Air quality in Europe shows significant improvements over the last two decades, study finds

A study led by the Barcelona Institute for Global Health (ISGlobal) has consistently estimated daily ambient concentrations of PM_2.5, PM₁₀, NO₂ and O₃ across a large ensemble of European regions between 2003 and 2019 based on machine learning techniques. The aim was to assess the occurrence of days exceeding the 2021 guidelines of the World Health Organization (WHO) for one or multiple pollutants, referred to as “unclean air days.”

The research team analyzed pollution levels in more than 1,400 regions in 35 European countries, representing 543 million people. The results, published in Nature Communications, show that overall suspended particulate matter (PM_2.5 and PM₁₀) and nitrogen dioxide (NO₂) levels have decreased in most parts of Europe.

In particular, PM₁₀ levels decreased the most over the study period, followed by NO₂ and PM_2.5, with annual decreases of 2.72%, 2.45% and 1.72% respectively. In contrast, O₃ levels increased annually by 0.58% in southern Europe, leading to a nearly fourfold rise in unclean air days.

The study also looked at the number of days on which the limits for two or more pollutants were exceeded simultaneously, a confluence known as a “compound unclean air day.” Despite the overall improvements, 86.3% of the European population still experienced at least one compound unclean day per year during the study period, with PM_2.5-NO₂ and PM_2.5-O₃ emerging as the most common compound combinations.

The results highlight the significant improvements in air quality in Europe followed by the decline of PM₁₀ and NO₂, while PM_2.5 and O₃ levels continue to exceed WHO guidelines in many regions, resulting in a higher number of people exposed to unclean air levels.

“Targeted efforts are needed to address PM_2.5 and O₃ levels and associated compound unclean days, especially in the context of rapidly increasing threats from climate change in Europe,” says Zhao-Yue Chen, ISGlobal researcher and lead author of the study.

“Our consistent estimation of population exposure to compound air pollution events provides a solid basis for future research and policy development to address air quality management and public health concerns across Europe,” points out Carlos Pérez García-Pando, ICREA and AXA Research Professor at the BSC-CNS.

Heterogeneous geographical distribution

The research team has developed machine learning models to estimate high-resolution daily concentrations of major air pollutants like PM_2.5, PM₁₀, NO₂ and O₃. This data-driven approach creates a comprehensive daily air quality picture for the European continent, going beyond sparsely distributed monitoring stations.

The models gather data from multiple sources, including satellite-based aerosol estimates, existing atmospheric and climate data, and land use information. By analyzing these air pollution estimates, the team calculated the annual average number of days in which the WHO daily limit for one or more air pollutants is exceeded.

Despite air quality improvements, 98.10%, 80.15% and 86.34% of the European population lives in areas exceeding the WHO recommended annual levels for PM_2.5, PM₁₀ and NO₂, respectively. These results closely match the European Environment Agency (EEA)’s estimates for 27 EU countries using data from urban stations only.

Additionally, no country met the ozone (O₃) annual standard during the peak season from 2003 to 2019. Looking at short-term exposure, over 90.16% and 82.55% of the European population lived in areas with at least 4 days exceeding WHO daily guidelines for PM_2.5 and O₃ in 2019, while the numbers for NO₂ and PM₁₀ were 55.05% and 26.25%.

During the study period, PM_2.5 and PM₁₀ levels were highest in northern Italy and eastern Europe, while PM₁₀ levels were highest in southern Europe. High NO₂ levels were mainly observed in northern Italy and in some areas of western Europe, such as in the south of the United Kingdom, Belgium and the Netherlands.

Similarly, O₃ increased by 0.58% in southern Europe, while it decreased or showed a non-significant trend in the rest of the continent. On the other hand, the most significant reductions in PM_2.5 and PM₁₀ were observed in central Europe, while for NO₂ they were found in mostly urban areas of western Europe.

The complex management of ozone

The average exposure time and population exposed to unclean air concentrations of PM_2.5 and O₃ is much higher than for the other two pollutants. According to the research team, this highlights the urgency of greater control for these pollutants and the importance of addressing the increasing trend and impact of O₃ exposure.

Ground-level or tropospheric O₃ is found in the lower layers of the atmosphere and is considered a secondary pollutant because it is not emitted directly into the atmosphere, but is formed from certain precursors—such as volatile organic compounds (VOCs), carbon monoxide (CO) and nitrogen oxides (NO_x)—that are produced in combustion processes, mainly in transport and industry. In high concentrations, ozone can damage human health, vegetation and ecosystems.

“Ozone management presents a complex challenge due to its secondary formation pathway. Conventional air pollution control strategies, which focus on reducing primary pollutant emissions, may not be sufficient to effectively mitigate O₃ exceedances and associated compound unclean days,” says Joan Ballester Claramunt, ISGlobal researcher and senior author of the study. “However, addressing climate change, which influences ozone formation through increased sunlight and rising temperatures, is crucial for long-term ozone management and protection of public health.”

The challenge of compound episodes

Despite improvements in air pollution, the research team reported that over 86% of Europeans experienced at least one day with compound air pollution events each year between 2012 and 2019, where multiple pollutants exceeded WHO limits simultaneously.

Among those compound days, the contribution of PM_2.5-O₃ compound days increased from 4.43% in 2004 to 35.23% in 2019, becoming the second most common type in Europe, indicating a worrying trend. They mainly occur in lower latitudes during warm seasons and are likely linked to climate change and the complex interplay between PM_2.5 and O₃.

Warmer temperatures and stronger sunlight in summer boost O₃ formation through chemical reactions. Subsequently, higher levels of O₃ will accelerate the oxidation of organic compounds in the air. This oxidation process leads to the condensation of certain oxidized compounds, forming new PM_2.5 particles.

Additionally, climate change increases the likelihood of wildfires, which further elevates both O₃ and PM_2.5 levels. “This complex interplay creates a harmful loop, highlighting the urgent need to address climate change and air pollution simultaneously,” explains Ballester Claramunt.