Last year in a Intro to Radiation Oncology lecture I was sitting in on, a physics professor at my school said that all a terrorist (or otherwise bad guy) would need to do to kill or at least harm a bunch of people is take a small amount of radioactive material and hide it somewhere in a desk. He said many people in whatever offices this was done in would eventually get sickened or killed by this little bit of material from coming near it every day when they went to work. I'm wondering if he was completely full of it, or if this is completely true, or anything in between. If it is, how wide a radius would the danger zone be, depending on the specific radioactive material? Please fill free to specify the specific radioactive atoms and the distances to them you think would be dangerous in this way. As for the reason why I'm wondering, part of it is curiosity, but part of it is I'm writing some fiction and I was thinking about using something like this. But I don't want to put it in if there isn't some legitimacy to it. Thanks to all replies!