Smog is a type of air pollutant. The word "smog" was made in the early 20th century as a portmanteau of the words smoke and fog to refer to smoky fog. This kind of smog is caused by the burning of timber and large amounts of coal within a city; this smog contains soot particulates from smoke, sulfur dioxide and other components. It is usually highly toxic to humans and can cause severe sickness, shortened life or death.