Mercury
What Is Mercury? As anyone who's ever broken a thermometer can attest,
mercury
is a fascinating substance. It also has an interesting history. Named
after the
fleet-footed Roman messenger of the gods, mercury has been used for
more than
2,000 years. Mercury ore cinnabar has even been found smeared
on Neolithic
skulls. Its first recorded mention is a reference by Aristotle
in the fourth
century B.C., a time when the silvery-white, heavy metal was
used in religious
ceremonies. Spanish miners used mercury to process gold ore
for ancient Rome;
when their mercury supply ran out, gold production dropped
and set in motion the
decline of the Roman Empire. Today mercury poses a
serious threat, this time
environmental. Human beings around the world cause
mercury to enter the
atmosphere primarily when they burn coal or incinerate
waste. Studies from
Sweden and Florida suggest that mercury also
evaporates from landfills, but only
0.0001 per cent. Groundwater
contamination from old dumps is of concern but
combustion is the major path
via which mercury pollutes the globe. Mercury
rarely exists in a free state
in nature; it's recovered from red mercuric
sulphide in geologically recent
volcanic rocks. The world inventory of mined
mercury is estimated at 600,000
tonnes, stored mostly in states of the former
Soviet Union. (Mercury has
not been mined in Canada since 1975.) Scientists
estimate that man-made
mercury releases are two- to four times greater than
those of nature. The
typical mercury content of lakes has increased up to
sevenfold since
industrialization. Acid rain dissolves lake-rock and releases
mercury to
water. (Granite contains about 0.2 p/m mercury.) With a melting point
of
minus 38.87°C, metallic or elemental mercury (the form used in
thermometers)
readily vaporizes and can be transported long distances. The
vaporization rate
of mercury doubles with every 10°C temperature increase and
its residence time
in the atmosphere is up to three years. Not surprisingly,
mercury condenses and
accumulates in cold climates such as the Arctic or
mountain regions. Though far
away from industrial activity, these areas have
become repositories of the
world's mercury emissions. Mercuric chloride, a
simple salt, is the predominant
form in many surface waters. Almost all the
mercury found in animal tissues is
methyl mercury, a water-soluble toxic
organic mercury compound. Persistent and
non-biodegradable, mercury
biomagnifies up the food chain. In the Arctic as few
as six steps stand
between ingestion by microscopic organisms and consumption by
human beings.
Pregnant women can pass mercury from contaminated fish to their
unborn
children. Toxic effects include central nervous system and kidney
damage,
mental retardation, blindness and cerebral palsy.