Smog
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, 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. ()

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Geoscience