All around the world large cities have the same problem. That problem is air pollution. Mexico City has very bad air. The air is dirty and very unhealthy. Cars are one reason for the dirty air. Many Mexicans now own their own cars and drive in the city. The factories in the area also cause air pollution. These factories put a lot of smoke into the air. It is not easy to clean up the air in a large city. The government has to make new laws and everyone has to help.