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
- unintentionally, from processes where mercury is an
unwanted impurity in raw materials, minerals and
particularly coal; and
- from soils, sediments, water bodies, landfills and
waste or tailings piles, contaminated previously by human
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
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
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.
4.3 How much mercury do we release into the environment?
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.
Much data exists about how much mercury some countries and
industries release into the environment, but the global picture
Fossil fuel power plants
and waste incinerators emit about 70% of the mercury released
into the atmosphere by human activity, for which there is
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
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
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
The relative importance of the two types of source in a
country or region depends upon:
- progress made in substituting mercury use in products
- reliance on
fossil fuels for energy
production, particularly coal;
- the size of the mining and mineral extraction
- waste disposal methods (incineration or landfill); and
- the use of technologies to remove mercury from
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