In order to halt the depletion of the ozone layer, countries around the world have banned the use of chlorofluorocarbons and other ozone-depleting substances. These compounds produce chlorine and bromine atoms high in the atmosphere, and these atoms react with ozone, destroying it.
When ozone depleting chemicals are released into the atmosphere, they eventually work their way into the upper layers. There, ultraviolet radiation breaks down their chemical bonds, converting the complex molecules into their component atoms. Chlorine and bromine are the most dangerous elements to the ozone layer, because they can break apart the bonds between the oxygen molecules in ozone.
Most countries banned the use of CFCs and other ozone-depleting chemicals by 1996. Unfortunately, the lifespan of these substances in the atmosphere can be measured in decades, and a single atom of chlorine can destroy thousands of ozone molecules before it breaks down. The concentration of these dangerous chemicals in the atmosphere has fallen since the ban took effect, but it may take a considerable amount of time for the ozone layer to fully replenish itself after the effects of pollution in the 20th century.
The ozone layer at the Earth's poles has become weaker than the layer around the rest of the planet. This has created a hole in the ozone layer over the Antarctic region that occasionally expands to allow ultraviolet radiation levels to increase to potentially harmful levels in South America.