Here is what wiki has to say about [Polonium radiological toxicity][1]:

> 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.

  [1]: https://en.wikipedia.org/wiki/Polonium#Biology_and_toxicity