While European authorities track down how former Russian spy Alexander Litvinenko acquired a lethal dose of the radioactive substance polonium-210, the rest of us go about our normal routines, not realizing we are taking in tiny amounts of the substance all the time. The people who take in the most are smokers and aficionados of reindeer meat and liver.
"It's the most lethal element in the periodic table," says toxicology professor Patricia Thomas of the University of Saskatchewan in Saskatoon, Canada. So it doesn't take a lot to be dangerous. The Health Physics Society estimates a lethal dose to be about a millionth of a gram, says health physicist Robert Whitcomb Jr. of the Centers for Disease Control and Prevention in Atlanta. That's about 100 times lighter than one sugar crystal.
Yet trace amounts of polonium-210 are everywhere -- in the soil, in the air. Much of it comes out of radon gas, those gas pockets created by uranium decaying in the Earth. Eventually, the radon gives way to radioactive lead-210, which sticks around in the environment for years, slowly decaying into polonium-210. "Because polonium-210 occurs naturally in the food we eat and the air we breathe, we all have some very low-level amounts of polonium-210 in our bodies," Whitcomb says. That sounds worse than it is: "The average amount of polonium-210 in the human body is more than a million times less than the lethal dose," he adds.
Even so, some people became concerned when reports surfaced in the 1970s that tobacco smoke contains the radioactive element. "Tobacco plants have especially sticky leaves that pick up lead-210," explains Beverly Cohen, a professor of environmental medicine at New York University. The radioactive lead decays into additional polonium while the tobacco is being aged, she says. The plants also collect polonium-210 from the soil and air.
Polonium-210 is a radioactive element that emits radiation known as alpha particles. These particles can't penetrate the skin's layer of dead cells, but they can damage cells that line the gut or lung when ingested or inhaled. While they can't travel very far, "they're like a heavy football player that knocks out everybody as he goes by," Cohen says. One alpha particle can potentially damage about five cells in close proximity in a tissue.
"When you inhale into [your] lungs, really, you're giving yourself a dose," Cohen says. "The issue is whether there's enough there to do damage." When Cohen autopsied the lungs of deceased smokers back in 1985, she couldn't find significant amounts of polonium-210. A more recent study showed that a smoker's dose of polonium-210 was about 12 times higher than non-smokers.
Thomas thinks that people who eat reindeer are giving themselves more polonium-210 than smokers. That's because caribou eat lichen, which accumulates polonium-210. Inhaling polonium-210 sends it to the lungs, whereas ingesting the element eventually concentrates it in the liver and kidneys. Polonium-210 sticks to proteins, especially the hemoglobin in red blood cells, so blood-laden organs are more at risk of damage from those bull-in-a-china-shop alpha particles.
Even so, "the risk of cancer in [caribou eaters] is quite low," Thomas says. Smokers and reindeer eaters probably have other things to worry about when it comes to their health than the small amounts of polonium-210 they accumulate. "The dose," Thomas says, "makes the poison."