304
Trace Elements in Abiotic and Biotic Environments
44.6 HUMANS
Normal concentrations of Ag in human tissues are low: in human body (70 kg) it is
approximately 2 mg (Emsley 2011), of which in the blood it is <0.0023 mg/L, in the
liver and kidney 0.05 mg/kg (Wan et al. 1991), and in the rib bone ≤0.011 mg/kg
(Zaichick et al. 2011).
Silver is not known to perform any essential function in humans. It may enter the
human body through the gastrointestinal tract, lungs, mucous membranes, and skin
lesions, when medicines containing Ag are also taken or applied to the skin or gums.
Generally, much less Ag enters the body through the skin than through the lungs or
stomach. Ingested Ag compounds are absorbed by the body at a level of about 10%, with
only 2%–4% being retained in tissues. Most of the Ag transported in the blood is bound
to globulins. In tissues, it is present in the cytosolic fraction, bound to metallothionein.
Silver is stored mainly in the liver and skin, and in smaller amounts in other organs.
Metallic Ag is not soluble in aqueous solutions nor is it readily solubilized by
any physiological mechanisms. Therefore, it is poorly absorbed after exposure and
is more likely to be excreted by the body than its soluble compounds. Finally, accumulated Ag can be oxidized to Ag sulfide or Ag selenide, resulting in blue-gray
pigmentation.
Soluble Ag compounds are absorbed by the body more readily as a result of their
ability to bind to proteins, DNA, and RNA. The majority of occupational exposures
are related to soluble Ag compounds, which seem to cause toxic effects at lower
concentrations than metallic Ag and insoluble Ag compounds (Drake et al. 2005).
Silver may be metabolized to all tissues other than the brain and the central nervous
system (Lansdown 2010).
Colloidal Ag protein has been used as an allergy and cold medication, in eye
drops to alleviate soreness, and for the treatment of various ailments. The adverse
effects from extended use of colloidal Ag protein may be discolored fingernails, ocular argyrosis, and generalized argyria. If there is overexposure, Ag can accumulate
in the skin, liver, kidneys, corneas, gingiva, mucous membranes, nails, and spleen.
Due to Ag affinity for the thiol groups in the liver, it binds to reduced glutathione and
is transported into the bile, thus depleting the amount of reduced glutathione available for biochemical pathways (Drake et al. 2005).
Most foods contain traces of Ag within the range of 10–100 μg/kg (Table 44.1).
Its low levels in drinking water, generally below 5 μg/L, are not relevant to human
health with respect to argyria. By contrast, Ag salts used to maintain the bacteriological quality of drinking water may be of some risk. Higher levels of Ag, up to
100 μg/L, could be tolerated, without risk to health (WHO 2011a). The mean intake
of Ag in Russia is <50 μg/day and is similar to levels found for most other countries,
70–100 μg/day (Gorbunov et al. 2004).
New problems for humans and animals create Ag in the form of nanoparticles
(AgNPs). Nano-silver is used in an increasing number of products. Three major
product categories are food, consumer products, and medical products. Nano-silver
products are mainly manufactured in Asian countries (81 products) and in the United
States (60 products); only a very small amount of these products have European
countries (Wijnhoven et al. 2009).