I see y'all don't have too much to say on this one.
Polonium 210 is highly radioactive and very toxic. By weight, it is about 250 million times as toxic as cyanide, so a particle smaller than a dust mote could be fatal. It would also, presumably, be too small to taste.
Polonium is extremely rare in nature. Named by its discoverer, Marie Curie, after her native Poland, it occurs in trace amounts in uranium ore and has been found in minute quantities in plants like tobacco, ...