SCENIHR publishes its preliminary opinion on nanosilver

Nanosilver and other forms of silver are widely used nowadays for their antibacterial activity.

The characteristics and the use of different forms of silver (including silver salts, silver oxides and silver materials appearing as silver wires, silver nanoparticles (Ag-NP) and others) are used in consumer and medical products may have different physico-chemical properties such as distinct solubility and surface-to-volume ratio, which all may affect their fate and biological activity.

In medical care forms of (nano)silver are used, for example in: wound dressings and catheters to reduce infections. In consumer products, forms of (nano)silver are used, for example in: sports and other textiles, washing powders and deodorants, where (nano)silver should reduce odours producing bacteria.

In view of the increasing use of (nano)silver compounds several questions were raised: Are these consumer and health care products hazardous for the environment and/or for human health; can micro-organisms become resistant against silver (including nanosilver and silver-based compounds); and is there any risk of an increased bacterial resistance with regard to their pervasive and widespread use.

Currently, products that contain forms of (nano)silver are difficult to track since they are marketed under numerous brand names, and, with a few exceptions, current labelling regulations do not specifically require listing nanomaterials as a constituent.

Consumer exposure depends on the location of the nanomaterial in consumer/medical products and/or the manipulation of the product. To determine the level of exposure, more information is needed on the concentrations of silver in products, the size and the form in which it is present and the probability of release of Ag-NP or Ag ions from the products.

When evaluating the end-of-life phase of Ag-NP containing products it is assumed that their waste management (recycling, wastewater treatment, landfilling, and incineration)is similar to conventional products. Consequently, silver content in non-recycled waste will ultimately end up in the environment, either as solid waste in landfills, emission from wastewater treatment plants, or as residual waste from incineration plants.

In humans, bioavailability of silver after oral and pulmonary administration of Ag-NPs has been shown. The main target organs for Ag-NP deposition after systemic availability are spleen, liver and kidney while there is less distribution to other organs. Additionally, high levels of silver were sometimes found in the testes.

The best-described adverse effects in humans of chronic exposure to silver are a permanent bluish-grey discoloration (argyria or argyrosis) of the skin and/or eyes. In several animal toxicity studies an increase of various liver enzymes was observed, indicating liver toxicity after a silver nanoparticle administration. However, liver toxicity could not be observed by histopathology. In vivo, oral exposure to silver nanoparticles did not consistently result in alterations of the non-specific immune responses. However, for iv exposure it was shown that the immune system is the most sensitive target for AgNP toxicity. In vitro, genotoxic effect of nanosilver has been reported. As the studies available on the in vivo genotoxicity of Ag-NPs are few, it was concluded that further studies are required to draw final conclusions.

It should be stressed that, in many studies, the release of ionic silver has been found to be the main cause of toxicity (in humans, in the environment and in hygienic applications), evertheless an increasing number of studies found that this release cannot alone account for the toxic effects observed.

Current human risk assessments are mainly based on the development of argyria. In workers the threshold limit value for metallic silver is 0.1 mg/m³ and 0.01 mg/m3 for silver salts. For the general population the WHO did set a No Observable Adverse Effect 5 Level (NOAEL) related to the sum of all exposure routes of 5 µg/kg bw/d. A detailed risk assessment of nanosilver has not been performed since too little information is available.

Ag-NP undergoes several transformations when it is released into the environment. Apart from aggregation and agglomeration, the important ones are dissolution and subsequent speciation, such as the formation of silver chloride and silver sulphide. Silver sulphide is particularly important because it is highly stable; sulphide is available in wastewater treatment plants and also in many freshwater bodies. The chemical species that are actually present determine the bioavailability and toxicity of silver in the environment. Bioavailability of Ag-NP in soils depends on both particle and soil properties. In general, the mobility of ionic silver in soil and sediments is very limited; however, particles may behave differently.

There is an effect of Ag-NP on the composition of bacterial flora and on the bacterial adaptation associated with certain conditions and uses. However, evidence is often fragmentary and focused on few specific cases. There is a paucity of information on potential resistance mechanisms to Ag-NP. Some of the genetic basis of bacterial resistance to ionic silver has been well documented, notably the expression of well characterised efflux systems. Recent transcriptomic and proteomic data suggest that a decrease in oxidative damage by regulation of anaerobic respiration may be important. Exposure to ionic silver and Ag-NP produces a stress-response and affects gene expression. More data is needed to better understand bacterial response to ionic silver and Ag-NP exposure.

Regarding the hazard associated with the dissemination of resistance mechanism following the use of Ag-NP, no documentation is available at this moment. This represents a serious gap in knowledge.

Source: SCENIHR

Image Source: Fotolia

Cited publication: SCENIHR (Scientific Committee on Emerging and Newly Identified Health Risks), Nanosilver: safety, health and environmental effects and role in antimicrobial resistance, Date of adoption

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