Pollution from motor vehicles has become an issue simply because of the steady increase both in the number of vehicles in use and the distance travelled by each vehicle each year. Since the 1960s, the number of motor vehicles in the world has been growing faster than its population. In 1950 there were 50 million cars for 3.5 billion people. There are now 600 m cars for 6 billion people, with a global production of 45 million cars per year. By 2020 there will be a billion cars. The net growth rate for all motor vehicles is now around 5%, compared to a population growth rate of 1-2% (Figure 3.1).