Mexico City’s air has gone from among the world’s cleanest to among the dirtiest in the span of a generation. Novelist Carlos Fuentes first novel took place here in 1959 and was entitled "Where the air is clear" - a title he has said is ironic considering the city’s now –soupy environment.
The average visibility of some 100 km in 1940s is down to about 1.5 km. Snow-capped volcanoes (Popocatepetl, Ixtacihuatl, and Paricutin) that were once parts of the landscape are now visible only rarely (fig.1.2). And levels of almost any pollutant like nitrogen dioxide (NO2) now regularly break international standards by two to three times. Levels of ozone (O3), a pollutant that protects us from solar radiation in the upper atmosphere but is dangerous to breathe, are twice as high here as the maximum allowed limit for one hour a year and this occurs several hours per day every day (fig.1.1).