Ozone is a gas that occurs in the atmosphere. It is formed in the stratosphere by the action of ultraviolet radiation from the sun on oxygen. The ozone molecule contains three atoms of oxygen. It is also formed near the Earth's surface in chemical reactions caused by man-made pollution. Ozone absorbs a part of the ultraviolet radiation from the sun, thereby reducing the potentially dangerous UV-B radiation reaching the surface of the earth.

Ozone is a strong oxidising agent. It will destroy any organic material on contact by oxidising the molecules of the cells. This property is often used to sterilise drinking and swimming pool water. Inhaling ozone can be harmful to mammals in small doses and toxic in large doses. In the lower atmosphere ozone is one of the detrimental components of photochemical smog that is produced as the result of pollution (Bad ozone). However, in the stratosphere, ozone absorbs ultraviolet radiation and protects the environment against the detrimental effects of the UV-B radiation (Good ozone). The radiation emitted by the sun contains an ultraviolet component. This covers the range with wavelengths from 100 to 400 nm and is divided into three bands: UV-A (315 - 400 nm), UV-B (280 - 315 nm) and UV-C (100 - 280 nm). As the sunlight passes through the atmosphere, no UV-C and approximately 10% of the UV-B is transmitted. They are absorbed mainly by ozone and oxygen. UV-A radiation is less affected by the atmosphere. Therefore, the ultraviolet radiation reaching the Earth's surface is composed of mainly UV-A with a small UV-B component. A decrease in the concentration of ozone in the atmosphere results in increased UV-B levels. DNA and other biological macromolecules absorb UV-B and can be damaged in the process.