People tend to associate mercury with its silvery liquid form - perhaps found in old thermometers. It was also used in electrical switches or relays that were built into machines until the mid-20th century, and it can be found in florescent lamps and even newer energy saving lamps.

This liquid form of mercury is especially dangerous because it vaporizes at room temperature. And when it vaporizes, it fills the air with tiny, invisible mercury atoms that are both scentless and soluble in oils or fats.

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If mercury vapor is inhaled, it is easily absorbed by the body, where it first gets into the lungs and from there into the blood and the brain. The nerve poison can cause sleep disorders, agitation, and paralysis.

Mercury is no longer used in thermometers but was once very common

Exposure to it can cause the neurological disorder erethism mercurialis - with at least one observable symptom, according to Thomas Gebel, a toxicologist at Germany's Federal Institute for Occupational Health and Safety in Dortmund.

"There are historically documented cases that describe how people's handwriting changed. They start to skid off at the end of the line and can't write straight," Gebel says.

Vapor is the most dangerous

Even a simple thing such as a broken thermometer can lead to mercury poisoning. It can be particularly dangerous in a children's playroom if liquid mercury seeps into cracks and corners and is left undisturbed to vaporize.

"It's not like with hot water - water vaporizes and is gone. But mercury vaporizes more slowly, it can take days or weeks, and could be inhaled over a longer period of time," says Gebel.

Some energy saving lamps are also considered to contain dangerous levels of mercury

Jochen Flasbarth, president of Germany's Federal Environment Agency, says children in some developing countries often expose themselves to the risks by disassembling florescent lamps and energy saving lamps to get at the metal inside.

"They just break open the lamps," recalls Flasbarth of a visit to India, "And the vaporized mercury ends up in their homes."

It can take weeks for the symptoms from mercury poisoning to clear up.

Fertility threat

In addition to liquid mercury, mercury salts and compounds are also dangerous, especially when they get into water systems through industrial waste.

"We've long known that mercury can accumulate in algae and fish," says Gebel. "No matter which mercury compound you introduce to the water, the mercury always comes back to us via things like seafood - and that in a concentrated form."

Mercury poisoning via heavily contaminated seafood and fish can harm a person's fertility as well as threaten their nervous system.

But while mercury is generally considered highly poisonous, doctors in the late 19th century gave patients significant amounts of the element to treat intestinal obstructions.

"Drinking mercury has a laxative effect," explains the toxicologist Gebel. "Its density cleans the intestine wonderfully."

The effect is completely different when mercury is inhaled. As a vapor, the mercury is inhaled as individual atoms and quickly absorbed by the lungs where its poisonous effects begin to develop.

If, however, you drink mercury, hardly any of it stays in the system - most of it exits the body once it has performed its function.

"Taken orally, without inhaling, there's almost no risk," says Gebel.

But you really shouldn't try drinking mercury - most of the patients in the 19th century didn't survive. It's all too easy to inhale mercury unintentionally along the way.