SAN FRANCISCO — Ricin, the toxin that preliminary tests show was discovered on a letter addressed to President Obama, is readily available, may be deadly in tiny amounts if properly prepared and can kill in 72 hours.

While it’s not easy to convert the substance into a useable poison, its attraction is its availability, said Jim Romagnoli, the vice president of emergency management at the North Shore- LIJ Health System in Great Neck, N.Y.

“This is a tricky thing for an amateur, although amateurs have done this with small amounts in the past,” Romagnoli said in a telephone interview. Ricin is found in castor beans, he said, and is part of the waste produced by making castor oil, so it’s “easily available.”

If the poison is partially purified or refined, it can be used in air, food or water, according to the Centers for Disease Control and Prevention. It works by preventing a person’s cells from making necessary proteins, thereby killing the cells.

“You must inhale or ingest it,” for the ricin to be poisonous, Romagnoli said. “You don’t get sick just from touching it.”

To process ricin from mash, it must first be dried into a powder. A powdery substance believed to be ricin was found on letters to the White House and Sen. Roger Wicker, R-Miss.

The lethal dose depends on how the ricin is processed and how people are exposed.

Inhaled or injected doses of as little as 3 to 5 micrograms per kilogram of body weight may be lethal, according to a December 2010 report from the Congressional Research Service. That translates into as little as 240 micrograms to 400 micrograms for a 175-pound person.

Those dosages are extrapolations of animal studies, which varied from animal to animal, said Eric Toner, a senior associate at the Center for Biosecurity at the University of Pittsburgh Medical Center. “If you had a pure, concentrated formulation and you could aersolize it, that dose is true. But those two conditions are really important. A state-sponsored program could do this, but I don’t think the average terrorist could.”

In 1978, a tiny pellet of ricin was used to assassinate Georgi Markov, a Bulgarian dissident, in London when it was injected into his skin using the tip of an umbrella.

Initial symptoms of poison from inhalation occur as early as four to six hours after the exposure, and include difficulty breathing and a cough. The symptoms progress rapidly to fluid within the lungs and respiratory failure. Deaths from the poison usually happen within 36 to 72 hours, according to the Congressional Research Service report.

While no antidote exists, doctors can counteract the effects of the poisoning by helping victims breathe or giving them fluids.

“It’s likely being used to terrorize people,” Toner said. “It’s an effective way to produce havoc and get a lot of attention, but not an effective way to kill people. Of course whether the perpetrator knows this or not, I don’t know.”