As police continue to investigate the death of UPMC neurologist Autumn Klein, toxicology experts noted that cyanide poisoning deaths, while gruesome, are extraordinarily rare.
John Trestrail, a pharmacist who specializes in studying criminal poisoning cases, said that in his database of known poisoning homicides, cyanide made up only 8 percent of the cases, and that over a 20-year period in the United States, all types of poisoning comprised less than two-tenths of 1 percent of total homicides.
Cyanide is used for suicides more often than homicides, he and other experts said, but even there, one 10-year study in New York City found that out of about 6,500 suicides, only 17 involved cyanide.
How cyanide kills
Cyanide prevents cells from using oxygen
The ability of oxygen to access the cytochrome oxidase enzyme (present on the mitochondria inside cells) is essential to normal, life-sustaining cellular respiration
Cyanide (CN) poisoning may disable the body's ability to use oxygen so it can be fatal despite the amount of oxygen available to the body.
Because CN prevents cells from using oxygen, supplemental oxygen alone may be insufficient to treat CN poisoning9
Moderate to high concentrations of CN can cause severe injury or death within minutes.
In massive acute CN poisoning, the mechanism of toxicity may involve other enzyme systems.
CN poisoning may also cause central nervous system side effects including intellectual impairment, Parkinson-type effects, and personality changes.
While some cyanide deaths in the U.S. come from workplace exposure and by smoke inhalation during fires, deliberate poisonings require people to gain access to what is usually a closely guarded substance.
Stephen Borron, an emergency medicine and toxicology professor at Texas Tech University Health Sciences Center, said cyanide "is certainly something that would be carefully kept out of reach of those who don't work with it every day. It's not something the average neurologist would find working in the neurology clinic, for instance."
On the other hand, Dr. Borron said, it is possible to buy cyanide compounds on the Internet without a huge amount of difficulty. He noted a 2003 case in which a Maryland teenager ordered cyanide on the Web, saying he wanted to use it to plate metal, and then spiked a romantic rival's soft drink with it, killing him.
While cyanide can kill a person within minutes, that is not automatic, he said.
In many cases, if patients can get to a hospital emergency room within two hours of being poisoned, they can be saved with an antidote such as Cyanokit, a substance that binds to cyanide in the bloodstream and converts it into Vitamin B12.
In one study he conducted in 2006 with French physicians, Dr. Borron showed that out of 11 patients who received potentially lethal doses of cyanide, the antidote saved seven.
Chemically speaking, cyanide is any compound that has a certain pair of carbon-nitrogen atoms, and it is found naturally in peach, apricot and nectarine pits as well as apple seeds. It also shows up in cigarette smoke in the form of hydrogen cyanide, the same gas that was used in the Nazi death chambers.
Our bodies have natural defenses against cyanide in the form of an enzyme that can turn it into thiocyanate, a safe compound. But when someone takes enough cyanide to cause acute poisoning -- about 250 milligrams, or less than a hundredth of an ounce -- it overwhelms the body's ability to convert it.
At that point, cyanide invades the cells and shuts down their ability to use oxygen, suffocating the brain and heart.
When pathologists suspect someone has died from cyanide poisoning, Dr. Borron said, the best way to detect it is in blood samples, but they have to get them quickly, because half the cyanide in blood is gone within an hour. It can also be found in special tests of heart, lung and spleen tissue, he said.
Some doctors also are able to sense the presence of cyanide by its bitter almond smell.
But Mr. Trestrail noted that only half the population is born with the genes that allow them to smell that aroma. "And I'm not one of them," he said.
Correction, posted May 3, 2013: An earlier version of this article listed the wrong state for the teenager accused of poisoning a romantic rival with cyanide.
Mark Roth: firstname.lastname@example.org or 412-263-1130.