WASHINGTON (AP) — Ever since the anthrax and terrorist attacks of Sept. 11, 2001, the poison ricin has at times been lumped in with other bioterrorism agents because it comes from a relatively common plant and seems easy to make.

But the reality is that ricin has created far more scares than victims and is more a targeted poison — an assassin's tool — than something to attack lots of people.

Preliminary tests of a suspicious substance in a letter addressed to President Barack Obama indicates ricin, the FBI said Wednesday. That comes after an envelope addressed to Sen. Roger Wicker, R-Miss., tested positive Tuesday for ricin after being received at a mail processing plant in suburban Maryland. In 2004, ricin was discovered in the sorting area of a mail room in a Senate office building.

Ricin is derived from the castor plant that makes castor oil. What makes it scary is that there is no antidote and it is at its deadliest when inhaled. It is not contagious.

Of all the biological and chemical terror agents, "it is one of the least significant; it is a poison," said University of Maryland bioterrorism expert Milt Leitenberg. 

Leitenberg said he was hard pressed to remember any case when an initial chemical test that showed the presence of ricin actually turned out to be ricin. Nearly every time it is a false alarm. The Centers for Disease Control and Prevention said there's a rapid detection test for ricin that takes 6 to 8 hours, but the more complete test — the ricin toxin test — takes about 48 hours to perform and the availability of cultured cells to do the test could be a problem getting it done that fast. That second test is "considered the best test for determining the presence of ricin," the CDC said in a fact sheet on ricin.

Still, a draft of a 2010 Homeland Security Department handbook lists only one person killed by ricin. And that was a political assassination, in 1978, of a Bulgarian dissident who was injected — via specialized secret-agent style umbrella— with a ricin pellet. The CDC said others have been killed, but also by injection.

People have been poisoned with ricin after eating castor beans, but it is not as well absorbed through the digestive track as it is when it is injected or inhaled, according to the CDC.

The CDC categorizes ricin as a "Class B" threat, which is the agency's second-highest threat level. It ranks behind anthrax, botulism, plague, smallpox, tularemia and viral hemorrhagic fevers.

It can be aerosolized, released into the air and inhaled. The Homeland Security handbook says the amount of ricin that fits on the head of a pin is enough to kill an adult if properly prepared. That's the issue with both the Obama and Wicker letters, where the concern is that the poison somehow gets from the letter into the body, usually by inhalation or by it getting on someone's hands and then into someone's mouth.

People need to put things in perspective, said Dr. Patricia Quinlisk,  the medical director of the Iowa Public Health Department who has served on several federal bioterrorism boards. "Making ricin into something that can be released from an envelope into the air, be the right size to be inhaled and stick in the lungs "is a lot to get right, especially if you are not a bioterrorism specialist and know how to do that. It's not something you can do in your garage."

"It's harder for these things to happen than most people think," Dr. Quinlisk said Wednesday. It isn't as easy as popular online handbooks say it is, Leitenberg said.

The list of ricin terror acts in the Homeland Security handbook includes several people who obtained or made ricin.

"Ricin is best suited for small-scale attacks rather than mass-casualty scenarios," the Homeland Security handbook said. It says the best "route of exposure" is injection into the bloodstream, like in the Bulgarian case.

The Homeland Security handbook also says inhaling ricin is more dangerous than eating it, but "formulating ricin powder to produce the necessary size to be efficiently disseminated via aerosol requires technical skill."

Ricin powder, the handbook says, "could also be delivered to indoor targets via letters or packages."

If a person is exposed to ricin, his or her clothing should be removed and the person should be washed vigorously with soap and water and get medical attention, the Homeland Security handbook says.

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