Thanks for all of your comments and questions in response to David Doss' post previewing tonight's special report on the murder of the former Russian spy. We've spent the past few days researching the medical angle of this story. Here is some of what we've found:

First off, polonium-210 can be found in harmless trace amounts everywhere. It naturally occurs in the earth's crust -- in rocks and minerals, even in our own bodies. If any of us took an alpha radiation detector and put it right next to our skin or if we took a urine test, we would most likely test positive for trace amounts of polonium-210. It's that common in our environment. That may sound frightening, but it's important to remember that in order to make it dangerous, you'd need a large amount.

As Dr. Robert Whitcomb, a Centers for Disease Control and Prevention health physicist told us: "In the toxicology world, dose makes the poison."

So, how much was used? Well, far more than what naturally occurs in the environment, but an amount still invisible to the naked eye. We're talking micrograms here. To best see it, you'd need a very powerful microscope.

"We can't see it, feel it, sense it," Whitcomb says.

You could make polonium-210 a few different ways: By extracting it from rocks containing radioactive uranium or by chemically separating it from radium-226. But, in order to get an amount that would be lethal, you'd have to produce it in a nuclear reactor laboratory. Hence, there are only a handful of people and places in the world capable of making polonium-210 such that it could poison someone.

What kind of threat does polonium-210 pose to the general public? The good news is that it poses very little danger. In fact, our skin provides natural protection from polonium-210. You'd have to sniff it or eat it in order for it to pose a health danger. You see, polonium-210 is an alpha emitter, the weakest form of radiation. Alpha radiation waves can be stopped by a piece of paper.

What kind of danger did the assassin face? The fact is the assassin risked very little by carrying the poison. The person could have carried polonium-210 wrapped in plastic or in a vial and been completely safe.

Why use it as poison? How long did it take doctors to determine it was polonium-210? These are just a few of the additional questions we'll try to answer in tonight's report. Please keep your questions and your comments coming. It's a medical mystery in the truest sense.