It is the Alpha particles emitted by the Polonium that can not pass through stuff. The contaminant on the aircraft is Polonium itself, which was presumably on the clothes of some passengers, or worse still in the sweat or other bodily fluids of contaminated individuals.
Also there is no reason why the assassin has to be dead. A drop or two of a Polonium containing solution in his soy sauce for example, and Bob's your uncle. Just don't eat it yourself.