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Assays / Trace
Metals/ Mercury
Mercury and its compounds have been used by man for many
hundreds of years and their toxicities are well recognised. Metallic
mercury is used in the manufacture of thermometers and scientific
instruments, and in the electrical, electronic and chemical industries.
Inorganic and organic compounds of mercury are widely used as anti-bacterial
and anti-fungal agents in the form of liquids, powders and paints.
Mercury is unique among the metals in being a volatile liquid at
ambient temperatures with a measurable vapour pressure.
Use of liquid mercury in religious practices of Caribbean
origin has been reported in the USA and the element may also be
a component of Indian sub-continental ethnic remedies.
Toxicity
Ingestion of metallic mercury appears to be
relatively harmless with little intestinal absorption. However,
metallic mercury presents a considerable hazard by virtue of the
possible inhalation of its vapour, even at ambient temperature.
Inhalation of a high concentration of mercury vapour may give rise
to pneumonitis and pulmonary oedema which can be fatal. Chronic
exposure to lower concentrations causes gingivitis and stomatitis
with excessive salivation, headaches and central nervous system
effects with tremor and personality changes. Ingestion of inorganic
salts produces abdominal pain, diarrhoea and vomiting and corrosion
of mucosal membranes. The kidney is the major target organ for inorganic
mercury following absorption, with renal tubular necrosis the result.
Some elemental mercury is oxidised to the inorganic form and may
produce renal toxicity. An idiosyncratic response to any form of
mercury exposure is glomerulonephritis with immune complexes detected
in the basement membrane.
Use of mercury is widespread in dentistry as a tooth
filling in the form of an amalgam prepared by mixing metallic mercury
with a silver-tin alloy. Increased urinary excretion of mercury
has only been observed immediately after extensive fillings and
there appears to be little evidence of long term hazard to the patients
involved, even when corrosion and break-down of the amalgam takes
place. However, the preparation of such amalgams can be hazardous
to dental personnel if insufficient care is taken; cases of poisoning
in dentists and their assistants have been reported. Symptoms from
mild long-term exposure may be very non-specific in the form of
headaches and irritability.
One of the most toxic mercury compounds is methyl mercury,
which is widely used as an antifungal agent. Its toxicity results
from its lipophilic properties, which allow its rapid penetration
through the body, particularly into the brain and nervous tissue.
It is slowly metabolised to inorganic mercury and the biological
half-life is some 70 days. Widespread toxicity has occurred in several
countries, where wheat treated with methyl mercury and intended
for use as seed was used to make bread. Inorganic mercury may be
converted by bacteria into methyl mercury, and such a transformation
was responsible for a major outbreak of poisoning in the Minamata
Bay area of Japan. In this case, inorganic mercury from an industrial
plant was released into a river, where it was transformed biologically
into methyl mercury, producing a high level in the fish consumed
by local people. Central nervous system toxicity was the main effect,
with ataxia, muscle weakness, tunnel vision, tremor, numbness and
tingling. Many of the effects were irreversible. Teratogenicity
and foetal death also occurred.
Laboratory Indices of Mercury Status
For suspected exposure to elemental mercury
or to inorganic mercury salts, determination of urinary mercury
output is the estimation of choice. An early morning urine sample
is appropriate in the first instance followed by a 24 hour collection
if the result is equivocal. In cases of exposure to organic mercury
derivatives, measurement of blood mercury concentration is preferred.
References:
Taylor A and Marks V. Diagnostic value of
urine mercury measurements. Annals of Clinical Biochemistry 1977;
14,: 297-300
Amin-Zaki L, Majeed MA, Clarkson TW, Greenwood MR.
Methylmercury poisoning in Iraqi children: clinical observations
over two years. BMJ 1978; 1: 613-616
UNEP/ILO/WHO. International Programme on Chemical Safety,
Environmental Health Criteria 118 Inorganic Mercury. WHO, Geneva,
1991
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