It depends. You mean atmospheric, land or water pollutant? The most important would be carbon dioxide (CO2) from literally, millions of automobiles and industries.
Mexico City, or the City of Mexico, is the capital and most populous city of Mexico.
Most people living in the Mexico City proper are born within Mexico City; there are some immigrants from central and southern states but most of them tend to live on the periphery, on greater Mexico City.
Mexico City - 19,231,829 inhabitants.
Mexico City would qualify as such.
Mexico City is the most known example.
Mexico City, with a population of 19,231,829.
It is Mexico City; and it is in Mexico, the Country.
Mexico City, Mexico
Mexico City.
Mexico City is the most populous on the North American Continent
from my research the most polluted city in china is Linfen.
Acapulco is a city in the state of Guerrero in Mexico. Acapulco is not a capital city. Chilpancingo is the capital city of Guerrero, Mexico. Mexico City is the capital and most populous city of Mexico.