Air pollution in the major cities became severe when coal use was recommended after the oil shock of the 1970s. Consequently, since 1985, this prohibition has banned the use of coal, coke, firewood, charcoal, and other solid fuel in regions that exceed or may exceed environmental standards. It is being applied to an increasing number of regions, and since 1999, the use of solid fuels has been banned in a total of 20 regions, which includes Seoul and the six metropolitan cities and 13 si areas of Gyeonggi-do.