Ricin scare: What makes the substance so potentially deadly? The U.S. Capitol Police announced on Wednesday that they had intercepted three suspicious letters this week, one of them addressed to President Obama, which may contain the poisonous substance ricin. In addition to the letter sent to Mr. Obama, two letters being tested for ricin were sent to Sen. Roger Wicker, R-Miss. At least one of the letters addressed to Wicker tested positive in a preliminary field test for ricin. Field tests are regularly inaccurate; therefore, the FBI is conducting more thorough tests on the letters. Ricin is a poison that is found in castor beans. It can be made from the waste produced when castor oil is made. It can be made into a mist, pellet, or powder and dissolved in water or weak acid. While it is still dangerous when it consumed, it is most deadly when it is inhaled because the body doesn't have stomach acids available to help diminish the effects. Ricin scare: FBI testing letter sent to Sen. Wicker The poison remains stable under most temperatures, but becomes deactivated when it is placed in temperatures above 176 degrees F. No antidote exists. Dr. Kenneth Spaeth, director of the Occupational and Environmental Medicine Center in the department of population health at North Shore University Hospital in Manhasset, N.Y., told CBSNews.com that deaths from ricin poisoning are very rare, but animal tests have shown just how fatal the substance can be. "Fortunately, it's not something that we see all the time," he explained. But, it does happen. The fact that it's naturally made and extracted from a plant means that there is access. Spaeth, who wrote "Bioterrorism Sourcebook," which details the effects of many poisons, has not advised authorities on the current ricin investigations. When ricin enters a person's system, it gets inside their cells and stops them from making the proteins the body needs. This causes cell death, which eventually leads to other symptoms. Typically, ricin can be very damaging to the lungs and make breathing very difficult if it is inhaled. It can cause asthmatic symptoms, chest tightening, shortness of breath and death. If it is ingested, ricin causes less severe symptoms but still toxic effects. This includes nausea, vomiting, diarrhea and stomach pain. Symptoms can appear as early as four to eight hours after contact and as late as 24 hours when ricin is inhaled. Subjects will typically feel effects in less than 10 hours upon consumption. It doesn't happen very often, but for what it's worth when it is severe, death typically happens two to three days later. There can be long term effects, like breathing problems. Before the letters, ricin made its way into pop culture as one of the substances Walter White makes to poison an enemy on the AMC TV show "Breaking Bad." "On the show, he is a very knowledgeable and great chemist, but in real life, it doesn't involve that much sophistication," Spaeth admitted. Spaeth added that it's very hard to weaponize ricin for a mass target because it requires specific technology, but instructions on how to make the poison are readily available on the Internet. The amount that is probably contained in the letters suggests that the amount of ricin being produced in these cases is on a smaller scale, but still dangerous. From what's there (in the reports), there's certainly enough. Health effects are possible from the amount that's inside an envelope," he cautioned.