Ozone is a unique pollutant in that it is exclusively a secondary pollutant. This means that it is formed through a complex series of chemical reactions initiated by the release of a primary pollutant such as gasoline vapors. When a volatile organic compound, such as the VOC’s in gasoline vapors, reacts with nitrogen dioxide (N0₂) in the presence of sunlight, OZONE is formed. Therefore the level or concentration of ozone formed is influenced by many factors, including concentrations of NO₂ and VOC’s in the area, the intensity of the sun and local weather conditions. Ozone is a colorless and odorless gas that can have adverse health effects.

The 1990 National Ambient Air Quality Standards (NAAQS) revisions recognized ozone as a potential health hazard and therefore set limits on the concentrations permissible in the atmosphere.

Ozone is the most prevalent chemical found in photo-chemical air pollution, or smog. Ozone is known to cause harmful effects in both newborn and unborn infants. Furthermore, ozone can be harmful on the elderly or other persons with respiratory problems. In addition, when compared to other air pollutants, ozone is found to cause the most damage to plants.

One control strategy for ozone is to regulate sources of VOCs and nitrogen oxides. Major sources of these pollutants include the products of incomplete combustion of motor vehicle exhaust, the burning of fossil fuels, and the use of petroleum compounds and organic solvents in manufacturing and cleaning processes (i.e. one solvent used in dry cleaning is a VOC).

Although ozone is harmful at ground level (troposphere), it is a necessary component of the upper atmosphere (stratosphere). At this level, ozone acts to protect the layer of atmosphere which is responsible for filtering out harmful ultraviolet rays produced by the sun. So, although ozone is harmful and must be controlled in the troposphere, it is protective and must be protected in the stratosphere.