Ground-level Ozone:
What is it? Where does it come from?

Ozone (O₃) is a gas composed of three oxygen atoms.  It is not usually emitted directly into the air, but at ground level is created by a chemical reaction between oxides of nitrogen (NO_x) and volatile organic compounds (VOC) in the presence of sunlight.  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.  "Good" ozone 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.  In the earth's lower atmosphere, ground-level ozone is considered "bad."

VOC + NOx + Sunlight = Ozone

Motor vehicle exhaust and industrial emissions, gasoline vapors, and chemical solvents as well as natural sources emit NO_x and VOC, that help to form ozone.  Sunlight and hot weather cause ground-level ozone to form in harmful concentrations in the air.  As a result, it is known as a summertime air pollutant.  Many urban areas tend to have high levels of "bad" ozone, but even rural areas are also subject to increased ozone levels because wind carries ozone and pollutants that form it hundreds of miles away from their original sources.

Ground-level Ozone

• Triggers a variety of health problems even at very low levels
• May cause permanent lung damage after long-term exposure
• Damages plants and ecosystems

The summertime pollutant

Peak ozone levels typically occur during hot, dry, stagnant summertime conditions.  The length of the ozone season varies from one area of the United States to another.  Southern and Southwestern states may have an ozone season that lasts nearly the entire year.

Ozone can be transported over long distances

Ozone and the chemicals that react to form it can be carried hundreds of miles from their origins, causing air pollution over wide regions. Millions of Americans live in areas where ozone levels exceed EPA's health-based air quality standards, primarily in parts of the Northeast, the Lake Michigan area, parts of the Southeast, southeastern Texas, and parts of California.

Ozone and the pollutants that form it can cause air quality problems hundreds of miles away.