An old problem, mercury pollution of freshwater fish, re-emerged during
the late 1980s in a form much more widespread and much more difficult
to control than previously seen.
Federal and state governments have done little to attack the problem,
partly because much of the bad news is recent, and partly because
government policies created much of the problem in the first place.
Mercury contamination first became known in the U.S. in 1970 when big
fish in the Great Lakes were discovered to be so laced with mercury
that they were poisonous to humans. The source was thought to be
industrial dumping of mercury directly into waterways. After various
industries reduced their mercury dumping into streams and lakes,
pollution control officials assumed the problem was under control and
basically went to sleep.
However, throughout the 1970s, Swedish researchers continued to report
high mercury levels in fish taken from remote lakes, far from any
industrial polluters. In the early 1980s, Minnesota officials began
examining fish from remote lakes and they too found fish with
dangerously high mercury concentrations. Then officials in Wisconsin
and Michigan made similar discoveries. In the Canadian province of
Ontario, researchers found high levels of mercury in 95% of all the
Scientists assumed the source of the mercury was natural geologic
deposits until recently when careful measurements of rainwater revealed
that the mercury is coming from the atmosphere, carried to earth by
rain. Mercury enters the atmosphere from two main sources: 65% from
combustion of coal, and 25% from solid waste incineration.
Recent measurements of mercury in the atmosphere over the oceans
indicate that total mercury in the atmosphere has doubled since the
19th century. Nature moves mercury into the atmosphere by various
means, such as volcanoes, but human sources are now three times as
large as all other natural sources of mercury. That is, humans
activities (chiefly burning coal and garbage) account for 75% of the
mercury entering the atmosphere today, whereas all non-human sources
account for only 25%. This new data has come as something of a shock to
scientists, who had thought humans were a rather puny actor on this
Economic projections by the Washington think tank, Resources for the
Future, indicate that humans are likely to double their mercury
releases during the next 50 years. If that should occur, some
scientists say freshwater fish in acidic waters everywhere will become
toxic to humans.
The mercury in the atmosphere is 97% elemental mercury, but the mercury
in fish is 95% to 99% methylmercury, which is elemental mercury with a
carbon atom and three hydrogen atoms attached. Methylmercury is much
more toxic than elemental mercury. Scientists now know that mercury
becomes methylated when it enters a body of water, especially an acidic
body of water, though the exact role of the acid is still unclear.
Big fish taken from acid waters are dangerous to humans already. And,
as other sources of protein become more expensive or come to be
regarded as unhealthful, such as red meat, eating fish becomes more
popular. The combination of more mercury entering the atmosphere,
increasing acidity of many waters because of acid precipation (rain,
snow, and even acid fog), plus increasing consumption of freshwater
fish adds up to a growing threat to humans.
Fish are relatively insensitive to mercury, so they can build up a
level in their tissues that is toxic to humans and other fish eaters.
Mercury mainly attacks the central nervous system, chiefly the brain.
In adults, the first symptoms of serious exposure are loss of sensation
in the extremities of the hands and around the mouth. If exposure
continues, an unsteady gait, slurred speech, tunnel vision (concentric
constriction of vision), loss of hearing, convulsions, madness and
The effect of mercury on humans began to be understood during the
1970s. The Chisso Corporation dumped tons of mercury into Minamata Bay,
a relatively small body of water in southern Japan, heavily
contaminating local fish. Some 2900 humans were severely injured, a
third of them killed outright. Recent court cases indicate that an
additional 10,000 individuals were permanently harmed. Based on
studies of adults contaminated at Minamata in the period 1953-1968, the
World Health Organization [WHO] in 1972 established 4300 nanograms per
kilogram of body weight per day (ng/kg-day) as the dose that caused
poisoning; they applied a "safety factor" of 10 and concluded that a
"safe" dose would be 430 ng/kg-day. (A nanogram is a billionth of a
gram; a kilogram is 1000 grams or 2.2 pounds; a gram is 1/28th of an
However, since that time, much new data has become available. In
northern Iraq during the winter of 1971-72, some 6530 patients were
admitted to hospital with mercury poisoning. The death of 459 of them
was attributed to mercury. Careful follow-up studies of mercury levels
in hair and blood of adults, children, and infants allowed medical
researchers to establish that infants are 5 to 10 times more sensitive
to mercury than adults are. Infants exposed before birth showed severe
brain damage whereas their mothers had mild or no symptoms. The fetus
is thought to be particularly sensitive during the first months of
pregnancy when cell migration and cell division are occurring at a fast
pace. Thus the WHO "safe" level is perilously close to a mercury level
known to harm fetuses and infants.
Dr. Thomas Clarkson at University of Rochester points out that Iraqi
infants exposed before birth showed symptoms when their pregnant
mothers had daily intakes of 600 to 1100 nanograms per kilogram.
Applying the standard "safety factor" of 10, Dr. Clarkson says the new
"safe" dose should be 60 to 110 nanograms per kilogram. Roughly 40%
of the American people already eat enough freshwater fish to exceed a
mercury intake of 60 ng/kg-day.
A careful survey of fish consumption (and therefore mercury intake) of
4864 Americans by the U.S. Department of Commerce showed that 0.1% of
the American people (250,000 individuals) exceed the WHO "safe" dose of
430 ng/kg-day. But if we accept Clarkson's new "safe" dose, we can
calculate that roughly 40% of the American people (100 million
individuals) are today eating levels of mercury in fish that would have
to be called unsafe.
Is anyone really being affected? Humans who eat a lot of freshwater
fish seem to be, and so do fish-eating birds and mammals. In 1978 a
neurological study of 592 Cree Indian people from three bands living
along James Bay in Quebec province revealed tremors, incoordination and
abnormal reflexes were prevalent neurological abnormalities, and other
major manifestations of mercury poisoning were seen occasionally,
including astereognosis (loss of the ability to judge the form of an
object by touch) and tunnel vision (concentric constriction of visual
field). Since that time, mercury in the blood of Cree people has
been cut in half by modifying their traditional diet to reduce intake
of freshwater fish, particularly big fish.
Ocean fish seem not so susceptible to mercury buildup in their tissues,
perhaps because the oceans are not acidic. Freshwater fish are the
Bass in the Florida Everglades have 4.4 parts per million of mercury;
U.S. Food and Drug Administration forbids interstate shipment of fish
containing more than 1 ppm mercury, and the state of Minnesota has set
a "safe" level in fish at 0.16 ppm, the strictest standard of any
state. Bass and other fish in the Everglades are definitely toxic
enough to pose a real threat to predators such as ospreys, eagles and
humans. Mercury has been confirmed as the cause of death in loons and
panthers in the Everglades. Furthermore, mercury is suspected as an
important cause of the reproductive failure being witnessed now among
eagles, mink, otter and other wildlife in the Great Lakes.
The mercury problem is heavily damaging the sport fishing industry in
at least 22 states and at least two Canadian provinces, where warnings
have been issued against eating certain species of fish from certain
waters. And, as we have seen, the problem shows every sign of growing
Given that the problem is atmospheric and international, it is not one
that states can completely solve alone, though each state could make
strides by banning garbage incineration and phasing out the burning of
coal. It will take an international accord to stem mercury poisoning.
This will require leadership from the federal government, which so far
has shown no interest.
Federal energy policy officially encourages coal-burning power plants
and incineration of municipal trash, for generation of electricity. The
Edison Electric Institute estimates it would cost $5 billion to capture
mercury from the nation's coal-burning plants, and it might cost $10 to
$30 million to fit an incinerator with mercury-capturing gear. (Coal
combustion provides a double whammy, acidifying waters while releasing
Against those costs, we must weigh loss of the sport fishing industry
and damage to the central nervous systems of large numbers of
Americans, especially infants and children.
 Janet Raloff, "Mercurial Risks From Acid's Reign," SCIENCE NEWS
Vol. 139 (March 9, 1991), pgs. 152-156.
 Keith Schneider, "Ancient Hazards of Mercury Re-Emerge," NEW YORK
TIMES August 26, 1991, pgs. A1, B5.
 F. Slemr and E. Langer, "Increase in global atmospheric
concentrations of mercury inferred from measurements over the Atlantic
Ocean," NATURE Vol. 355 (Jan. 30, 1992), pgs. 434-[437.]437.
 William D. Watson, Jr., "Economic considerations in controlling
mercury pollution," in J. O. Nriagu, editor, THE BIOGEOCHEMISTRY OF
MERCURY IN THE ENVIRONMENT (NY: Elsevier/North-Holland Biomedical
Press, 1979), pgs. 41-77.
 David E. Sanger, "Japan and the Mercury-Poisoned Sea: A Reckoning
That Won't Go Away," NEW YORK TIMES January 16, [1991,] pg. A3.
 Thomas William Clarkson, "Human Health Risks From Methylmercury in
Fish," ENVIRONMENTAL TOXICOLOGY AND CHEMISTRY Vol. 9 (1990), pgs. 957-
 Clarkson, cited above, pg. 958.
 William F. Fitzgerald and Thomas W. Clarkson, "Mercury and
Monomethylmercury: Present and Future Concerns," ENVIRONMENTAL HEALTH
PERSPECTIVES Vol. 96 (1991), pgs. 159-166.
 Gail E. McKeown-Eyssen and John Ruedy, "Prevalence of Neurological
Abnormality in Cree Indians Exposed to Methylmercury in Northern
Quebec," CLINICAL & INVESTIGATIVE MEDICINE Vol. 6, No. 3 (1983), pgs.
Descriptor terms: mercury; resources for the future; chisso
corporation; japan; world health organization; thomas clarkson;
everglades; who; fish; wildlife; water pollution; minamata; drinking