This week, federal authorities announced that the notoriously lethal poison ricin had been found in two letters - one addressed to President Obama and the other to U.S. Senator Roger Wicker (R-Mississippi). According to news accounts, the letter to Wicker was postmarked in Memphis, Tennessee and sent by someone who often relayed dissatisfaction to government officials. And the letter to Obama was reportedly postmarked in a similar fashion.

Both letters were caught in post office screening facilities and neither came close to the intended target. The Federal Bureau of Investigation is currently looking into reports of additional suspicious letters sent to government facilities. But my interest here is less focused on what we all hope will be a fast-moving and fast-concluding government investigation.

My interest instead is in the poison used and its rather notorious history.

Ricin is a naturally occurring poison - another reminder that some of the most toxic compounds we know derive not from industrial processes, but from plants ginning up some poisonous chemistry to defend themselves from predators. Technically classified as a glycoprotein, ricin is found in the seed of the castor oil plant (native to the Mediterranean region), which is widely grown for medicinal and other purposes. Ricin is, in fact, a common waste product in castor oil processing.

The plant is common enough in the wild, though it's been studied as a threat to grazing animals and to pets who like to occasionally nibble on leaves. A European report on ricin as an animal feed contaminant notes that cattle tolerate it pretty well, that sheep and horses get very sick, and that dogs suffer a nasty combination of lethargy and vomiting.

For people, ricin is very deadly - especially if injected or inhaled. A 2009 analysis, for instance, on "Ricin as a weapon of mass terror," pointed out that it's slightly less dangerous when swallowed, where it tends to cause damage to the mucusol lining of the stomach but is not absorbed as effectively. This would suggest that mailing it - raising the possibility that ricin particles might be inhaled by someone opening envelope - would lead to a very effective poisoning. But as the same study notes, inhalation poisoning is also tricky because the ricin particles must be at a uniquely tiny size for this method to work well. According to Wikipedia, which has a very detailed entry on ricin (with some solid citations, which is why I'm referencing it) the U.S. government considered using ricin as a chemical weapon in World War I, with the goal of generating a poisonous cloud of ricin dust. That turned out not to work.

None of this diminishes the fact that ricin is a murderous compound, a cell-destroyer; as the CDC notes, it kills effectively by any method of delivery. It binds to mammalian cells, it breaks down their ability to synthesize proteins, and it kills through the resulting cellular death. A review of its cytotoxic properties described ricin like this: "It is exquisitely potent to mammalian cells, being able to fatally disrupt protein synthesis by attacking the Achilles heel of the ribosome."

The purified toxin from a single castor bean, according to some accounts, is lethal enough to kill at least a thousand people given an effective delivery system.

This, of course, is why the U.S. government considered it as a chemical agent during World War I and, as the CDC also notes, during World II. It was most famously adopted by the Soviet Union's spy agency, the KGB, during the Cold War. The case that caught worldwide attention was the 1978 murder of a Bulgarian dissident, Georgi Markov, who was killed when a "weaponized umbrella" shot a ricin pellet into his leg while he walked down a busy London street. The KGB was later identified as the source of the pellet.

But, and we see this with letter mailing issues, spy agencies are not needed to acquire ricin. A comprehensive review of ricin pois0ning by the CDC in 2005 noted: "The ease with which the native plant can be obtained and the toxin abstracted makes ricin an attractive weapon." The CDC report followed a series of ricin-laced letters -- sent to government officials and postal authorities -- in 2003 and 2004. Despite a $100,000 reward offered by the FBI, no arrest was ever made in that case.

It has turned up, occasionally, in mysterious ways, such as in a Las Vegas hotel room. But the earlier cases - and those this week - illustrate a couple of key points. Ricin is easy to acquire, yes. But we are very aware of that, standard tests are in place for it, and we are efficient at detecting it. In mailings, as occurred this week, the letters were flagged at processing places far from the named targets. The primary risk was to workers in those places. And they did not die - again making the point that while ricin is undoubtedly dangerous, we know how to protect ourselves against it. In fact, we've also improved our ability to detect ricin in very small amounts, both inside and outside a body.

So today's events also remind us that we've learned to smarter about poisons like this. That we've learned to keep our guard up. That we should be grateful that such systems work - that no one died from ricin poisoning this week. And that - although I doubt we need this reminder - there will be no letting down of our guard anytime soon.

Image: ricin structure/Wikipedia