The EPA recently conducted a survey in which San Bernardino, and its county, has the worst air pollution rating of any city\county in the U.S. I'm sorry, I don't have the numbers, though.
El Paso!
The city of Los Angeles is generally considered to have America's worst air pollution problem. Most of California's metro areas have bad air pollution.
The cities that have the worst air pollution is Pittsburgh and Los Angeles
Mexico City
the largest city is Mexico City, it also has the dubious reputation for having the worst pollution in the world.
America
It is measured by the pollution index
Mexico City and Sao Paulo are amongst the worst of the major world cities.
Detroit is the worst overall city. For larger cities, Chicago is statistically pretty bad. Chicago has some of the worst stats involving taxes, unemployment, youth violence, traffic congestion, pollution, corruption....on and on.
Pittsburgh, Pennsylvania, replaced Los Angeles, California for the most polluted city in America.
New York, Las Vegas, Hollywood, Nevada, and so many more!
China