Overview: Good Ozone, Bad Ozone

Ozone is a gas that is present in small amounts in Earth's atmosphere - 0.00006%. While ozone in the troposphere is a powerful oxidant and harmful to human and other life, stratosphereic ozone plays an important role in protecting life on the ground from harmful ultraviolet (UV) radiation.
Near Earth's surface, reactions between such gases as nitrogen oxides and hydrocarbons create ozone and other ingredients of a dangerous pollutant called photochemical smog.
In the troposphere, ozone is a pollutant. It can harm plant and animal tissues and it can degrade rubber and plastic.
However, ozone in the stratosphere is beneficial. It blocks harmful ultraviolet rays from the sun, protecting life on Earth.