New research has led to the development of a global database of annual, population-weighted concentrations for PM2.5, NO₂ and ozone (O3), as well as fossil fuel CO₂ (FFCO2) emissions across 13,189 urban areas.
The study, undertaken by a team from George Washington University, focused on the years spanning from 2005-2019, using data from satellite observations, ground-based measurements and computer models to measure city-level air pollution and the average amount of FFCO2 emitted.
Susan Anenberg, professor of environmental and occupational health at the University’s GW Milken Institute School of Public Health said: ‘This study provides a powerful snapshot of how urban environments are evolving across the globe. It also shows that progress is possible but uneven, with some cities seeing worsening pollution while others are experiencing cleaner air over time.’
Across the period studied, global O3 concentrations increased by 6%, while PM2.5, NO₂, and FFCO₂ per capita showed little to no overall change (+0%, −1%, and +4%, respectively). Across most cities, these pollutants were positively correlated, suggesting they were being created by the sources.
The team identified regional differences in trends, with high-income countries seeing consistent declines in all four pollutants, a trend the team put down to effective air pollution and climate policies.
In contrast, South Asia and Sub-Saharan Africa experienced significant increases in pollution and emissions, with many cities showing strongly positive correlations among pollutant trends – a symptom of rapid industrialisation, urbanisation and fossil fuel use outpacing mitigation efforts.
In Latin America and the Middle East the long-term trends of the different pollutants were not consistently moving in the same direction, suggesting that each is influenced by different local factors – specific industrial activities, energy sources, traffic patterns or environmental regulations.
The team also worked with different definitions of what areas are included by the term ‘urban’, using GHS-SMOD (a standardised global urban dataset) and C40 Cities’ definition which is based on administrative city limits.
They found that the way the physical boundaries of a city are defined can affect pollution estimates, particularly for NO₂ and FFCO2 emissions per person as these can vary widely, depending on how a city’s boundaries are drawn. NO₂ tends to concentrate near roads and traffic areas, while CO₂ emissions per person are influenced by where people live and the types of activities within those areas, such as industry or commuting from suburbs.
The team point to this as emphasising the importance of choosing boundary definitions that are aligned with specific research or policy objectives.
Following the research, the team created an interactive map and dashboard to track air pollution in cities worldwide.
The full research can be read here.
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