Ozone is a gas that is present in small amounts in Earth's atmosphere. There are about three molecules of ozone for every 10 million air molecules. Depending on its location in the atmosphere, ozone has very different roles. Ozone near the ground is often called “bad ozone”, while ozone in the stratosphere is called “good ozone”.

In the troposphere, ozone is a pollutant. It can harm plant and animal tissues and it can damage rubber and plastic.

However, ozone in the stratosphere is beneficial. It blocks harmful ultraviolet rays from the sun, protecting life on Earth. Without the ozone layer in the stratosphere, more of the Sun's UV light would penetrate the atmosphere and would reach the Earth's surface.