[dmatre]

Not only UV will degrade tires, but ozone as well.

Storing the tires in plastic bags reduces or prevents ozone degredation.

Ozone is produced by the arcing in any electric motor (think garage door opener, air compressor, refridgerator/freezer, PC polisher, etc).

While ozone will still be produced, having the tires in plastic will prevent the ozone degredation. (plastic doesn't have to be sealed, as there is very little airflow through a folded or rolled plastic bag).