Pea soup fog

Pea soup fog is a very thick, often greenish-yellow fog produced by air pollution, carrying tarry soot particulates and sulphur dioxide from the burning of soft coal for home heating and in industry. Historically common in cities, it could be lethal to vulnerable people such as the elderly, infants and those with respiratory conditions, and became known in Britain as a London particular or London fog, a name that later lent itself, in reverse, to a thick pea and ham soup.