Can a Leica Summitar Be Restored?

[reddesert]

Not all radiation is the same. Both the energy and the type of particle matter; particle type matters because of the penetrating power - or lack thereof. The alpha particles from thorium are easily stopped by your skin, as others said up thread. For this reason owning a thoriated glass lens isn't particularly risky. (Reference for does estimates: https://www.orau.org/health-physics.../products-containing-thorium/camera-lens.html ) But modifying it in such a way that you could ingest some of the glass by breathing (such as polishing, grinding, breaking it) is not a good idea.

The reason radon gas is of concern in people's houses is because it is a gas, and thus can migrate out of the soil into your house air, at some low but measurable concentration, and be breathed in. And people spend a lot of time in their houses so both the concentration and exposure are significantly higher than they are outdoors.

Radiation and carcinogen risk are generally cumulative - for ex, radon exposure is much worse for people who are already smokers. So "There's radon in basements anyway so it doesn't hurt any more if I grind on a radioactive Summicron" is not a valid argument. Just use the Summicron (which I realize is a different lens from the OP's lens), but don't polish it.

Disclaimer: I am a physicist, but not a health physicist.