Silver
(endorsed 1996)
Guideline
Based on health considerations, the concentration of silver in drinking water should not exceed 0.1 mg/L.
General description
Silver concentrations in natural source waters are generally very low, less than 0.0002 mg/L. In some countries silver and silver salts are used for disinfection and preservation of water, and this can result in higher silver concentrations.
Silver is a precious metal and is used in the production of tableware, jewellery and coins. It is also used in batteries, mirrors, as a chemical catalyst, and as an antiseptic agent.
Traces of silver can be found in most foods. The daily dietary intake has been estimated at between 0.03 mg and 0.09 mg.
Typical values in Australian drinking water
Australian drinking water supplies have not been routinely monitored for silver.
Treatment of drinking water
Silver can be readily removed from drinking water by conventional coagulation or lime softening.
Measurement
The concentration of silver in drinking water can be determined by graphite furnace atomic absorption spectroscopy or inductively coupled plasma emission spectroscopy (APHA Method 3500-Ag Parts B or C 1992). The limits of determination are 0.001 mg/L and 0.01 mg/L respectively.
Health considerations
Although silver can be found in many biological substances, it is not considered an essential trace element for mammals.
It has been estimated that less than 10% of dietary silver is absorbed by the gastrointestinal tract. Silver is stored mainly in the liver and skin and is capable of binding to amino acids and proteins.
The best-known clinical condition of silver intoxication is argyria, which results in a bluish-grey metallic discolouration of the skin, hair, mucous membranes, mouth and eye. Most cases have been associated with self-administration of silver preparations, or occupational exposure to silver and silver compounds.
Experiments with laboratory rats and mice have reported similar results. Very high concentrations of silver in drinking water (over 600 mg/L) for a lifetime caused discolouration in the thyroid and adrenal glands, the choroids of the brain and eye, and the liver and kidney. Some hypoactive behaviour was also reported.
No data are available on the carcinogenicity of silver. Silver salts are not mutagenic in tests with bacteria, but can induce damage in mammalian DNA.
Derivation of guideline
The guideline value for silver in drinking water was derived as follows:
where:
0.4 mg/day is derived from a human lifetime no-effect level of 10 g (Hill and Pillsbury 1939).
0.5 is the proportion of total daily intake attributable to the consumption of drinking water.
2 L/day is the average amount of water consumed by an adult.
No additional safety factors were used, as the calculation was based on a human no-effect level.
It is unlikely that silver concentrations in drinking water would ever reach a concentration that could cause adverse effects. Silver or silver salts should not be used as antimicrobial agents unless no other disinfectants are available.
References
APHA Method 3500-Ag Part B (1992). Silver: Atomic Absorption Spectrometric method. Standard Methods for the Examination of Water and Wastewater, 18th edition. American Public Health Association, Washington.
APHA Method 3500-Ag Part C (1992). Silver: Inductively Coupled Plasma method. Standard Methods for the Examination of Water and Wastewater, 18th edition. American Public Health Association, Washington.
Hill WR, Pillsbury DM (1939). Argyria, the Pharmacology of Silver. The Williams and Wilkins Co., Baltimore, Maryland.
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