I recently learned of this as well. I had no idea until someone mentioned it in conversation.
Some of the info I've read said that they can be up to 3mR/hr, which as I understand it means that the lens would have to be in your pocket, 8 hours a day, 40 hours a week, for 6 months straight before you'd have the recommended annual "dose".
Don't hold that lens close to your jewels if you want kids
"Photography, like surfing, is an infinite process, a constantly evolving exploration of life."