Good and bad ozone

Ozone occurs naturally at low concentrations throughout the Earth's atmosphere. In the stratosphere, which extends from about 10 to 50 km above the Earth's surface, ozone is 'good' as it protects life on Earth from the sun's ultraviolet rays.

This is why the world is trying to stop the manufacture and use of substances that damage the stratospheric ozone layer.

In the troposphere, near the Earth's surface, human activities lead to ozone concentrations several times higher than the natural background level. Too much of this ground-level ozone is 'bad' as it is harmful to breathe and also damages vegetation.