Pea soup in the context of "Pea soup fog"

Pea soup fog (also known as a pea souper, black fog or killer fog) is a very thick and often greenish-yellow fog caused by air pollution that contains tarry soot particulates and the poisonous gas sulphur dioxide. This very thick smog occurs in cities and is derived from the smoke given off by the burning of soft coal for home heating and in industrial processes. Smog of this intensity is often lethal to vulnerable people such as the elderly, infants, and those with respiratory problems. The result of these phenomena was commonly known as a London particular or London fog; in a reversal of the idiom, "London particular" became the name for a thick pea and ham soup.