Air Inequality: Global Divergence in Urban Fine Particulate Matter Trends

Fine particle air pollution (PM_2.5) is the largest global environmental risk factor for ill-health and is implicated in >7% of all human deaths. Improved air quality is a key policy goal for cities, yet in-situ PM_2.5 measurements are missing for >50% of the world’s urban population. Here, we apply satellite remote sensing to develop a 21-year time series of ground-level PM_2.5 concentrations for the 4231 urban areas with populations >100,000 (2.9 billion people) from 1998 -2018. Globally, we find the most polluted cities are generally small (<1 million population) and lack PM_2.5 monitors. Since 1998, we observe a growing divide in urban air quality between cities in lower and higher-income regions, with the PM_2.5 disparity increasing by >50% (from 25 to 39 µg m^-3) between the highest- and lowest income quartiles of world cities. Within Asia, a sharp divergence is underway, with sustained PM_2.5 increases in South Asian cities (+48%) contrasted against dramatic improvements in Chinese cities (-40% since 2011). While 85% of the world’s urban population experiences PM_2.5 higher than World Health Organization guidelines, urban PM_2.5 concentrations are tightly linked to regional conditions, suggesting that city-level efforts alone may be insufficient to address this major health threat.