Geographic sources of ozone air pollution and mortality burden in Europe.

Ground-level ozone (O 3 ) is a harmful air pollutant formed in the atmosphere by the interaction between sunlight and precursor gases. Exposure to current O 3 levels in Europe is a major source of premature mortality from air pollution. However, mitigation actions have been mainly designed and implemented at the national and regional scales, lacking a comprehensive assessment of the geographic sources of O 3 pollution and its associated health impacts. Here we quantify both national and imported contributions to O 3 and their related mortality burden across 813 contiguous regions in 35 European countries, representing about 530 million people. Imported O 3 contributed to 88.3% of all O 3 -attributable deaths (intercountry range 83-100%). The greatest share of imported O 3 had its origins outside the study domain (that is, hemispheric sources), which was responsible for 56.7% of total O 3 -attributable mortality (range 42.5-87.2%). It was concluded that achieving the air-quality guidelines set out by the World Health Organization and avoiding the health impacts of O 3 require not only the implementation of national or coordinated pan-European actions but also global strategies.