Smog is caused by many factors.  The smog that enters the atmosphere consists of over 100 chemicals, many coming from different sources.

Particulates present in smog include carbon monoxide, dirt, soot, dust, and ozone.  To really create the smog effect, sunlight, hydrocarbons, and nitrogen oxides have to mix together.

Major producers of smog include automobiles, fires, waste treatment, oil production, industrial solvents, paints, and coatings.

Car engines, especially diesel engines, as well as gas stations that allow gas to be leaked out are huge contributors to the smog problem.  Gas vapor that gets away from gas pumps contributes to the hydrocarbons needed to form smog.  Diesel engines emit particles of soot that enter the atmosphere.

Lead is also a major problem, especially when found in gasoline that is combusted in automobiles.  Though the US now uses unleaded gas, Third World nations remain dependent upon cheaper leaded gas.

Today, the smog problems created by cars are becoming increasingly severe.  As gas prices decline, consumers are buying cars that use more gas, and hence pollute the atmosphere to a greater extent.