https://doi.org/10.1351/goldbook.S05716
The term originated in Great Britain as a popular derivation of '@S05719@-@F02470@' and appears to have been in common use before World War 1. It originally referred to the heavy pollution derived largely from coal burning (largely @S05719@ filled air, rich in sulfur dioxide), and it probably was largely a reducing atmosphere. More common today in cities is an oxidizing atmosphere which contains ozone and other oxidants.
See:
photochemical smog