Too many cars have caused a lot of serious problems in our world. Besides congestion, accidents and fast fuel consumption, cars are responsible for a good part of air pollution in big cities. One possible solution is to design and develop clean cars and clean fuels. In Shanghai, some of the public buses begin to run on natural gas, which does not give off as much carbon dioxide as petrol. Another solution is to develop modern public transportation systems and restrict the use of private cars. If the price of petrol rises constantly and the public vehicles are efficient and convenient enough, most people will not buy private cars. Thus, the total number of cars in big cities will reduce greatly. On the whole, the elimination of air pollution needs the collective efforts from the government, the public and the environmentalists.