text: A visibly persistent example of the effects of pollution from burning coal is the “smog and soot which had serious health impacts on residents of growing urban centers.” Smog is a colloid that is formed when sunlight reacts with gaseous nitrogen oxides and at least one volatile organic compound (VOC) latent within the atmosphere. Smog is often visible to the naked human eye, and exists as a hazy, white smoke sometimes causing acute irritation of the eyes, nose, and respiratory system. The two main sources that cause smog are primary and secondary pollution. Primary pollution is emitted directly from a source, such as driving a car, burning coal, burning fossil fuels, etc., whereas secondary pollution is formed in the atmosphere, such as natural chemical reactions between sunlight and ozone. According from the Water and Air Pollution  "The great smog of 1952 was caused by the mass production of burning coal, killing 4,000 people in londonWater and Air Pollution, and in “1948 there was severe industrial air pollution created deadly smog that asphyxiated 20 people in Donora Pennsylvania and made 7,000 more people sick.”