To the people worrying about airport security - if you put some of this stuff in your eye drop bottle, without getting any on the outside, then screwed the lid on it, there would be nothing to detect. Alpha particles as mentioned several times, cannot escape a container.
Also, Alpha particles are only harmful if emitted from inside the body. Unless you have somehow got the Polonium from a contaminated seat, into your body then you are at zero risk. If you have managed to get it in your body - perhaps by licking the seat or something, I'd say the dosage is likely to be too low to do any significant harm. (This is based on my undertstanding that the seat contamination has simply come from the sweat of contaminated individuals). Yes the stuff is harmful even at very very low doses. But the amount secreted in a bit of sweat is going to be a very very tiny amount of an already very very small dose.
Incidentally, you breath in Radon every time you sit watching TV in your lounge. Radon decays by Alpha emission too.
Also, just over 10% of cosmic radioation is made up of Alpha particles. Mind you, they don't even make it as far as the aircraft. It is the protons, and more seriously the Beta particles and Gamma rays you have to watch out for. Cosmic radiation is a subject that is taken far too lightly by the airlines in my view, and yet we as pilots and passengers don't seem to worry about it too much either. But we certainly should.