"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."
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.
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.