How does Silver get into your water?
Silver is present in silver compounds primarily in the oxidation state +1 and less frequently in the oxidation state +2. A higher degree of oxidation is very rare. The most important silver compounds from the point of view of drinking water are silver nitrate (AgNO3) and silver chloride (AgCl).
Silver occurs in our soils mainly in the form of its insoluble and therefore immobile chloride or sulfide. As long as the sulfide is not oxidised to the sulfate, its mobility and ability to contaminate the aquatic environment are negligible. Silver in river water is "dissolved" by complexation with chloride and humic matter.
Ambient air concentrations of silver are in the low nanogram per cubic metre range.
Average silver concentrations in natural waters are 0.2–0.3 μg/litre. Silver levels in drinking water in the Britain that had not been treated with silver for disinfection purposes varied between “non-detectable” and 5 μg/litre.
In a survey of Canadian tap water, only 0.1% of the samples contained more than 1–5 ng of silver per litre.
Water treated with silver may have levels of 50 μg/litre or higher; most of the silver will be present as nondissociated silver chloride.
Most foods contain traces of silver in the 10–100 μg/kg range.
The Health Implications of Silver
The median daily intake of silver from 84 self-selected diets, including drinking water, was 7.1 μg. Higher figures have been reported in the past, ranging from 20 to 80 μg of silver per day.
The relative contribution of drinking water is usually very low. Where silver salts are used as bacteriostatic agents, however, the daily intake of silver from drinking water can constitute the major route of oral exposure.
Silver may be absorbed via the gastrointestinal tract, lungs, mucous membranes, and skin lesions. The absorption rate of colloidal silver after oral application can be as high as 5%.
Most of the silver transported in blood is bound to globulins. In tissues, it is present in the cytosolic fraction, bound to metallothionein. Silver is stored mainly in liver and skin and in smaller amounts in other organs. The biological half-life in humans (liver) ranges from several to 50 days. The liver plays a decisive role in silver excretion, most of what is absorbed being excreted with the bile in the faeces. In mice, rats, monkeys, and dogs, cumulative excretion was in the range 90–99%. Silver retention was about 10% in the dog, <5% in the monkey, and <1% in rodents.In humans, under normal conditions of daily silver exposure, retention rates between 0 and 10% have been observed.
The estimated acute lethal dose of silver nitrate is at least 10 g.
The only known clinical picture of chronic silver intoxication is that of argyria, a condition in which silver is deposed on skin and hair, and in various organs following occupational or iatrogenic exposure to metallic silver and its compounds, or the misuse of silver preparations.
