Not fogging. Lens elements made of thoriated glass eventually yellow. They're still clear. The yellowing can be removed by letting the lens bask for a while, sometimes a long while, under a UV lamp.
Originally Posted by Tony D
Aero Ektars aren't the only lenses that contain radioactive elements. I've cleared three TTH tessar type process lenses and one Repro Claron with a compact 20w UV-B fluorescent. The process took nearly a month with that dim little bulb. Some people have reported quicker clearing by setting lenses to bask in the sun. I was reluctant to do that because of the crud that falls from the sky.