What is smog?its harmful effects?its causes?how to prevent it?

Smog is a kind of air pollution; the word "smog" is a portmanteau of smoke and fog. Clic smog results from large amounts of coal burning in an area caused by a mixture of smoke and sulfur dioxide. Modern smog does not usually come from coal but from vehicular and industrial emissions that are acted on in the atmosphere by sunlight to form secondary pollutants that also combine with the primary emissions to form photochemical smog.