The ozone layer is a band of ozone gas in the stratosphere, roughly 15 to 35 kilometers above the surface. It absorbs most of the Sun's ultraviolet radiation. Without it, life on land would be impossible. The layer is thin. If compressed to surface pressure, it would be only a few millimeters thick.

Ozone forms when ultraviolet light splits oxygen molecules. The freed atoms recombine with other oxygen molecules to form ozone. The process is continuous. Ozone is also destroyed naturally by reactions with nitrogen and hydrogen compounds. The balance keeps concentrations steady.

Human-made chemicals disrupted that balance. Chlorofluorocarbons and similar compounds released chlorine and bromine that destroyed ozone faster than it could form. The Montreal Protocol phased out these chemicals. The layer is now recovering. Scientists monitor it from satellites and ground stations. The ozone layer remains a global commons that requires continued protection.

Key facts

• Located in the stratosphere
• Absorbs ultraviolet radiation
• Forms and breaks down continuously
• Damaged by CFCs and halons
• Recovering under the Montreal Protocol