Here's why:

* Oxygen (O2) is the stable form of oxygen, the one we breathe and is abundant in the atmosphere.

* Ozone (O3) is a molecule with three oxygen atoms. It's highly reactive and unstable because the third oxygen atom is easily detached, making it a powerful oxidizer.

Ozone is important in the stratosphere, where it protects us from harmful ultraviolet radiation. However, in the troposphere (lower atmosphere), ozone is a pollutant that can damage human health and the environment.