Mercury is a naturally occurring element that's found in soil and rocks as well as bodies of water. It's both a metal and a liquid at room temperature and normal pressure, but it's not the only one. Four other elements — gallium, cesium, francium and bromine — are also liquids under these conditions.

It is also now well understood to be a serious health concern. As Environment Canada says on its website "Chronic exposure to mercury can cause damage to the brain, spinal cord, kidneys, liver and developing fetus. Exposure to mercury while in the womb can lead to neurodevelopmental problems in children."

Mercury is far less ubiquitous today than it used to be but it still plays a big role in many industrial processes from chlorine production to small-scale gold refinement to the performance of atomic clocks. Compounds containing small amounts of mercury are also used as a preservative in some medical preparations such as vaccines.

Historically, the liquid metal, also called quicksilver, has been regarded as a curiosity, and used in lucky charms, toys and reflecting pools. It also was thought to have medicinal properties, such as curing syphilis when ingested, before its toxic nature was fully understood.

In the home, mercury can be found in:

Thermometers: Medical thermometers containing mercury are being phased out in favour of electronic thermometers or those containing coloured alcohol. It is still used in some high-temperature thermometers.

Thermostats and light switches: Some older non-digital thermostats and "silent" light switches contain a large drop of mercury in a glass vial. These should be disposed of with care when they breakdown and delivered to toxic waste facilities where possible.

Fluorescent lights: Fluorescent lights work by using electricity to excite mercury vapour into a glowing plasma. Both compact fluorescent lights and fluorescent tubes contain small amounts of mercury, about the same as you would find in the average watch battery. They too should be disposed of with care when they are no longer working.

Batteries: Button-style batteries used in watches, calculators, toys and hearing aids contain mercury.

Dental fillings: Dental amalgam fillings contain a mixture of mercury and other metals such as silver, tin, copper and zinc.

Health Canada says amalgam fillings are the largest single source of mercury exposure for average Canadians.

But the agency says levels released into the body are not enough to cause illness.

However, it notes, that since mercury can cross the placental barrier and can impair kidney function at sub-clinical levels of exposure, pregnant women and people with kidney disease should avoid amalgam fillings.

Health Canada does not recommend the removal of amalgam fillings from people who do not show signs of adverse effects.

On June 4, 2008, the U.S. Food and Drug Administration warned that mercury in dental amalgams may pose health risks to children, fetuses and pregnant women. The FDA also says in the vast majority of cases, amalgam fillings should not be removed.

Many dentists now use a white plastic resin filling instead of amalgam fillings. Sweden and Norway have banned the use of mercury amalgam fillings, citing health concerns.

Paints and pesticides: Older paints can contain mercury as a pigment or antifouling agent. Some older insecticides also contain mercury.

A high level of mercury damages our nervous systems and can inhibit brain development in young children. It's not clear what the long-term effects of extremely low levels of mercury are. But North Americans absorb trace amounts of it from dental fillings, air and water pollution and from the food we eat. Fish tend to be our largest source of mercury.

Why is mercury a problem in fish?

Mercury binds tightly to the proteins in fish tissue. Most fish will contain trace amounts of mercury, depending on the level of mercury in their environment and their place in the food chain. The bigger the fish and the higher up the food chain it is, the more mercury it will tend to contain. Large predatory fish species tend to have higher levels than non-predatory fish or species further down the food chain.

Health Canada guidelines for mercury in fish call for a concentration of no more than 0.5 parts per million - about half the limit considered safe in the United States. The Canadian Food Inspection Agency regularly tests fish sold commercially. Most fish come in well under the limit, with the exception of swordfish, shark and fresh or frozen tuna, which is why officials recommend limits on the consumption of those species.

Is mercury a problem in freshwater fish?

It can be, depending on the body of water the fish calls home. The CFIA tests fish caught by commercial freshwater fishermen and if it finds high levels, it will shut down or severely restrict the fishery.

Recreational fishermen should check with their provincial or territorial governments for information on safe consumption standards and guidelines. One of Canada's top salmon-fishing rivers is the Credit River, which flows into Lake Ontario in Mississauga, just west of Toronto. The Ontario Ministry of Natural Resources advises people not to eat any Chinook salmon caught in the river that are more than 55 centimetres (22 inches) long. The ministry recommends no more than four meals a month of Chinook salmon smaller than that. For Coho salmon caught in the river, the ministry advises not to eat anything longer than about 35 cm (14 inches).

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