Why are some dentures radioactive?

["Bjorn Cedervall" <bcradsafers@hotmail.com>]

Radsafers,

I tried to send the text below when it arrived a few days ago. For some 
unknown reason (probably my fault) it must have failed:

Bjorn Cedervall        bcradsafers@hotmail.com
---------------
   The Learning Kingdom's Cool Fact of the Day for January 19, 2000
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                  Why are some dentures radioactive?
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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).

More about sources of radiation in the human environment:
http://www.em.doe.gov/fs/fs1f.htmlQuestions and answers about dentures:
http://www.ada.org/consumer/faq/bridges.htmlAnother Cool Fact about 
dentures:
http://www.cool-fact.com/archive/1999/03/11.html
A Cool Fact about naturally radioactive people:
http://www.cool-fact.com/archive/1999/12/17.html
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