Last week, a Swiss investigation found evidence to support the idea that Yasser Arafat was poisoned with polonium-210 — a radioactive element that’s safe to carry around in a container, but causes unstoppable death if swallowed. NPR sat down with Deborah Blum, a science writer who specializes in the chemistry of poisonings, to talk about what makes polonium-210 a particularly handy way to off somebody and why it’s so hard to bring a polonium poisoner to justice.