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4. Where is mercury found?

  • 4.1 How does mercury cycle through the biosphere?
  • 4.2 Do local releases cause global effects?
  • 4.3 How much mercury do we release into the environment?
  • 4.4 How is mercury released naturally?

4.1 How does mercury cycle through the biosphere?

Mercury moves from the Earth’s crust into the biosphere as a result of both natural processes and human activities.

Examples of natural processes include the weathering of rocks and volcanic activity.

Human activity can release mercury

  • when it is intentionally mined, processed or used in products;
  • unintentionally, from processes where mercury is an unwanted impurity in raw materials, minerals and fossil fuels, particularly coal; and
  • from soils, sediments, water bodies, landfills and waste or tailings piles, contaminated previously by human activities.

Once released, mercury enters the air, water and soil and can continue to move between them over long periods of time, depending on its chemical form (see section 1.3).

Mercury is only removed from the biosphere when it reaches sediments deep under the oceans or when it is immobilised in controlled landfills. This implies that, even as we gradually eliminate mercury releases from human activity, levels in the environment will take several decades or longer to go back down. However, improvements may be quicker in places where local or regional contamination is a major source. More...

4.2 Do local releases cause global effects?

Airborne mercury may deposit into water or onto soil close to its source of emission, or even on the other side of the world, depending on its chemical form. Several studies have concluded that the amount of mercury deposited at any particular place can come from both local and global sources. Virtually any local source contributes to the global mercury pool in the biosphere.

Most of the mercury emitted to the air through human activity is elemental mercury vapour, which can stay airborne long enough to cross continents. Other forms of mercury, such as inorganic mercury, fall to earth within roughly 100 to 1 000 km. However, how far the mercury travels also depends on whether mercury converts from one form to another in the air.

Computer modelling has estimated that 50% of the mercury, released by human activity and deposited in North America, comes from elsewhere. This figure is 20% for Europe and 15% for Asia.

Mercury can also be re-released from water and soil, prolonging the time it stays in the biosphere. One study suggests that around 20% of the amount deposited can be re-released over a two-year period. More...

4.3 How much mercury do we release into the environment?

4.3.1 Human activity (see 5.1) is now the main source of mercury to the atmosphere, water and soil. A recent study has suggested that the amount of mercury in the atmosphere has tripled because of this.

On average around the globe, there are indications that human activity has raised the rate of mercury deposition by 1.5 to 3 times since pre-industrial times. In and around industrial areas, the deposition rates have increased by 2 to 10 times during the past 200 years. More...

4.3.2 Much data exists about how much mercury some countries and industries release into the environment, but the global picture is incomplete.

Fossil fuel power plants and waste incinerators emit about 70% of the mercury released into the atmosphere by human activity, for which there is data.

These releases are expected to increase unless alternative energy sources or technologies to control emissions are further developed and widely used.

Mercury production from mines has been decreasing from about 6 000 to about 2 000 tonnes per year during the 1980s and 90s. Thus, releases from mining and mercury use may also be in decline. Small-scale gold and silver mining can be an important source in some countries4.

Emissions from a number of major sources have decreased in North America and Europe. In Canada, for example, emissions into the air were reduced from about 33 to 6 tonnes between 1990 and 2000. More...

4.3.3 Emissions from human activities come both from the intentional use of mercury, and the unintentional releases of mercury impurities. The share of emissions into the air emanating from intentional uses ranges between 10% and 80% for different countries.

The relative importance of the two types of source in a country or region depends upon:

  • progress made in substituting mercury use in products and processes;
  • reliance on fossil fuels for energy production, particularly coal;
  • the size of the mining and mineral extraction industry;
  • waste disposal methods (incineration or landfill); and
  • the use of technologies to remove mercury from emissions.


4.4 How is mercury released naturally?

Mercury occurs naturally in forms that are volatile, so mercury continuously evaporates into the atmosphere, from both soils and water. Mercury-rich rocks and soils can lead to elevated mercury levels across wide areas. The weathering of rocks, volcanic activity and forest fires all contribute to the natural emission of mercury into the air.

Actual natural mercury emissions are very difficult to determine, because total emissions of mercury from soil and water surfaces come from both natural sources and from re-emission of previously deposited mercury. This, in turn, will have come from both natural sources and human activity.

Natural mercury emissions are beyond our control, and it is currently estimated that less than 50% of total mercury releases are from natural sources. However, it is important to keep them in mind as significant sources of mercury in the environment. More...

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