The gases and particles from car exhaust pipes can create photochemical smog. This form of smog is particularly common in hot, dry and sunny cities such as Athens in Greece, Cairo in Egypt and Mexico City. The car exhaust pipes pump out gases such as nitrous oxide, carbon monoxide and ozone that react together in the sunlight. The result is a smoggy haze over the city.
Los Angeles in California, USA, has suffered badly in the past from smog. The smog was due in part to the exhaust fumes from millions of cars, but also from the local geography. The city lies by the coast and is surrounded by mountains. In certain weather conditions, the smoggy air would be trapped over the city, and air quality would fall steadily until atmospheric conditions changed and the smoggy air was replaced by cleaner air. A hazy layer can still be seen over Los Angeles, but the level of pollutants has fallen dramatically due to new legislation and improved pollution control on cars.