Safety of Polonium Isotope source I saw a YouTube video by a guy demonstrating Geiger Counter use and one of his radioactive test sources was a disk with Polonium. He casually mentioned that this was the poison used to kill that Russian defector (Alexander Litvinenko). I am referring to "antiproton" (see http://www.youtube.com/watch?v=EMGF-nnNdL8 for example).
How dangerous are isotopic sources like that? The NRC summary of the isotope says "If swallowed or inhaled, this form can be fatal in very small amounts." They go on to say "The chance of someone acquiring a lethal dose of polonium-210 is very small. Both the form and amount of radiation in exempt sources mean they would pose no health problem if ingested. It would take tens of thousands of these sources to make a fatal dose."
This would seem to imply that the source the YouTube guy had was so dilute that it is harmless, even if ingested. Is that correct?
 A: Let's suppose you have swallowed one of the Po-210 sources from this student kit. Its activity is 3700 Bq (0.1 μCi). Based on the Table 6 in the meta-study [1], it is probably safe to ingest up to 0.02 MBq/kg of the Po-210. 
This means, that for 80 kg person, it is probably safe to ingest 1.6 MBq of the Po-210, so you "need" to eat approx. 400 of these sources. This is a rather conservative estimate since the Po-210 in these sources is not "free" but firmly attached to some matrix which makes its digestion much less efficient.
[1] Scott, B. R. (2007). Health Risk Evaluations for Ingestion Exposure of Humans to Polonium-210. Dose-Response, 94–122. http://doi.org/10.2203/dose-response.06-013.Scott
A: Here is what wiki has to say about Polonium radiological toxicity:

By mass, polonium-210 is around 250,000 times more toxic than
hydrogen cyanide (the LD50 for 210Po is less than 1 microgram for an
average adult (see below) compared with about 250 milligrams for
hydrogen cyanide[66]). The main hazard is its intense radioactivity
(as an alpha emitter), which makes it very difficult to handle safely.
Even in microgram amounts, handling 210Po is extremely dangerous,
requiring specialized equipment (a negative pressure alpha glove box
equipped with high performance filters), adequate monitoring, and
strict handling procedures to avoid any contamination. ...
The median
lethal dose (LD50) for acute radiation exposure is generally about 4.5
Sv.[71] The committed effective dose equivalent 210Po is 0.51 µSv/Bq
if ingested, and 2.5 µSv/Bq if inhaled.[72] So a fatal 4.5 Sv dose can
be caused by ingesting 8.8 MBq (240 µCi), about 50 nanograms (ng), or
inhaling 1.8 MBq (49 µCi), about 10 ng. One gram of 210Po could thus
in theory poison 20 million people of whom 10 million would die. The
actual toxicity of 210Po is lower than these estimates, because
radiation exposure that is spread out over several weeks (the
biological half-life of polonium in humans is 30 to 50 days[73]) is
somewhat less damaging than an instantaneous dose. It has been
estimated that a median lethal dose of 210Po is 15 megabecquerels
(0.41 mCi), or 0.089 micrograms, still an extremely small amount.

