Rapid urban growth has also critically affected people and the environment through pollution. Mexico City and cities on the U.S.-Mexico border have grown so quickly that tens of millions of people live in areas without sewage systems. A confidential study conducted by a United Nations task force in the late 1980s calculated that "600 tons of solid human waste are dumped into [Mexico City's] air daily... the 'number of colonies of micro-organisms per cubic meter [are] uncountable." The source of this waste material is "the deposited wastes of about six million people and two million dogs" on the outskirts of Mexico City. The waste comes into Mexico City in the form of dust which is blown into the city, especially in the months of February, March, and April, at a rate of 20 tons per square kilometer per month.

[...]

The forests in the mountains surrounding the valley of Mexico have been dying due at least in part to the severe air pollution caused by 3,000,000 cars and trucks and 25,000 industrial plants in Mexico City. Each year, five million tons of chemicals and suspended particles are dumped into the air in Mexico City. According to "government-approved figures" this includes "3,720,000 tons of carbon monoxide, 525,000 tons of hydrocarbons, 411,600 tons of sulphur dioxide, 153,800 tons of suspended particles, 132,0000 tons of nitrogen oxides and 18,250 tons of lead. The blood level of lead of the average person in Mexico City is four times that in Tokyo, twice that of persons in Baltimore, Stockholm, Lima, and Zagreb. Sternberg reports successive winters in which birds dropped dead out of the air.  

David A. Sonnenfeld, "Mexico's "Green Revolution," 1940-1980: Towards an Environmental History" en Environmental History Review, Vol. 16, No. 4 (Winter, 1992).