Cross-border ozone pollution significantly increases mortality rates in Europe

In a recent study published in the journal Nature Medicine, researchers identified the geographic sources of ozone air pollution and estimated ozone-related mortality rates in Europe.

Ground-level ozone is formed in the troposphere by the interaction of sunlight and greenhouse gases (GHGs) emitted from both natural and anthropogenic sources, especially nitrogen oxides and volatile organic compounds.

Ground-level ozone is a very harmful air pollutant. It is associated with many respiratory complications, including asthma, chronic obstructive pulmonary disease, and lung infections. Moreover, excessive ozone exposure is the leading cause of air pollution-related hospitalizations and premature deaths worldwide.

According to the European Environment Agency, more than 95% of the European population is exposed to ozone levels that exceed air quality guidelines set by the World Health Organization (WHO).

Tropospheric transport of ozone and its precursors from distant sources, otherwise known as imported ozone, is the main determinant of ground-level ozone. Therefore, concerted action among countries is needed to effectively reduce ground-level ozone and the associated health effects.

In the current study, the researchers assessed the health impacts of ground-level ozone exposure in Europe. They also quantified mortality rates associated with both domestic and imported ozone in 813 contiguous regions of 35 European countries, representing 530 million people.

O3 levels and associated mortality in the warm season (May–September) 2015–2017.
A. Daily mean maximum 8-hour O3 (µg/m³).
B. Mortality (annual deaths per 1 million population) attributable to O3.
A,b. Histograms display both the color legend and the number of regions for each value.

The average ground-level ozone concentration in European countries was estimated at 101.9 μg/m³. Ozone concentrations in southern European countries were higher than in the north, which can be explained by the warmer climate of this region.

During the warm seasons of 2015–2017, 72 annual deaths per million inhabitants were reported. The highest death rates were recorded in densely populated and southeastern European countries.

About 88.3% of all ozone-related deaths were due to exposure to imported ozone, with a cross-country range of 83-100%. Hemispheric sources were the major contributors to imported ground-level ozone, which was responsible for 56.7% of all ozone-related deaths.

Exposure to imported ozone produced by other European countries was responsible for 20.9% of all ozone-related deaths. Imported ozone from ocean and marine sources also contributed to 7.2% of all deaths in smaller southern European countries.

The most populous and industrialized countries were the main contributors to mortality associated with imported ozone. Significant effects of ozone originating from France were noted on mortality in neighboring countries, including Luxembourg, Switzerland, Belgium, Liechtenstein, Spain, and Germany. Similarly, ozone from Germany significantly affected mortality in Luxembourg, the Czech Republic, the Netherlands, Denmark, Austria, Belgium, and Poland.

O3-attributable mortality by O3 emission sources in 35 European countries in 2015–2017. Only days when the daily mean maximum 8-hour O3 value exceeded 70 μg/m³ were included in the analysis. Horizontal bars represent the 95% empirical confidence interval of the overall O3-attributable mortality (i.e. the sum of the contributions from the five sources).

In the southwestern countries, the impact of transboundary ozone transport was less pronounced. The highest mortality rates due to national ozone production were recorded in Spain, France and Portugal.

A sensitive analysis assessing ozone-related mortality at a safe threshold of 70 µg/m³ showed a three-fold reduction in deaths to 23 annual deaths per million inhabitants during the warm seasons of 2015–2017.

The broad geographic coverage of the current study allowed the researchers to determine the impact of ground-level ozone on overall mortality across the continent. The largest mortality burden in Europe was due to hemispheric ozone transported from other countries. In comparison, only a small proportion of mortality was due to ozone production at the national level.

Ozone transported from other European countries also significantly affects mortality. A significant contribution of ozone emissions from marine sources to mortality has been noted in some coastal regions and small Mediterranean countries.

The study highlights the need for transboundary assessment of pollution sources and associated health impacts to effectively manage air pollution. However, most current mitigation efforts are focused at the national and regional levels.

The observed impact of marine emissions on mortality highlights the need for nitrogen emission control areas to reduce nitrogen oxide emissions, which has been successfully implemented in the North and Baltic Seas.

Current projections indicate that global warming may increase ground-level ozone. In addition to directly triggering ozone production, global warming may also increase emissions of ozone precursors, which may further contribute to overall tropospheric ozone concentrations. Climate change mitigation is therefore necessary to improve air quality and reduce the health impacts of air pollution.

Taken together, the study findings highlight the need for national or coordinated pan-European actions and global strategies to reduce the deadly effects of ozone exposure.