Earlier this week, the U.S. Post Office intercepted letters addressed to Sen. Roger Wicker (R–Miss.) and Pres. Barack Obama that contained a mysterious white powder. The substance turned out to be ricin, a deadly toxin that can kill within days. But just how dangerous were these attacks?

Since the Obama administration was first warned about the dangers of new ricin attacks in 2010, it has requested periodic updates on the white, powdery substance—from where it is being produced to the places it’s being shipped. In 2011 U.S. counterterrorism officials received word that al Qaeda’s affiliate in Yemen was making efforts to get large amounts of castor beans, the plant source from which the toxin is produced. That same year four American men—Frederick Thomas, Dan Roberts, Ray Adams and Samuel Crump—were arrested for plotting to poison hundreds with ricin as well as blow up government buildings.

The biology of ricin may explain the compound’s popularity with terrorists. A protein with two chains, ricin targets human ribosomes, the protein-makers of human cells. The first chain binds to ribosomal walls, breaking them open; the second enters the ribosome and stops protein synthesis. Without protein, the body loses its ability to function. As the major structural component of all cells in the body, protein is necessary to form blood, regulate body functions and conduct other duties essential for life. Virtually all bodily hormones are proteins—for instance, insulin, central to regulating carbohydrate and fat metabolism, and progesterone, involved in pregnancy and the female menstrual cycle, are proteins.

For ricin to have any hazardous effect, however, it first must enter the bloodstream. One way is via direct injection, which is how Bulgarian dissident journalist Georgi Markov was assassinated in 1978, via a shot from a micro gun apparently hidden in an umbrella tip by a member of the Soviet intelligence agency, the KGB. Less than two milligrams of the protein, in its purified form, could kill. If not injected, ricin may be ingested (eaten) or if sufficiently powdery, inhaled.

Only a small amount of ricin is needed to kill. “When it’s ingested along the gut, a little goes a long way,” says Dan Brown, a nutritional toxicologist at Cornell University. Whereas it would take consuming eight whole castor beans to kill an adult, 2,000 milligrams of the purified protein would be lethal if ingested.

If ricin is simply placed in an envelope where it could be inhaled, however, Brown says the poison “isn’t much of a weapon.” That stands in stark contrast to anthrax, which, as bacteria, can be lethal when inhaled; in 2001 five people were killed by weaponized anthrax placed in mailed envelopes. Ricin, as a plant-based compound, has little potential for biological warfare, Brown says: The toxin would have to be inhaled directly and for a long enough period to result in harmful exposure.

In the case of the letters received by off-site White House and Senate mail rooms this week, Brown says the letter openers would have had to place their faces almost directly into the envelopes while opening them to experience negative effects. “Either that, or there would have to be a pretty big puff,” he adds.

If anyone should be concerned about the public health effects of ricin, Brown says, they should be focused on ingestion. According to the U.S. Centers for Disease Control and Prevention, swallowing more than 2,000 milligrams of ricin could lead to death in as few as 36 hours. Once a person begins vomiting and experiencing diarrhea, which would dehydrate the body and bring down blood pressure to dangerously low levels, the victim’s liver, spleen and kidneys would stop functioning.

In developing countries, where people use castor beans for cooking and as jewelry, the threat is particularly acute, Brown says. Although a castor bean, when whole, could theoretically make it through the entire gastrointestinal tract without causing damage, most are either chewed or punctured (for the purposes of stringing the bean on a necklace). The poison could then enter the gut, where it would overwhelm the body’s barriers to toxins and wreak havoc.