Millions of people wear some type of dentures (false teeth). Some of them, especially the oldest ones, are made out of porcelain that includes a trace of uranium, a radioactive element.
The uranium was added to these old-style dentures because it flouresces (glows in response to certain wavelengths of light), giving the dentures a bright, white look. Without it the porcelain teeth would appear dingy and greenish under artificial light.
A full set of porcelain dentures releases about 1,500 millirems of radiation in a year, which is about four times the natural background level. Other factors that can lead to similar levels of exposure include living at a high altitude (from cosmic rays) and smoking (from elements in the smoke).
Source: The Learning Kingdom’s Cool Fact of the Day for January 19, 2000