Smog, according to Science Officer Spock, was the presence of "moderately industrialized pollution containing substantial amounts of carbon monoxide and partially consumed hydrocarbons."
He made this statement after conducting a tricorder scan of the planet 892-IV in 2268, and noted that its composition was similar to the atmosphere found on 20th century Earth. Doctor Leonard McCoy noted that a colloquial term describing the phenomenon was "smog", which surprised the Vulcan, who hadn't thought McCoy was such a historian. McCoy clarified that he only said it to get Spock to stop his lecture. Automobiles such as the Jupiter 8 produced smog. (TOS: "Bread and Circuses")
When the episode "Bread and Circuses" was produced in 1968, smog was a major problem in United States cities. Most major cities had air quality which by modern standards was considered deadly, caused by unregulated dumping of chemicals and pollutants into rivers and lakes, as well as unchecked industry destroying the air with smoke and harmful waste. In Los Angeles, where Star Trek was filmed, the city was often clouded in a haze of smog, especially in the summer. [1] In at least one case, the presence of smog benefited a Star Trek production, specifically the episode "A Private Little War", where Los Angeles can be seen briefly on the horizon of the planet Neural, but is so obscured by smog that the television quality of the time made the city barely noticeable.[2]