Most polluted capital cities in the world

30: Mexico City is the 30th most-polluted capital city in the world, according to the latest ranking from two groups monitoring air pollution. The study by IQ AirVisual, a Swiss-based group that gathers air-quality data globally, and Greenpeace, found the Mexican capital has a PM2.5 concentration of 19.7. PM2.5, or particles of 2.5 micrometers or less in diameter, are dangerous because they can lodge deep in the lungs.