According to the Environmental Protection Agency, chlorofluorocarbons (CFCs) and other complex chemicals make their way into the upper reaches of the stratosphere where they decay and release chlorine and bromine atoms that destroy ozone. The chemical reactions caused by these substances break apart the ozone atom, removing its protective capacity and increasing the amount of ultraviolet radiation that can pass through the Earth's atmosphere.
One of the reasons CFCs and other ozone-destroying substances are so dangerous is that they're extremely stable molecules that can last for years or decades. When these gases are released at ground level, it potentially takes many years for them to make their way through the atmosphere into the upper reaches of the ozone layer. There, the increased energy from the sun helps to break apart these complex molecules, creating free chlorine and bromine to react with the ozone molecules. A single molecule of chlorine can destroy as many as 100,000 ozone molecules during its lifespan.
In 1987 the Montreal Protocol banned the use of CFCs and other ozone-depleting substances, vastly reducing their output in the countries that signed the treaty. Unfortunately, the long-lived nature of these molecules means that a reduction in the upper atmosphere is only becoming apparent decades later, and it may take still more time for the ozone layer to recover.