Environmental Engineering Reference
In-Depth Information
in 1966 in view of the drastic reduction in the wild bird population attributed to
treated seed. In Yakima, Washington, early recognition of the hazard prevented
illness when 16 members of an extended family were exposed to organic mercury
poisoning in 1976 by the consumption of eggs from chickens fed mercury-treated
seed grain. The grain contained 15,000 ppb total mercury, an egg 596 and 1,902
ppb, respectively, of organic and inorganic mercury. Blood levels in the family
ranged from 0.9 to 20.2 ppb in a man who ate eight eggs per day. A whole-blood
level above 20 ppb may pose a mercury poisoning hazard.
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It is also reported that crops grown from seed dressed with minimal amounts
of methylmercury contain enough mercury to contribute to an accumulation in
the food chain reaching humans. The discovery of moderate amounts of mercury
in tuna and most freshwater fish and relatively large amounts in swordfish by
many investigators in 1969 and 1970 tended to further dramatize the problem.
Methylmercury has an estimated biological half-life of 70 to 74 days in
humans, depending on such factors as age, body mass, and metabolism, and
is excreted, mostly in the feces, at the rate of about 1 percent per day. Mercury
persists in large fish such as pike for one to two years.
Elemental metallic mercury volatilizes on exposure to air, especially if heated,
and in that state poses a distinct hazard. Mercury spills and the mercury from
broken thermometers and barometers must be meticulously cleaned and the space
ventilated and isolated until the mercury vapor level is no longer detectable by
a “mercury sniffer” or similar device. Metallic mercury should never be incin-
erated; toxic gases would be released. Mercury should normally be stored and
handled in an airtight enclosure with extreme care. Laboratory use must be care-
fully controlled and monitored.
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Certain mercury compounds may be absorbed
through the skin, gastrointestinal tract, and respiratory system (up to 98 per-
cent), although elemental mercury and inorganic mercury compounds are not
well absorbed through the digestive tract.
Mercury is ubiquitous in the environment from both natural and manmade
sources. Natural sources include leachings, erosion, and volatilization from
mercury-containing geologic formations. Carbonaceous shales average 400 to
500 ppb Hg, up to 0.8 ppm in soil. Manmade sources include waste discharges
from chlor-alkali and paper pulp manufacturing plants, mining and extraction of
mercury from cinnabar, chemical manufacture and formation, the manufacture
of scientific instruments, mercury seals and controls, treated seeds, combustion
of fossil fuels, atmospheric deposition, and surface runoff. As a result of these
processes, mercury may end up in lakes, streams, tidal water, and the bottom
mud and sludge deposits.
Microorganisms and macroorganisms in water and bottom deposits can
transform metallic mercury, inorganic divalent mercury, phenylmercury, and
alkoxyalkylmercury into methylmercury. The methylmercury thus formed
and perhaps other types, in addition to that discharged in wastewaters, are
assimilated and accumulated by aquatic and marine life such as plankton, small
fish, and large fish. Alkaline waters tend to favor production of the more volatile
dimethylmercury, but acid waters are believed to favor retention of the dimethyl
