Chapter 3. Uptake and Metabolism of Silver in the Human Body

Abstract

Silver is a highly reactive metal which ionises in the presence of moisture, body fluids and exudates to be absorbed by ingestion, inhalation and percutaneous absorption. Up to 10% of silver ingested may be absorbed gastro-intestinally, but uptake is dependent upon the quality of the diet and concentration of dietary fibre and silver-binding substances. Inhalation of silver from industrial fumes and particulate matter in the air was a potential risk of higher body silver levels and consequent symptoms of argyria, but the problem is reduced by environmental control procedures. Percutaneous uptake of silver is low on account of the strong metal-binding capacity of epidermal keratins, precipitating anions on the skin surface and sebaceous exudates. Uptake of silver sulphadiazine is higher in thermally damaged skin but blood silver levels of 310µg/L have not been associated with pathological changes. Insignificant levels of percutaneous absorption can be expected where silver is used as an antibiotic in hygiene textiles. Uptake of Ag+ from silver acupuncture needles, intraparenteral insertion silver-coated venous catheters, cardiac prostheses, surgical needles and other devices has rarely been a cause of argyria, and has not been associated with fatality. The implications of Ag+absorption from silver nitrate as used in early topical and periorbital anti-infective therapies and anti-parasitic procedures is discussed.Silver is metabolised in the human body as a complex with metal carrier proteins to be deposited in bone and soft tissues. Thus, metallothioneins are induced by silver and serve as major carrier proteins and cytoprotective agents. Macrophages sequestrating silver acting as an additional cytoprotective mechanism for the body; silver precipitates are bound in lysosomal vacuoles. Silver is not regarded as a cumulative toxin and is excreted mainly in the bile and via the urinary tract. A small proportion of silver remains in the body bound to hydroxyapatite in bone. The principle site for deposition of silver precipitates (as sulphide or selenide) is as argyria discoloration in dermal regions of the skin. These are cosmetically undesirable but of minimal toxicological significance.