Pea soup, or a pea souper, also known as a black fog, killer fog or smog is a very thick and often yellowish, greenish, or blackish fog caused by air pollution that contains soot particulates and the poisonous gas sulfur 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, the very young and those with respiratory problems.