Tropospheric ozone is one of the leading causes of premature mortality linked to air pollution in Europe. A study published in Nature Medicine reveals that more than 88% of these deaths are linked to exposure to ozone from other countries, whether European or not, and only about 12% to exposure from domestic sources.
Tropospheric ozone is a harmful pollutant formed in the lower layer of the atmosphere by the interaction between sunlight and precursor gases, mainly nitrogen oxides (NOx) and volatile organic compounds (VOCs), of natural and anthropogenic origin. High levels of this ozone in ambient air are harmful to health, as they cause respiratory problems that can lead in the most serious cases to hospitalization and death. For the first time, researchers from Inserm, ISGlobal1 and the BSC-CNS2 have quantified the impact of this exposure on the scale of the European continent. Their work, published in Nature Medicine, reveals that the majority of this pollutant is imported from other countries.
To monitor concentrations of tropospheric ozone and its precursors (NOx and VOCs), the scientists used an air quality system called Caliope, developed at BSC-CNS, which tracks these pollutants formed or emitted in 35 European countries, representing coverage of a total population of around 530 million people. To obtain data outside the study area, a modelling approach was used, which made it possible to monitor the dispersion and transport of air pollutants over long distances. The analysis was based on data from the warm season, from May to September, when ozone exposure values are highest. Data from Eurostat, the statistical office of the European Union, were used to estimate mortality.
The study estimates that there were nearly 115,000 deaths attributable to ozone between 2015 and 2017, across the entire area of the 35 European countries considered, representing a mortality rate of 72 deaths per million inhabitants per year. More than 88% of these deaths are linked to exposure to ozone from other countries and only about 12% to exposure of national origin. This situation is explained by the fact that unlike its precursors such as nitrogen dioxide (NO2) which have a shorter lifespan, tropospheric ozone has the ability to persist over long distances in the free troposphere during transport. In detail, 60.2% of all observed deaths are linked to ozone outside Europe, 20.9% originate in the other European countries analyzed, while the remaining 7.2% are linked to ozone from maritime transport.
It also appears that the most industrialized and populated countries are the main contributors to mortality attributable to ozone transported across borders. Thus, ozone from France caused 4,003 deaths in neighboring countries during the hot season between 2015 and 2017. More specifically, this ozone caused 32.3% of deaths attributable to this pollutant in Luxembourg, 29.3% in Switzerland and 24.4% in Belgium. For its part, ozone from Germany caused 3,260 deaths in its European neighbors.
Pan-European and global strategies for struggle must be pursued
The study also highlights the importance of prevailing winds, particularly those coming from the west and heading towards the east, on the transport of ozone and therefore on deaths. Consequently, the countries of south-west Europe are less affected by the health effects of transboundary ozone transport, and conversely are those where mortality attributable to national ozone production is the highest. The latter represents 53.7% of deaths attributable to ozone in Spain, 47.1% in France and 46.2% in Portugal.
The results of this work highlight the need to implement coordinated pan-European and global strategies to combat air pollutants, such as tropospheric ozone. Future research should refine this study by analyzing the contribution to mortality of different sectors or economic activities (energy, industry, transport, residential, agriculture, etc.), by country, of transboundary ozone transport.
Limiting global warming is also an essential step in reducing the impact of ozone, as explained by Hicham Achebak, a researcher at Inserm and ISGlobal: “Global warming will enhance the conditions for tropospheric ozone formation in the future, as photochemical ozone formation mechanisms are enhanced during heat waves and periods of high solar radiation. Combating global warming is necessary to achieve lasting improvements in air quality. Further research will also be important to better identify the sources of pollution that contribute most to mortality, whether ozone or other pollutants that have equally harmful effects on health.”
1 ISGlobal: Institute for Global Health in Barcelona
2 BSC-CNS: Barcelona Supercomputing Center – Centro Nacional de Supercomputación
