148 Ecology and Applied Environmental Science
Organic mercury occurs in the form of methyl mercury CH 3 Hg + (soluble,
an important pollutant of water bodies) and dimethyl mercury (CH 3 ) 2 Hg
(insoluble). Methyl mercury accumulates in tissues and is particularly toxic.
It reaches humans mainly through fish in the diet. It accumulates in the
nerve cells, attacks the metabolism of the nervous system and disturbs the
action of various enzymes. Its half-life in the human body is around 70 days.
The symptoms of poisoning may appear weeks or months after ingestion
and include numbness and pain in the lips or hands and feet, ataxia, vision
and hearing impairment, and emotional disturbances. This type of illness
is known as “Minamata disease” because it appeared for the first time in
epidemic form in the small town of Minamata in Japan in the 1950s. There
the mercury came from a chemical plant that released its wastes into the sea.
It ended up in the bodies of people and animals via the marine food chain.
In 1970, 121 serious cases of poisoning including 46 deaths had been officially recognised, but it would appear that the disease affected, to a greater
or lesser degree, a much larger number of people. Children born by affected
mothers suffered from genetic defects. Other cases of toxic effects have been
reported from Iraq, Pakistan and Guatemala, due to consumption of seeds
sprayed with a fungicide containing mercury.
In 1969 it was discovered that inorganic mercury can be converted to
organic (methyl and dimethyl) mercury by microorganisms that are very common on the beds of seas and other water receptors. In the United States and
Canada a problem arose of potential poisoning of humans when unusual concentrations of organic Hg were found in Lake St. Clair’s fish. It was believed
that this process was responsible for the formation of organic mercury.
The United States and the European Union have set rules for maximum
permissible concentrations of mercury in the bodies of fish used for food,
and in drinking water, sea water and freshwater.
7.4.3 Cadmium and Zinc
Cadmium (Cd) and zinc (Zn) are metals with similar chemical properties
that occur together in nature. Their ecological difference lies in the fact
that zinc is a trace element essential for life (the body of an adult human
contains 1.4–2.3 grams of zinc), whereas cadmium is not used in biological processes, but is strongly toxic. It has the tendency to displace zinc
from many enzymes. Zinc becomes toxic only in very large concentrations,
i.e. 10–15 times higher than normal in the organism.
Cadmium is used in metal plating, in the manufacture of plastics,
batteries, etc. Substantial amounts are released into the air during the
smelting of metals and burning of plastics, as well as during the burning of
solid and liquid fuels containing cadmium. The quantities of cadmium and
zinc released into the environment from mines are many times greater than
Organic mercury occurs in the form of methyl mercury CH 3 Hg + (soluble,
an important pollutant of water bodies) and dimethyl mercury (CH 3 ) 2 Hg
(insoluble). Methyl mercury accumulates in tissues and is particularly toxic.
It reaches humans mainly through fish in the diet. It accumulates in the
nerve cells, attacks the metabolism of the nervous system and disturbs the
action of various enzymes. Its half-life in the human body is around 70 days.
The symptoms of poisoning may appear weeks or months after ingestion
and include numbness and pain in the lips or hands and feet, ataxia, vision
and hearing impairment, and emotional disturbances. This type of illness
is known as “Minamata disease” because it appeared for the first time in
epidemic form in the small town of Minamata in Japan in the 1950s. There
the mercury came from a chemical plant that released its wastes into the sea.
It ended up in the bodies of people and animals via the marine food chain.
In 1970, 121 serious cases of poisoning including 46 deaths had been officially recognised, but it would appear that the disease affected, to a greater
or lesser degree, a much larger number of people. Children born by affected
mothers suffered from genetic defects. Other cases of toxic effects have been
reported from Iraq, Pakistan and Guatemala, due to consumption of seeds
sprayed with a fungicide containing mercury.
In 1969 it was discovered that inorganic mercury can be converted to
organic (methyl and dimethyl) mercury by microorganisms that are very common on the beds of seas and other water receptors. In the United States and
Canada a problem arose of potential poisoning of humans when unusual concentrations of organic Hg were found in Lake St. Clair’s fish. It was believed
that this process was responsible for the formation of organic mercury.
The United States and the European Union have set rules for maximum
permissible concentrations of mercury in the bodies of fish used for food,
and in drinking water, sea water and freshwater.
7.4.3 Cadmium and Zinc
Cadmium (Cd) and zinc (Zn) are metals with similar chemical properties
that occur together in nature. Their ecological difference lies in the fact
that zinc is a trace element essential for life (the body of an adult human
contains 1.4–2.3 grams of zinc), whereas cadmium is not used in biological processes, but is strongly toxic. It has the tendency to displace zinc
from many enzymes. Zinc becomes toxic only in very large concentrations,
i.e. 10–15 times higher than normal in the organism.
Cadmium is used in metal plating, in the manufacture of plastics,
batteries, etc. Substantial amounts are released into the air during the
smelting of metals and burning of plastics, as well as during the burning of
solid and liquid fuels containing cadmium. The quantities of cadmium and
zinc released into the environment from mines are many times greater than