Ozone (O₃) is a highly reactive gas composed of three oxygen atoms. It is found in both the upper atmosphere and at ground level. Ozone can be "good" or "bad" for your health and the environment, depending on its location in the atmosphere. Click here for more information.
The atmospheric or "good" ozone layer extends upward from about 6 to 30 miles (10 to 48 km) and protects life on the Earth from the sun's harmful ultraviolet (UV) rays. Atmospheric ozone is formed naturally through the interaction of solar UV radiation with molecular oxygen (O₂).

The ground-level or "bad" ozone layer is the layer closest to the Earth's surface. It forms primarily from reactions between two major classes of air pollutants: volatile organic compounds (VOCs) and nitrogen oxides (NO_x). Ozone is harmful to breathe and it damages crops, trees, and other vegetation.

Ambient Ozone Regulations
Both the U.S. Environmental Health Association (EPA) and the State of California Air Resources Board (CARB) regulate ozone concentration in Ambient Air Quality Standards, which define the maximum amount of ozone that can be present in outdoor air without harming human health.  
Click here to check the National Ambient Air Quality Standards (NAAQS)
Click here to check the California Ambient Air Quality Standards (CAAQS)