On Capitol Hill today, politicians are holding yet another hearing about the risks of mercury. This one focuses on the metal's use in dental fillings. It's part of the latest wave of concern about mercury, which also turns up in fish, air pollution and in some vaccines. These fears aren't new. As NPR's Jon Hamilton reports, the dangers of mercury have been known for hundreds of years.
Spain was among the first nations to mine the liquid metal known as quicksilver. John Emsley of Cambridge University says it's not surprising that Spanish royalty were among the first people to be poisoned by it.
"In some of the medieval palaces in Spain, they had mercury pools as an ornamentation -- you know it was a rather dramatic thing, a mercury pool, because it was reflective," says Emsley. "But it was found that people who lived in those palaces suffered severe effects of constantly being exposed to this constant background of mercury" -- effects like tremor, drooling and paranoia, says Hamilton. The problem was that the pools of mercury filled the palace -- and the royal lungs -- with mercury vapor.
Emsley, author of the book Nature's Building Blocks, says inhaling the metal sends it straight to the bloodstream. From there, it blazes a toxic trail through internal organs, as well as the brain and nervous system.
Emsley says the lessons from Spain apparently weren't passed on to the British, who had their own mercury problems in the 1800s:
"People who made hats generally made them of felt. And felt, of course, is made from rabbit fur and rabbit fur is very short fiber. In order to get the very short rabbit fibers to mat together to produce felt, you soaked them in mercury nitrate," he tells Hamilton.
Mercury nitrate is a form of inorganic mercury. It doesn't get absorbed by the body as easily as other varieties. But it was toxic enough to addle the brains of many hatmakers -- including the one that Alice encounters at a tea party in Wonderland.
Industries gradually learned to limit workers' direct exposure to mercury. But they didn't always avoid dumping it into the environment, says Hamilton.
Factories in Japan making products such as chlorine used to dump tons of mercury into Minamata Bay. Emsley says the pollutant was absorbed by tiny microbes, which transformed it into methyl mercury, a particularly dangerous form. Eventually that methyl mercury got into the local fish.
"The first thing they noticed was that the cats, who lived off fish, started behaving erratically," according to Emsley. "But it wasn't long after that that people started to go down with symptoms. Ten thousand people were affected by what was called Minamata disease, and it was all traced to this very dangerous form of mercury getting into the environment from the chemical plant."
Methyl mercury is dangerous because it passes easily through a barrier that usually protects the brain and nervous system. In Minamata, that caused hideously deformed children, poisoned while they were still in the womb.
Such experiences have led most governments to limit the use of mercury. Streetlights, thermometers and nearly all childhood vaccines no longer contain mercury. But Dr. Howard Frumkin, an environmental and occupational medicine specialist at Emory University, says there's still some around.
"We have small amounts of mercury exposure from air, especially if we live downwind of a power plant," he says. "We have small amounts of mercury exposure from preservatives, we have a little bit from the fillings in our teeth. Put it all together and we probably have more exposure than we'd like. Is it enough to cause health problems? That's the part we don't fully understand yet."
Any lingering health problems will be hard to spot, Frumkin says, because unlike those of the past, they are likely to be subtle and rare.
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