(WHO/SDE/WSH/03.04/13).12.106

Silver occurs naturally mainly in the form of its very insoluble and immobile oxides, sulfides and some salts. It has occasionally been found in groundwater, surface water and drinking-water at concentrations above 5mg per liter. Levels in drinking-water treated with silver for disinfection may be above 50mg per liter. A recent estimate of daily intake is about 7mg per person. Only a small percentage of silver is absorbed. Retention rates in humans and laboratory animals range between 0 and 10%.The only obvious sign of silver overload is argyria, a condition in which skin and hair is heavily discolored by silver in the tissues. An oral NOAEL for argyria in humans for a total life time intake of 10 g of silver was estimated on the basis of human case reports and long-term animal experiments. The low levels of silver in drinking-water, generally below 5mg per liter, are not relevant to human health with respect to argyria. On the other hand, special situations exist where silver salts may be used to maintain the bacteriological quality of drinking-water. Higher levels of silver, up to 0.1 mg per liter (this concentration gives a total dose over 70 years of half the human NOAEL of 10 g), could be tolerated in such cases without risk to health. There is no adequate data with which to derive a health-based guideline value for silver in drinking-water. History of guideline development The 1958, 1963 and 1971 WHO International Standards for Drinking-water did not refer to silver. In the first edition of the Guidelines for Drinking-water Quality, published in 1984, it was not considered necessary to establish a guideline value for silver in drinking-water. No health-based guideline value for silver was proposed in the 1993 Guidelines. Where silver salts are used to maintain the bacteriological quality of 12. CHEMICAL FACT SHEETS 435 drinking-water, levels of silver up to 0.1 mg per liter can be tolerated without risk to humans.