Polonium is an element and has been linked to the deaths of everyone from Yasser Arafat to Russian dissidents. The most common form is 250,000 more toxic than hydrogen cyanide. It's radioactive and causes damage by the release of alpha particles (they don't get along with organic tissues). Alpha particles can't pass through the skin, so polonium needs to be ingested or injected into the victim. However, if that happens, it doesn't take much. One theory states that a gram of polonium 210 could kill as many as ten million people if injected or ingested, via radiation poisoning or cancer later on. Let's all be glad it's not on Amazon Prime...