Ground-Level Ozone and Smog

Smog is a mixture of pollutants made up mostly of ground-level ozone that can damage the respiratory system.  They form through chemical reactions among nitrogen oxides (NOx) and volatile organic compounds (VOC).  Most of the air pollutants come from vehicles, power plants, refineries, chemical plants, and other industrial sources that create combustion from fossil fuels,  which chemically react with sunlight to form ozone.