In Mexico City the smog in here isn't as bad as I thought it might be. I understand that during the really cool times of the year, it is worse because there isn't enough heat to move the air up and carry the smog with it.
Mexico City and the surrounding areas is one of the most populated places in the world. It has a population of over 19 million. This beats New York with a population around 18 million. Tokyo and Sheol are the only two cities with a larger population than Mexico City. There are about 10,000 people living in every square mile. By way of comparison, Los Angles has about 1,000 people living in every square mile.
Mexico City is built in a valley surrounded by mountains. The mountains create something of a "bowl" effect trapping in all the smog. Several years ago the city's mayor, Manuel Camacho, announced a plan to install giant fans to blow the smog out of the city. They were going to create large incinerators that would heat the air in the most polluted parts of the city and raise the air off the ground so the fans could blow it away.
There were also some ideas that involved cutting large holes in the nearby mountains to let fresh air in. I don't know what happened with any of these ideas. Currently older vehicles that run less efficiently are only allowed to drive every other day based on their license plate number.