Why are aerosol cans bad for the environment?

They used to be bad for the environment but now the problem has been addressed by propellant changes.

The propellant used to force the aerosol out of the can was a CFC, which damaged the ozone layer. Aerosol cans now use a hydrocarbon gas, nitrogen or air depending on the material to be sprayed. There are still potential environmental and safety issues but the ozone related problems have been addressed.

CFC's are still used in some applications (e.g. asthma inhalers) but the number of these exceptions is limited.