During the last 5 years, several policies and actions were enacted in order to decrease pollutants emissions, among them, the banning of leaded gasoline and heavy oil fuels, and the availability of diesel with reduced sulfur contents. The overall results were the lowering of pollutant air concentrations below the allowed limits (INE, 1996 and INE, 1997). From the ZMCM emissions inventory (DDF, 1996), it can be deduced that the main sources of contaminant criterion emissions are those related to transportation. It is known that 75% of such emissions come from transportation activities, while the industry contribution is only 10%, and natural origin emissions are up to 12%. The gasoline supplied to the ZMCM, specially Magna (unleaded), has been improved. Nova Plus (leaded) is off the market since 1997. The olefins, benzene, and other aromatic hydrocarbons (some of the so-called volatile organic compounds; VOC) contents have been substantially reduced.