The article ’Silver soils’ (Chemistry World, February 2012, p36) highlights some interesting recent events relating to the environmental impact of silver residues. While a wealth of evidence exists to show that silver ions are toxic to lower organisms, there is less evidence that nanocrystalline or other forms of silver present a health risk to higher animals or humans. I have studied the toxicology and use of silver for more than 20 years and apart from rare cases of allergy, the major adverse responses to silver are the cosmetically and socially undesirable conditions known as argyria and argyrosis, resulting from deposition of silver sulfide or silver selenide in the skin and eye respectively.
In management and estimation of risks of silver contamination and silver efficacy as an antibacterial/antifungal agent, it has to be understood that nanocrystalline silver not only has a much greater solubility (or ionisation) in water compared to metallic silver, but possibly very different physicochemical properties. All forms of metallic silver are relatively inert in the presence of biological materials but the silver ion released in the presence of water, body fluids and tissues is central to silver’s action as an antibacterial agent or preservative.
The technology, safety and efficacy of silver in textiles and other clothing is well discussed in Biofunctional Textiles and the Skin (U Hipler and P Elsner, 2006), where it is emphasised that the levels of silver residues washing down sewers from this source are minute. Far higher levels derive from the silver-copper filtration systems that have been in use for many years in water purification. In the view of the World Health Organization, a total lifetime consumption of 10g of silver should be considered as a human no-effect level and the concentrations permitted in drinking water (0.1mg/l) are well tolerated without risk to health after 70 years’ exposure.
With regard to the term ’colloidal silver’, I suspect the article refers to a distribution of minute silver particles in water but a range of colloidal silver antibiotics were introduced in the 19th century as ’germ fighters’. These early colloidal silver preparations based upon silver-protein complexes were classified as strong or mild type depending on the level of silver ion release. Widespread cases of serious argyria and argyrosis are reported and there is no substantive evidence for the clinical efficacy of these preparations. Although no longer legally sold, they are available through clandestine means.
Apart from a greying of the complexion and eyes as a consequence of long term unprotected industrial exposure or use of the now illegal colloidal silver antibiotics, silver should be considered one of the safest metals in the periodic table.
A Lansdown FRSC
Imperial College London, UK