To identify the most polluted cities in the world, 24/7 Wall St. ranked nearly 3,000 cities worldwide based on the annual average concentration of PM2.5 per square meter of air. We only considered the most recent air quality data from the WHO Global Ambient Air Quality Database (update 2018). Only cities for which PM2.5 concentration was directly measured and not imputed from other measures of air pollution were considered. Though it was not factored into the ranking, we also included PM10 concentration. Pollution data came from the World Health Organization. Population figures also came from the WHO Global Ambient Air Quality Database (update 2018). For cities not included in the U.N. report, population estimates came from a range of sources.