Ozone is a gas found throughout the atmosphere.
I. High in the atmosphere, in a region called the stratosphere, naturally-occurring ozone is formed when intense sunlight causes oxygen molecules to break up and reform as unstable ozone molecules. Ozone is also found in the lower atmosphere, a region called the troposphere, but normally at very low concentrations that are not harmful. A small amount of tropospheric ozone is transferred down from the ozone formed naturally in the stratosphere. The remainder is formed in the lower atmosphere by the action of sunlight on natural and human-made substances. If the ozone present in the air were produced only from naturally-produced substances, it would not be of concern.