What problems does Mexico City have because of overpopulation?

Mexico City has a very troublesome pollution problem due to both the high population as well as the city's location within a valley region. The sheer number of people means there is a high output of pollutants and carbon emissions. The location in a valley means those emissions cannot easily escape the area and instead tend to "pool" and "sink" within the valley resulting in very thick and unhealthy smogs.