Here's why:
* CFCs are very stable: They don't break down easily in the lower atmosphere, allowing them to reach the stratosphere where the ozone layer resides.
* UV radiation breaks them down: Once in the stratosphere, ultraviolet radiation from the sun breaks down CFCs, releasing chlorine atoms.
* Chlorine destroys ozone: These chlorine atoms act as catalysts, destroying ozone molecules in a chain reaction. One chlorine atom can destroy thousands of ozone molecules.
While CFCs have been largely phased out due to international agreements like the Montreal Protocol, they linger in the atmosphere and continue to damage the ozone layer.
