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