Smog was caused by lingering fog combining with smoke from coal fires. (PROSE: Amorality Tale)
2024 native Ruby Sunday believed that London had gotten rid of its smog in the 1950s, though the Fifteenth Doctor noted that there was a "really bad smog" in December 1962. On 11 February 1963 in an alternate timeline, Ruby mistook Maestro's darkening of the Earth for smog. (TV: The Devil's Chord [+]Loading...["The Devil's Chord (TV story)"])