Top 10 Dirtiest Cities in the World in 2018

5. Mexico City, Mexico

Our Index Score: 33.0
Mexico’s altitude is about 7350 feet above sea level, thus making it to have little oxygen supply. It is also the home of approximately four million vehicles. Because of these factors, it is prone to incomplete combustion of hydrocarbons, especially the diesel emissions that eventually lead to high levels of air pollution. In other words, the volcanic crater and mountains lock in the pollutants, resulting in smog clogging the city. Unhealthy ozone emissions remain constant during the whole year at about 85%.