A really bad nineteenth century fog appeared early in the morning as a thick white mist, like the country fog, only dirtier. With the lighting of the fires it would soon become yellow and pungent, irritating the throat and eyes, till midday the continued outpouring of chimney products would have turned the fog a sooty brownish black causing the darkness of night. -- L.C.W. Bonacina
The problem of "pea soupers" continued into the twentieth century, culminating in the Great Smog of 1952, which may have caused as many as 12,000 fatalities due to respiratory problems. The Clean Air Act 1956, which banned coal fires in many cities and relocated power stations away from urban areas, finally brought an end to serious smog in London.