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2. What are the impacts of mercury on human health?

  • 2.1 What are the potential health effects of mercury?
  • 2.2 How are we exposed to mercury?
  • 2.3 What levels of mercury might cause harm?
  • 2.4 How great are the risks from mercury today?

2.1 What are the potential health effects of mercury?

The toxicity of mercury depends on the form of mercury to which people are exposed.

Although mercury and its compounds are toxic substances, there is ongoing debate about exactly how toxic they are. Toxic effects, especially in the case of methylmercury, may be taking place at lower concentrations than previously thought, but this is proving difficult to establish because the suspected toxic effects are subtle and their mechanisms complex. Methylmercury is of particular concern because it can accumulate in the food chain to reach high concentrations (biomagnification). More...

2.1.1 Methylmercury is special among organic mercury compounds because large numbers of people are exposed to it and its toxicity is better understood. Methylmercury in food, such as fish, is a particular health hazard because it is easily taken up into the body through the stomach and intestines.

It is a poison for the nervous system. Exposure during pregnancy is of most concern, because it may harm the development of the unborn baby’s brain. Some studies suggest that small increases in exposure may affect the heart and circulatory system.

Moreover, there is some evidence at present that methylmercury can cause cancer in humans, but it is far from conclusive: the International Agency for Research on Cancer (IARC) has classified methylmercury as "possibly carcinogenic to humans" (Group 2B).

Methylmercury’s poisonous potential was highlighted by an incident in Minamata (Japan), in the 1950s, where wastes from a chemical factory using mercury were discharged into the local bay. More...

2.1.2 Elemental mercury is also poisonous to the nervous system. Humans are mainly exposed by inhaling vapours. These are absorbed into the body via the lungs and move easily from the bloodstream into the brain. However, when elemental mercury is ingested, little is absorbed into the body.

The inhalation of elemental mercury vapours can cause neurological and behavioural disorders, such as tremors, emotional instability, insomnia, memory loss, neuromuscular changes and headaches. They can also harm the kidneys and thyroid. High exposures have also led to deaths. However, there is no evidence at present that elemental mercury causes cancer in humans and it has been classified by IARC into Group 3 "unclassifiable as to carcinogenicity in humans" More...

2.2 How are we exposed to mercury?

The main source of elemental mercury vapour is dental amalgam (a tooth filling).

Diet, particularly fish, is generally the main source of both inorganic and organic mercury. Methylmercury is by far the most common organic form, and is especially found in fish and other seafood.

For some people, the workplace may also be an important source of exposure. Examples include chlor-alkali plants, mercury mines, thermometer factories, refineries and dental clinics, as well as the mining and manufacturing of gold extracted with mercury.

People can also receive extra doses in specific situations, such as when mercury compounds are used in skin-lightening creams, soaps and traditional medicine. Exposure may also arise from localised pollution through air and water, and from mercury spills at home or work (e.g. from certain old gas meters containing mercury). More...

2.3 What levels of mercury might cause harm?

For methylmercury, the US Environmental Protection Agency (US EPA) has estimated a safe daily intake level of 0.1 µg/kg body weight per day. This was based on a study in the Faroe Islands, where fish containing significant levels of mercury form a large part of the diet. The study compared development test scores for children whose mothers had been exposed during pregnancy1. A European Union scientific review, in 2001, has supported this safe daily intake level.

For elemental mercury vapour, several studies show that long-term workplace exposures– at around 20 µg/m3 of air or higher – have subtle toxic effects on the central nervous system.

Other adverse effects of various forms of mercury have been seen in humans, but either the findings are less consistent or the doses involved are much higher.

The Working Group that prepared this assessment, in line with its mandate, did not assess the potential effects of exposures to elemental mercury vapour from dental amalgams or reach any conclusions about whether or not dental amalgams cause adverse effects. This remains a matter of scientific debate2. More...

2.4 How great are the risks from mercury today?

2.4.1 Whilst the diet and amalgam fillings in teeth are respectively the main sources of methylmercury and mercury vapour exposure for most people, sources such as local pollution, exposure at work, cultural practices and traditional medicines are important in some regions (see 2.2).

Assessments of mercury exposure have been made in various parts of the world. For example, a recent study of 1700 women in the USA found that about 8% of them had mercury concentrations in their blood and hair exceeding the levels that correspond to the US EPA’s estimated safe dose.

Data indicate that exposures in Greenland, Japan and some other areas are generally higher than in the USA. On the other hand, measures have been taken in recent decades to reduce emissions of mercury in various countries (see 6.3). More...

2.4.2 Fish is the main food source in many parts of the world and provides nutrients that are not easily replaced. Mercury contamination adds health risks to this important food supply.

Many countries, international organizations and scientific investigations have reported mercury concentrations in fish between about 0.05 and 1.4 mg/kg of fish tissue, depending on the water and the fish.

Predator fish and marine mammals that eat other fish tend to have higher levels of mercury because mercury bioaccumulates in fish and is biomagnified up the food chain (see 3.1). Mercury levels are thus higher in such fish as king mackerel, pike, shark, swordfish, walleye, barracuda, large tuna (as opposed to the small tuna usually used for canned tuna), scabbard and marlin, as well as in seals and toothed whales.

Moderate consumption of fish with low mercury levels is not likely to result in worrying levels of exposure for humans. However, people who consume higher amounts of contaminated fish or marine mammals may be highly exposed to mercury and are, therefore, at risk. Indeed, high concentrations of mercury in fish have led governments in a number of countries to give warnings to consumers. These advise people, especially sensitive groups (such as pregnant women and young children), to limit or avoid consumption of certain types of fish from specific areas. More...

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