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Introduction

The health hazards of mercury compounds have been recognized for quite some time. In the early part of the 19th century, hat makers used a solution of mercury salts to soften animal hairs in the production of felt. Hat makers were known to exhibit bizarre behavior and terms such as "hatter's shakes" arose due to the neurological symptoms of chronic (long-term) mercury poisoning. Consequently, the Mad Hatter in "Alice in Wonderland" got his name because hatters in Lewis Carroll's day often displayed quite erratic behaviour.

Today, the main effects of mercury exposure to humans are understood to be neurological, renal (kidney), cardiovascular and immunological impacts. 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 can impair the ability to feel, see, move and taste, and can cause numbness and tunnel vision. Long-term exposure can lead to progressively worse symptoms and ultimately personality changes, stupor, and in extreme cases, coma or death. Recent findings have described adverse cardiovascular and immune system effects at very low levels.

Most people are exposed to mercury as a result of normal activities such as the inhalation of air, contact with water and soil and/or exposure to substances or products containing mercury such as dental amalgam. The main pathways for mercury intake in humans are through the consumption of food, especially fish, and the application of dental amalgams as a tooth restorative. Recent research has indicated that exposure to mercury vapour resulting from past spills of liquid mercury in the home may also be an important pathway.

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Exposure Due to Food Consumption

Mercury emissions from natural and anthropogenic sources enter the global mercury cycle and are distributed in the environment locally and globally through various processes. Atmospheric emissions of mercury can enter the environment through deposition onto soils and water. When mercury enters fresh water bodies and the oceans, or settles into sediments and soils, it can become involved in biogeochemical cycles, be transformed into the highly toxic form of methylmercury, and bioaccumulate in the food chain. Predatory fish may attain high levels of methylmercury that may be almost completely absorbed by human or animal consumers.

While fish consumption is the main source of exposure to methylmercury, Health Canada advises that the benefits and risks of eating fish should be balanced, since fish are an excellent source of high-quality protein and omega-3 fatty acids, and are low in saturated fat. In order to protect Canadians from mercury poisoning, a guideline of 0.5 parts per million, or 0.5 micrograms (1 microgram = one millionth of a gram) of total mercury per gram of fish tissue (ug/g) has been established by Health Canada for most commercial fish. Commonly consumed marine species such as salmon, cod, pollock, sole, shrimp, mussels, scallops and canned tuna, have mercury levels that generally fall below this limit and are therefore considered safe for consumption. The Canadian Food Inspection Agency is responsible for testing mercury levels in canned tuna and rejects any shipments bound for commercial sale in Canada if the 0.5 ppm limit is exceeded.

Shark, swordfish and fresh or frozen tuna, which generally have concentrations ranging from 0.5 to 1.5 ppm, are exempt from the 0.5 ppm limit because they are considered "gourmet" fish not eaten in quantities sufficient to pose a health hazard. Instead, Health Canada advises a consumption limit of one meal per week of these fish for the average Canadian, and recommends that women of child bearing age, pregnant women, and children should not consume these fish more than once per month. Fish containing over 1.5 ppm are not considered suitable for human consumption at any time.

Predatory, freshwater fish species such as pike, bass and walleye have also been know to attain elevated methylmercury levels in various lakes and river systems. If fish constitutes a staple part of the diet, such as for indigenous peoples and recreational anglers, consumption of these species may pose an elevated risk of mercury exposure. Federal, provincial and territorial agencies have issued fish consumption advisories for the protection of the public, suggesting limited consumption of specific fish species from certain lakes. In Canada, thousands of advisories on fish species from individual water bodies are put in place in every year due to excessive mercury contamination of fish tissues. Province-wide advisories are in effect in New Brunswick and Nova Scotia while other provinces have advisories for specific lakes and/or species.

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Exposure Due to Inhalation

Based on available science, normal ambient air concentrations of mercury vapour, averaging 1.6 nanograms per cubic meter of air, do not appear to be a cause for concern (1 nanogram = one billionth of a gram). However, inhalation can be a significant route of exposure when mercury-silver amalgam is used in dental fillings, particularly for individuals that are hypersensitive to mercury. Health Canada advises that mercury fillings should not be placed in the teeth of pregnant women or people with impaired kidneys, and wherever possible, non-mercury fillings should be used for children.

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Exposure Due to Contact

Dermal contact is also a route of exposure to mercury with alkyl mercury compounds being particularly notorious. While few Canadians come into direct contact with mercury or its compounds, skin absorption can be lethal. In 1997, a researcher named Karen Wetterhahn, from Dartmouth College in New Hampshire, died when a single drop of dimethylmercury passed through her protective latex glove and through her skin.

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Metabolism

When an individual is exposed to mercury, a certain percentage is absorbed, depending on the route of exposure and the form of the mercury. About 80% of elemental mercury is absorbed when inhaled, however less than 1% of ingested liquid mercury is absorbed. Methylmercury, on the other hand, is readily absorbed irrespective of the exposure pathway. Approximately 95% of ingested methylmercury is absorbed, and absorption through the lungs and skin is also believed to be quite high. Both elemental and methylmercury can cross the blood-brain and placental barriers. The critical target organ for elemental mercury is the adult and fetal brain, and the critical target organs for methylmercury are the brain and the kidneys.

Inorganic mercury compounds do not readily migrate through the blood-brain or placental barriers, but do accumulate in the kidneys. Absorption of inorganic mercury varies with the type of inorganic salt. Within the body, the kidneys accumulate the highest concentrations of all forms of mercury, yet mercury can also concentrate in the brain, the central nervous system, the liver, and indeed in most organs in the body.

Mercury is predominantly excreted from the body in urine and feces, but usually at a slower rate than that of uptake, leading to the accumulation of mercury in living tissue. Mercury is deposited in hair as it grows, and it may also be found in breast milk. This may result in high concentrations in infants whose mothers are heavily exposed. The unborn child also receives some of the maternal mercury body burden because mercury compounds cross the placental barrier, yielding equal or higher blood concentrations in the fetus than in the mother.

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Toxic Effects

The severity of mercury's toxic effects depends on the form and concentration of mercury and the route of exposure. Exposure to elemental mercury can result in effects on the nervous system, including tremor, memory loss and headaches. Other symptoms include bronchitis, weight loss, fatigue, gastro-intestinal problems, gingivitis, excitability, thyroid enlargement, unstable pulse, and toxicity to the kidneys.

Exposure to inorganic mercury can affect the kidneys, causing immune-mediated kidney toxicity. Effects may also include tremors, loss of co-ordination, slower physical and mental responses, gastric pain, vomiting, bloody diarrhea and gingivitis.

Symptoms of methylmercury toxicity, also known as Minamata disease, range from tingling of the skin, numbness, lack of muscle coordination, tremor, tunnel vision, loss of hearing, slurred speech, skin rashes, abnormal behaviour (such as fits of laughter), intellectual impairment, to cerebral palsy, coma and death, depending on the level of exposure. In addition, methylmercury has been classified as a possible human carcinogen by the U.S. Environmental Protection Agency. More recently, additional findings have described adverse cardiovascular and immune system effects at very low exposure levels.

Prenatal exposure to organic mercury, even at levels that do not appear to affect the mother, may depress the development of the central nervous system and may cause psychomotor retardation for affected children. Mild neurological and developmental delays may occur in infants ingesting methylmercury in breast milk. Affected children may exhibit reduced coordination and growth, lower intelligence, poor hearing and verbal development, cerebral palsy and behavioural problems.

Much of our knowledge of mercury toxicity comes from studying cases of occupational or acute community poisonings. In 1956, Minamata disease (later found to be methylmercury poisoning) was officially "discovered" in a Japanese community near a polyvinyl chloride plastic plant which was discharging untreated effluent containing methyl mercury chloride into Minamata Bay and the Shiranui Sea. Once in the bay's sediments, the mercury was readily absorbed by marine species, which resulted in the contamination of the entire ecosystem. Many of the local residents consumed fish and seafood from the mercury-contaminated waters as a staple part of their diet. Thirty-nine years later, 2 252 patients had been officially diagnosed with Minamata disease, and over 1 000 had died.

The largest ever outbreak of mercury poisoning occurred in Iraq in 1971 and 1972 when the population accidentally ate bread made from seed-grain treated with a mercury-based seed treatment pesticide. About 6 000 cases of mercury poisoning were diagnosed. Over 500 cases were fatal. Instances such as these highlight the critical need for governments to implement mercury management strategies and to educate the public on the risks of mercury exposure.

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