronounced heterogeneity across demographic groups, causes of death, and over time. These results highlight the importance of accounting for compound climate-air pollution risks in public health surveillance. Integrating temperature and air quality information into early warning systems and climate adaptation strategies is essential to reduce preventable mortality and protect vulnerable populations in a changing climate.