The third major source of air pollution is automotive emissions. As recently as the 1940's, emissions from the internal combustion engine were not an appreciable problem, because there were relatively few motor vehicles. However, during the early 1950's, a population shift among Americans put a spike in the vehicle ownership statistics, as people began to move from cities to the surrounding suburbs. There became an unprecedented need for personal vehicles, since most suburbs lacked mass transit of any significance. This need for transportation in turn created an attractive market for automobile manufacturers, and they dramatically increased production to take advantage of (and encourage) the big changes in supply and demand. Since these mid-century demographic developments, the number of vehicles sold, along with a marked increase in new highways constructed to support them, added considerably to the number of internal combustion engines on the road between cities and the suburbs.
As the automotive boon continued, another new phenomenon emerged: multi-vehicle families which placed emphasis on each member having his own set of wheels. As might have been expected, this increase in vehicle usage was being matched by pollutant levels that rose in direct proportion. The air, land and water were now getting dirty faster as suburbanites drove daily to and from their employment.
As people's lifestyles changed and took new turns, the changing environment produced a new twist of its own: an altogether new kind of air pollution; a foggy, smoky haze was forming and, at times, remained suspended over certain cities around the country. At first this "smog," derived from the words "smoke" and "fog," was blamed on industrial sources, but it was soon concluded that automobile tailpipes were the primary culprit. The synthesis of this new form of air pollution was discovered to result from high levels of everyday automobile emissions when exposed to sunlight for a period of time. The complex chemical reactions that took place yielded the smog.
More specifically, it was found that smog was a photo-chemical layer that developed when certain oxides of nitrogen (NOx) and unburned hydrocarbons (HC) from automobiles were exposed to sunlight. It was discovered that smog was more severe when it would remain stagnant over an area in which a warm layer of air would settle over the top of a cooler air mass at ground level; the warm air would trap and hold the automobile emissions, instead of the emissions being dispersed and diluted through normal air flows. This type of air stagnation was named a temperature inversion.