seen an explosion of population in the valley along with the growth of industry. Since 1900, the population has doubled every fifteen years. Today, around 21 million people live in the Mexico City metropolitan area, which extends throughout almost all of the valley into the states of Mexico and Hidalgo and is one of the most populous urban areas in the world.

The growth of a major urban industrial centre in an enclosed basin has created significant air and water quality issues for the valley. Wind patterns and thermal inversions