What is Ozone?

Ozone is a gas composed of three oxygen atoms. Ozone has the same chemical structure whether it occurs miles above the earth or at ground level and can be "good" or "bad," depending on its location in the atmosphere. Ozone that is considered “good” occurs naturally in the stratosphere approximately 10-to-30 miles above the earth's surface and forms a layer that protects life on earth from the sun's harmful rays.

However in the earth's lower atmosphere, ground-level ozone is considered "bad." Ground-level or "bad" ozone is not emitted directly into the air but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. Sources of these chemicals can include:

Ozone Pollution


  • Motor vehicle exhaust from automobiles, trucks, buses, aircraft, and locomotives
  • Emissions from construction equipment
  • Emissions from lawn and garden equipment
  • Sources that combust fuel, such as large industries and utilities
  • Small industries such as gas stations and print shops
  • Consumer products, including some paints and cleaners

Ground-level ozone is the primary constituent of smog. The combination of sunlight and hot weather cause ground-level ozone to form in harmful concentrations in the air. Ozone can also be transported long distances by wind. For this reason, even rural areas can experience high-ozone levels.

Breathing ground-level ozone can trigger a variety of health problems. Ground-level ozone also damages vegetation and ecosystems. In the United States alone, ozone is responsible for an estimated $500 million in reduced crop production each year.

Under the Clean Air Act, the U.S. Environmental Protection Agency has set protective health-based standards for ozone in the air we breathe. The Airnow Web site provides daily air quality reports for many areas. These reports use the Air Quality Index (or AQI) as an indicator of how clean or polluted the air is. State or local air quality agencies may declare an Air Quality Action Day for Ozone when ozone levels are forecast to reach unhealthy levels.