People tend to associate mercury with its silvery liquid form — as found in old thermometers. But it was also used in electrical switches or relays that were built into machines until the mid-20th century. Later, it was florescent lamps and some early energy saving lamps.

The 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.

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. It's a nerve poison that can cause sleep disorders, agitation and paralysis.

Exposure to mercury can cause a neurological disorder called erethism mercurialis, which can cause irritability and depression.

Mercury vapor is its most dangerous form

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.

Fortunately, many thermometers, even household ones, are digital now. In the European Union, mercury "measuring instruments" such as thermometers have long been banned. In other regions, such as the United States and Asia, some mercury thermometers are still used in industry, but they are being phased out. 

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

Risk from mercury for fertility

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

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

The World Health Organization says people are mainly exposed to methylmercury when they eat fish and shellfish that contain the compound.

Methylmercury is very different to ethylmercury. Ethylmercury is used as a preservative in some vaccines and does not pose a health risk.

Deadly: Historical use of mercury

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

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 and is less of a health risk than if you inhaled it.

But you really shouldn't drink mercury — under any circumstances — most of the patients in the 19th century didn't survive.