The third major source of air pollution is automotive emissions. Years ago, emissions from the internal combustion engine were not an appreciable problem because of the small number of registered vehicles and the nation's small highway system. However, during the early 1950's, the trend of the American people was to move from the cities to the surrounding suburbs. This caused an immediate problem in the transportation area, since most suburbs were not afforded mass transit convenience. This lack of transportation created an attractive market for the automobile manufacturers, which resulted in a dramatic increase in the number of vehicles produced and sold, along with a marked increase in highway construction between cities and the suburbs. Multi-vehicle families emerged with much emphasis placed on an individual vehicle per family member. As vehicle ownership and usage increased, so did the pollutant levels in and around the cities. This was due, in large part, to the suburbanites' daily driving to their employment in the city, then returning at the end of the day to their homes in the suburbs.
It was noted that a fog and smoke type haze was formed and, at times, remained in suspension over the cities without quickly dissipating. At first this "smog'', derived from the words "smoke'' and "fog'', was thought to result from industrial pollution, but it was determined that the automobile emissions were largely to blame. It was discovered that as normal automobile emissions were exposed to sunlight for a period of time, complex chemical reactions would take place.
It was found that smog is a photo-chemical layer which develops when certain oxides of nitrogen (NOx) and unburned hydrocarbons (HC) from the automobile emissions are exposed to sunlight. The problem intensifies when the smog remains stagnant over an area in which a warm layer of air settles above a cooler air mass at ground level. This results in the trapping and holding of automobile emissions, instead of the emissions being dispersed and diluted through normal air flow. This type of air stagnation was given the name "Temperature Inversion''.