What is “good” and “bad” ozone?
Ozone occurs in two layers of the atmosphere. The ozone that is commonly associated with ozone depletion and the ozone “hole” is in the stratosphere, a layer that extends upward from about 10 to 30 miles (15 to 50 kilometers) above the earth. This “good” ozone shields life on earth from the sun's harmful ultraviolet rays (UV-b).
The layer surrounding the earth's surface is the troposphere. Here, ground-level or "bad" ozone is an air pollutant that damages human health, vegetation, and many common materials. It is a key ingredient of urban smog. The troposphere extends from the surface to about 10 miles (15 kilometers) above the surface. Ground level, or “bad ozone”, is also called tropospheric ozone while “good ozone” is called stratospheric ozone. About 80-90% of all ozone in the atmosphere is found in the stratosphere.
(adapted from Ozone: Good Up High, Bad Nearby)