Smog
There are two types of pollution smog: wintertime smog and summertime smog. Wintertime smog was once much more common that it is today. It is made up of a mixture of smoke and sulphur dioxide pollution, and fog. When the majority of homes still used coal for heating, and urban factories were not required to disperse pollution through tall chimneys, smoke and sulphur dioxide pollution could build up to sometimes dangerous levels within a town or city. When the weather was foggy and winds were low, the pollution became trapped over the cities. The smoke and fog mixed to produce smog. More coal was burnt in wintertime for heating and so smog pollution was greatest at this time of year. The Great London Smog of 1952 caused 4000 deaths and persuaded the Government to clean up Britain's air.
Summertime smog is now more common than wintertime smog. It mostly consists of ozone that forms when other air pollutants from cars mix and react together in the air on warm sunny days. It is sometimes called "photochemical smog". Los Angeles in America is a prime example of a city that has a big problem with summertime smog. The city lies in a basin and so pollution becomes trapped.
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