A term chosen by the Glasgow public health official Des Voeux at the beginning of the twentieth century to describe the smoky fogs that characterized coal-burning cities of the time. The word is formed by adding the words smoke and fog together and has persisted as a description of this type of urban atmosphere. It has more and more been used to describe photochemical smog, the haze that became a characteristic of the Los Angeles Basin from the 1940s. "Smog" is sometimes even used to describe air pollution in general, even where there is no reduction in visibility at all. However, the term is most properly used to describe the two distinctive types of pollution that dominated the atmospheres of late nineteenth century London, England, known as winter smog, and twentieth century Los Angeles, called summer smog.

The city of London burned almost 20 million tons of coal annually...