Since the Industrial Revolution and the great increase in the use of fossil fuels, industrial areas and great cities have experienced pollution in the form of smoke and sulphur dioxide in the atmosphere. This visible air pollution has been much reduced in the second half of the 20th century with the growing use of electrical energy. In large British towns, such as London, Birmingham, and Manchester, despite the increase of pollution from vehicle exhausts, the air is now much cleaner and fogs less dense and frequent than was the case in the early decades of the century.

However when electricity is produced in large thermal power stations burning oil and coal the gases released from the high chimneys are carried downwind for long distance. It is thought that this is one of the main causes of the acid rain which in quite rural areas has been blamed for leaf loss or the death of trees and the destruction of fish in freshwater lakes where this acid rainwater accumulates. Areas particularly affected are parts of Scandinavia, central Europe, and eastern Canada, and this is blamed on industrial emissions from factories and power stations in Britain, central and eastern Europe and the United States.

When pollution is trapped near the ground by a temperature inversion, smog can form. Smog is a thick dirty poisonous fog. Some of the chemicals in pollution are changed by the action of sunlight to produce an even more dangerous photochemical smog. Controls on vehicle exhaust emissions help to reduce smog but it is a growing problem and cities on every continent are now afflicted by it. Quite apart from the problems caused by reduced visibility, smog is a medical hazard. People suffering from chest illnesses are especially risk. During the 1950s smogs killed thousands of Londoners and hospitalised many more.