LOL, all those people who are worried about radiation of thorium show that they don't have the slightest clue how radiation works. Take a geiger counter and measure the inside of your house, then measure the lens, It might not even be detectable trough the background radiation you receive every single moment of your life. Also, the particles it emits are Alpha and Beta particles, while Beta particles are a little more powerful they cannot go trough things like thick lens glass, nor can the alpha's. Both particles are only dangerous when they float freely in the air as dust.
Keep in mind that people can have far more dangerous substances in their homes. Remember those old vintage wall clocks that glow in the dark?? well if they are old enough (like WWII area or just beyond that) they most likely contain Radium, and that dear people is a real Gamma emitter!! Far more powerful and dangerous then Thorium used in lenses. I have 2 tabs (which they used to place above the light switches in the 1960's to see the switches in the dark) filled with Radium and while the emissions don't go very far (I use them to test Geiger counters that are part of my NBC collection) I don't want that to float around in the form of dust (the Radium in the tabs I have are safely sealed in Plexiglass). The most dangerous part of radioactivity is when it's released in the form of dust, then it can get everywhere, when it's in the glass it can go nowhere.