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srt180 I was a Chemistry & Radiological Controls officer in the USN in my youth. Thoriated glass was not allowed in eyepieces for microscopes, binoculors, etc...becasue of proximity to the cornea of the eye. This is one place where low kinetic highly ionizing ALPHA particles can cause damage from outside of the body. Of course, ingestion of ALPHA is deadly in sufficient quantities. It is highly unlikely that either is a problem if one uses the lens as it is intended. Ingestion of soluble thorium from glass is not realistic. Neither is walking around with the rear element next to your eyeball for days on end. As long as the lens is attached to the camera body or protected with an end cap, this should be no issue in terms of dangerous levels of exposure. Alpha's are stopped by the dead layer of skin on a body's surface. Gamma is also emitted from one of the decay chain daughters of Th-232, specifically Thallium 208. However, it is a relatively low energy gamma. Also, some Beta, but again...beta is shielded by the camera body, other lens glass that is not thoriated, etc. Beta can be stopped by a sheet of alu foil. Lastly, you will get very high reads right next to the surface of the rear element. However, read from a foot distance and the counts will drop significantly. This is because the Alphas will not travel very far away from the rear element before being attentuated by the air space. I own two Super Taks (50/1.4, 55/1.8), which are great lenses IMO.