Mercury in the Environment
Mercury is an element that occurs naturally
in the environment and exists in several forms. These forms can be
organized under three headings: metallic mercury (also known as elemental
mercury), inorganic mercury, and organic mercury.
Mercury is a known toxicant, affecting growth, reproductive
success, and development.
Metallic mercury is a shiny, silver-white metal that
is frequently used in thermometers and some electrical switches.
Inorganic mercury compounds or mercury salts occur when mercury
combines with elements such as chlorine, sulfur, or oxygen.
Most inorganic mercury compounds are white powders or crystals,
except for mercuric sulfide (also known as cinnabar), which is red
and turns black after exposure to light.
When mercury combines with carbon, the compounds
formed are called "organic" mercury compounds. By far the
most common organic mercury compound in the environment is methylmercury.
Sources of Mercury in the U.S. Environment
The Environmental Protection Agency estimates that
50 to 75 percent of the mercury released annually comes from human
activities. Of approximately 200,000 tons of mercury emitted
to the atmosphere since 1890, about 95 percent resides in soils, 3
percent in the ocean surface waters, and 2 percent in the atmosphere. Estimates of the annual total global input
to the atmosphere from all sources is about 5,500-6,000 tons.
The human activities that release mercury into the
environment are a complex combination of (i) activities that directly
emit or inject mercury into the air, soil, or water; and (ii) industrial
use of products that contain mercury and may eventually be returned
to the environment through landfills, combustion, or other means. Mercury
is utilized in the electric industry (switches, thermostats, batteries);
dentistry (dental amalgams which are 50% mercury); medicinal products
(i.e. mercurochrome, laxatives, worming medications, teething powders,
and measuring devices such as thermometers); numerous
industrial processes; and many other uses.
In the United States, approximately 158 tons of mercury
are emitted annually into the atmosphere by human activities. The majority of the mercury entering lakes, streams, rivers,
and oceans comes from the atmosphere. Eight-five percent of all mercury
pollution in the U.S. is released by (1) power plants burning coal
and (2) municipal and medical waste incinerators burning mercury-tainted
trash. In the Gulf of Mexico, discharges of drilling muds (lubricating
fluids) from the Gulf's 4,000 oil and gas production platforms are
also a significant source of mercury.
Movement of Mercury in the Environment
When mercury is released into the air, wind currents
carry it until it settles down over land and water. In the water, mercury is absorbed by micro-organisms
and transformed, by a process called “methylation,” into an even more
dangerous compound called methylmercury. In this toxic form, mercury easily makes its way up the food
chain: microscopic organisms feed off contaminated sediments, which
are then eaten by smaller fish, which are eaten by larger predator
fish, which are, in turn, eaten by humans.
At each level of the food chain, a higher concentration of
methylmercury is found, a process known as “bioaccumulation.”
For
information on the Regulation of Mercury Use and Release and other
Issues regarding Mercury in the Environment go to:
The United State Environmental Protection Agency
website: http://www.epa.gov/mercury/index.html
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