Ozone is a colorless gas that occurs both in the earth’s upper atmosphere and at ground level. Depending on its location in the atmosphere, ozone can be categorized as "good" or "bad".
Tropospheric ozone is created by chemical reactions between oxides of nitrogen (NOx), volatile organic compounds (VOC), and sunlight.
Sources of NOx include automobile emissions and industrial sources such as power plants and industrial boilers.
Man-made VOCs come from household and commercial products, as well as cigarette smoke, pesticides, and gasoline emissions.