Head light discolour
 

What causes the headlights , mainly on older xc's to 'fog over' or 'degrade'?
I was of the understanding that the sun over a period of time causes the plastic to become faded and unclear- so here's my question, when visiting mallorca i noticed quite a few xc90's both early and late versions, all of them had as new crystal clear headlights.
They get far more sun than we get and at a much greater heat, so why do so many of early xc90's here that you see around have faded lights,(i say many but not all?)

any thoughts?

My thoughts are that it's to do with winter weather and the crap on the road hitting them say that I saw a Audi yesterday who's whole headlights were cloudy and I would be very surprised of they passed it's next Mot.