Ozone is exceedingly rare in our atmosphere, averaging about 3 molecules of ozone for every ten million air molecules.
Nonetheless, atmospheric ozone plays vital roles that belie its small numbers.
Ozone is mainly found in two regions of the Earth's atmosphere:
While the ozone in these two regions is chemically identical (both consist of three oxygen atoms and have the chemical formula "O3") the ozone molecules have very different effects on humans and other living things depending upon their location.
With the two different aspects of ozone (stratosp./troposp.) come two separate environmental issues, controlled by different forces in the atmosphere.