Yesterday, an envelope addressed to Senator Roger Wicker, Republican of Mississippi, was found to contain a white granular substance that was identified as ricin. Today, a similar letter addressed to President Obama was found. These envelopes were intercepted off-site--they never got anywhere near their targets--but as a precaution, Capitol Police have shut down mail service until they can figure out what's going on.

In the meantime, let's talk about ricin!

How poisonous is it?
Oh, man. Very. It's dangerous in just about any way it gets into your system, though ingesting (eating) it is about the least dangerous way. Injecting or inhaling requires about a thousand times less ricin to kill a human than ingesting, and that's a very small amount indeed. An average adult needs only 1.78mg of ricin injected or inhaled to die; that's about the size of a few grains of table salt--which ricin resembles visually.

Castor Seeds: The seeds of the castor plant, from which castor oil (and ricin) are extracted.  Wikimedia Commons

How does it work?
Ricin, a toxic protein, infects cells, blocking their ability to synthesize their own protein. Without cells making protein, key functions in the body shut down; even in survivors, permanent organ damage is often the result of ricin poisoning. It's a highly unpleasant way to be poisoned: within six hours, according to the Center for Disease Control, victims who have ingested ricin will feel gastrointestinal effects like severe vomiting and diarrhea, which can lead to serious dehydration. Then the ricin infects the cells of the vital gastrointestinal organs as they pass through the body, leading to the failure of the kidneys, liver, and pancreas.