Ground-level ozone is a secondary pollutant that is created in the lower atmosphere when primary pollutants called volatile organic compounds (VOCs) and nitrogen oxides (NOx) chemically react in the presence of heat and sunlight. It is the primary component of urban and regional smog.

Major sources of VOCs include motor vehicles, evaporation of gasoline from refueling and spillage, gasoline-powered lawn equipment, and consumer products such as oil-based paints, aerosol sprays and lighter fluid.

Major sources of NOx include motor vehicles, diesel engines, gasoline-powered lawn equipment and industrial facilities.

Ground-level ozone is different from stratospheric ozone, which exists naturally in the Earth's upper atmosphere and acts as a protective filter from the sun's harmful ultraviolet radiation.

Weather plays an important role in ground-level ozone formation and the declaration of Knozone Action Days. In the spring and summer when the temperature reaches 90 degrees or more and there is with little or no wind, ground-level ozone can build up quickly. Local and state air quality officials will then closely monitor pollution levels. If pollution levels are forecasted to be unhealthy for sensitive groups, a Knozone Action Day is declared.

All nine central Indiana counties (Boone, Hamilton, Hancock, Hendricks, Johnson, Madison, Marion, Morgan, Shelby) were designated by the U.S. Environmental Protection Agency as non-attainment of the 8-hour ozone standard in June 2004. Click here (direct to internal standards page) for more information.