Automakers have succeeded in cleaning up most of the harmful substances emitted by vehicles. Compared with unregulated vehicles 30 years ago, today's new cars generate 98% fewer hydrocarbons, 96% less carbon monoxide and 90% fewer nitrous oxides. However, vehicles still emit a great deal of air pollution, including carbon monoxide, nitrogen dioxide, particulate matter, volatile organic compounds and carbon dioxide. Vehicle exhaust also contributes to ozone formation.