Is Your Lens Radioactive?

Short answer: Probably not.

Unless you own a decrepit, fungus farm of a lens that was made from the 1940's to the 1970's, you're most likely in the clear. In the case you do have some really old lenses lying around, mainly made by Kodak, you might want to have this checked out.

Kodak Aero-Ektar Lens

Many lenses from that time used Thorium Oxide as a component for their glass because it had the high refraction and low dispersion properties similar to Florite which is used today. Although it was a good material to make cheaper, low dispersion lenses with less chromatic aberration, there's a reason why they don't use it anymore...Thorium Oxide is radioactive. Now does this mean that using the lens will give off enough gamma radiation to make you the Incredible Hulk of photography? Typical radiation levels can reach 10 mR/hr on the surface, which is similar to your average x-ray. No Hulk smash for you. That goes without saying, any amount of radiation isn't necessarily good for you. Although most smaller lenses made with thorium are typically harmless, some eye pieces were also made of the same material. Having a radioactive substance in such close proximity to an eye could lead to cataracts and other generally bad things with long term exposure.

Might want to keep that away from your head buddy.

The biggest manufacturer of thorium lenses was Kodak who produced them until the 60's. Their Aero-Ektars are probably the most well known; being mounted on World War II bombers to take aerial photographs of Europe and then becoming popular with the photojournalist crowd because of their fast apertures. Although state of the art at the time, the radiation unfortunately causes the lens to take on a brownish shade, making them not as fast as they used to be. Could say that about a lot of older things.  Although semi-hazardous, you can still find them if you look hard enough. One photographer recently got their hands on a big ol' radioactive lens and created a focusing adapter to attach it to a GH2. They even used a 3D printer to create the lens mount! That's a lot of dedication for some heavy vignette ;)

Fortunately for you, lenses aren't made of radioactive material anymore. Your behemoth smart phone on the other hand might be something to be concerned about...