Here's why:

* CFCs are very stable: They don't break down easily in the lower atmosphere.

* They reach the stratosphere: Where the ozone layer is located.

* UV radiation breaks them down: This releases chlorine atoms.

* Chlorine atoms destroy ozone: They catalytically break down ozone molecules, each chlorine atom destroying thousands of ozone molecules.

While CFCs were the primary culprit, other ozone-depleting substances (ODS) include:

* Halons: Used in fire extinguishers.

* Methyl bromide: Used as a fumigant.

* Hydrochlorofluorocarbons (HCFCs): Used as refrigerants and in aerosols.

Thanks to international agreements like the Montreal Protocol, the production and use of these substances are being phased out, leading to a slow but steady recovery of the ozone layer.