As in many large cities, and especially in ones located in valleys with limited ventilation, Mexico City experiences air pollution problems, especially ozone and suspended particles. Stringent controls since 1990 have resulted in major reductions of sulfur dioxide emissions. Sulfur in diesel fuel has been reduced from to . Only one industrial complex still uses residual oil, and it is slated to soon change to gas. Gasoline-powered vehicles were required to have catalytic converters after 1990, and unleaded fuel was introduced at that time to provide cleaner emissions. Within the Distrito Federal, the central core of Mexico City that has its own government, many old diesel.
