Mercury
(endorsed 1996)
Guideline
Based on health considerations, the concentration of total mercury in drinking water should not exceed 0.001 mg/L.
General description
Natural release of mercury into drinking water is extremely low, but contamination can result from industrial emission or spills. Mercury compounds fall into two categories: inorganic mercury salts, many of which are very insoluble in water; and organic mercury compounds, the most notable being methyl mercury. Inorganic mercury can be converted into methyl mercury, possibly by the action of bacteria in sediments, and can then readily enter the food chain.
Mercury is used widely in electrical components including cells, lamps, arc rectifiers and switches. It is also used in dental amalgams, fungicides, antiseptics, preservatives and pharmaceuticals.
Concentrations of total mercury in natural water are generally so low that accurate analysis is difficult. Studies overseas have reported concentrations of less than 0.0005 mg/L, with some sources less than 0.00003 mg/L (30 ng/L). The highest value was 0.0055 mg/L from some wells in Japan.
Food is the main route of exposure, with highest concentrations found in fish and fish products. The average Australian adult dietary intake of mercury is approximately 0.004 mg per day. Drinking water is likely to constitute only a small fraction of total intake.
Typical values in Australian drinking water
In major Australian reticulated supplies, the concentrations of total mercury range up to 0.001 mg/L, with typical concentrations usually less than 0.0001 mg/L.
Treatment of drinking water
Coagulation is moderately effective in reducing the concentration of inorganic mercury in drinking water. Granular activated carbon is effective in removing both inorganic and organic mercury from water.
Measurement
The concentration of total mercury in drinking water can be determined by the cold vapour atomic absorption method (APHA 3500-Hg Part B 1992). The limit of determination is 0.0001 mg/L.
Health considerations
Inorganic mercury
Less than 15% of inorganic mercury in drinking water is absorbed by the gastrointestinal tract. Inorganic mercury compounds accumulate in the kidney and have a long biological half-life, probably many years.
An extensive review and summary of the human and animal toxicity data for inorganic mercury is available (IPCS 1991).
Many studies have looked at groups of workers occupationally exposed to mercury, and have reported health effects including tremors, mental disturbances and gingivitis (inflammation of the mucous membrane surrounding the teeth). The main toxic effects are to the kidney, leading to kidney failure.
In animal studies, the principal target organs of mercury toxicity are the kidney and the central nervous system. Some disruption to ovulation in female rats has also been reported.
Various reports indicate that inorganic mercury binds to, and damages, mammalian DNA. Some evidence of carcinogenicity in rats has been reported.
Organic mercury
Organic mercury compounds are unlikely to be found in uncontaminated drinking water; however, the toxic effects are more severe than those of inorganic mercury.
An extensive review and summary of the human and animal toxicity data for methyl mercury is available (IPCS 1990).
Methyl mercury compounds are almost completely absorbed by the gastrointestinal tract. Methyl mercury has greater lipid solubility than inorganic mercury and can cross biological membranes, especially in the brain, spinal cord, peripheral nerves and placenta.
The main effects of methyl mercury poisoning are severe irreversible neurological disorder and mental disability.
In Japan, two major epidemics of methyl mercury poisoning, known as Minamata disease, were caused by the industrial release into Minamata Bay of methyl mercury and other mercury compounds. The compounds accumulated in fish, which were subsequently eaten by humans. Other countries have reported cases of poisoning caused by mercury contamination of bread and cereal.
Animal studies with rats, cats, monkeys and squirrels have shown similar results, with the main effects of long-term exposure being behavioural changes, neurological disturbances and disturbances to the movement of legs and tails.
Data are insufficient to determine the carcinogenic effects of methyl mercury; however, it is active in inducing chromosomal aberrations in vivo.
Derivation of guideline
The guideline value for mercury in drinking water was derived as follows:
where:
0.00047 mg/kg body weight per day is the maximum tolerable daily intake to ensure that adverse effects will not occur (WHO 1988).
70 kg is the average weight of an adult.
0.1 is the proportion of total daily intake attributable to the consumption of water.
2 L/day is the average amount of water consumed by an adult.
The maximum tolerable daily intake value includes adequate safety factors. No additional safety factors are necessary.
The guideline value was set on the basis of the toxicity of methyl mercury, as this is the most toxic form. It is likely that methyl mercury would be less than 10% of the total mercury concentration.
The guideline value should be sufficient to protect pregnant women and nursing mothers, who are at greatest risk from the adverse effects of methyl mercury. Data are insufficient to determine a separate value for this group.
References
APHA Method 3500-Hg Part B (1992). Mercury: Cold vapor Atomic Absorption method. Standard Methods for the Examination of Water and Wastewater, 18th edition. American Public Health Association, Washington.
IPCS (International Programme on Chemical Safety) (1990). Methyl Mercury. Environmental Health Criteria, 101. World Health Organization, IPCS.
IPCS (International Programme on Chemical Safety) (1991). Inorganic Mercury. Environmental Health Criteria, 118. World Health Organization, IPCS.
WHO (World Health Organization) (1988). Toxicological evaluation of certain food additives and contaminants: Methylmercury. 33rd meeting of the Joint FAO/WHO Expert Committee on Food Additives. WHO Food Additive Series, 24, 296–328.
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