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Mercury [Hg, 80]
to inorganic Hg, which accumulates primarily in the liver and kidneys. MeHg is
estimated to have a biological half-life of approximately 50 days. The majority of Hg
in the body is excreted through the feces, with a small amount excreted as inorganic
Hg in the urine.
The primary effects associated with oral exposure to organic Hg compounds are
neurological effects and developmental neurotoxicity; effects are similar for both
acute and chronic exposure. Because MeHg is hydrophobic, it easily penetrates the
cell membrane and inactivates the enzymes that participate in protein synthesis
within the central nervous system. Especially, it disrupts cell division, formation of
microtubule and movement of neuroaxis. Symptoms of organic Hg toxicity include
a tingling sensation in the extremities; impaired peripheral vision, hearing, taste,
and smell; slurred speech; muscle weakness and an unsteady gait; irritability; memory loss; depression; and sleeping difficulties. Exposure of a fetus or young child to
organic Hg can result in effects on the development of the nervous system, affecting
fine motor function, attention, verbal learning, and memory. The gastrointestinal
effects include nausea, vomiting, and abdominal pain; higher doses can cause diarrhea and an exposure-related colitis. Other symptoms include the discoloration of
the gums (similar to poisoning with inorganic compounds), sialorrhoea, and perioral
paraesthaesia. Central nervous effects after slight exposure include numbness in the
limbs; as the level of exposure increases, there are tremors, ataxia, dysarthria, and
visual field constrictions (Langford and Ferner 1999).
The chronic toxicity of MeHg is best exemplified by the epidemic poisonings in
Japan and Iraq. In Minamata and Niigata, Japan, MeHg poisoning resulted from
the ingestion of fish that had accumulated MeHg and other Hg compounds that
were released from industrial sources into surface water. By a process of biomagnification, the MeHg accumulated to sufficient amounts in predatory fish to poison
over 800 people, who relied on the fish for their nutrition. Many of them were
infants in utero.
In Iraq, over 6000 individuals were hospitalized and 459 individuals died, as a
result of consuming bread prepared with flour made from wheat and barley treated
with a methylmercurial fungicide. Methyl Hg concentration in the wheat flour ranged
from 4.8 to 14.6 mg/kg. The clinical symptoms included paresthesia, visual disorders, dysarthria, and deafness.
International Agency for Research on Cancer (IARC) in 1993 determined that
MeHg compounds are possible human carcinogens (Group 2B), based on inadequate
human data but sufficient animal data showing a link to certain cancers, particularly
renal cancer. Metallic Hg and inorganic Hg compounds were determined to be not
classifiable as to their carcinogenicity (IARC 1993, 2013).
WHO established a provisional tolerable weekly intake (PTWI) for inorganic Hg
of 4 μg/kg bw. The previous PTWI of 5 μg/kg bw for total Hg was withdrawn. The
new PTWI for inorganic Hg was considered applicable to dietary exposure to total
Hg from foods other than fish and shellfish (WHO 2011b). European legislation set a
maximum level for total Hg in fish and fishery products of 0.5 mg/kg FW, except for
some predatory species with limit 1.00 mg/kg FW (EC 2006).
Total Hg levels in foods other than fish products are generally low (range 0.0001–
0.050 mg/kg). The highest levels are found in fungi (Table 27.4).