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Stensberg et al. Page 4 Assessments of Ag NP release into the environment Current models indicate that environmental Ag+ and Ag NPs comprise nearly 15% of the biocidal compounds released as a point source from the plastics and textile industries [35]. Ag NPs have also been identified in the effluent waste from mining, photographic and electronic industrial processes [28,29]. Although there are currently no published data concerning the fate of Ag NPs in hospital waste, the potential for accumulation of Ag NPs from this industry is large considering the increased use of Ag NPs in wound dressings [39], surgical equipment [44], catheters [45,46] and even paint on the walls of hospital rooms [47]. It is estimated that the amount of Ag NPs released into aquatic ecosystems is currently on the order of 10–100 ng/l, a level that is likely to be exceeded within the next decade [48]. Silver nanoparticles are commonly discharged into the environment as a point-source pollutant and thus may be collected in municipal wastewater treatment plants (WWTPs) [26]. However, approximately 7% of Ag NPs entering WWTPs will be accumulated in sludges that are later deployed as agricultural fertilizers [35]. A majority of the Ag NPs in sludge remain in the upper soil layers and can potentially enter surface waters via runoff or groundwater tables [49]. The fate and transport of Ag NPs is also complicated by the fact that while some materials can be complexed as silver sulfide [42], removed by precipitation [50] or agglomeration [21,51], other forms may pass through WWTPs unaltered [35] and/or released as Ag+ [28]. It is important to note that in addition to the potentially adverse effects of increased biological exposure to Ag NPs, there is a risk that an increased release of Ag NPs into the environment may stimulate a rise in bacterial strains with heightened resistance to silver [19]. At present, most studies have focused on planktonic microbes [24,29,49] and very little information is available to outline the effect of Ag NPs on aquatic or terrestrial microbial populations [17]. One study did find a high sensitivity of a crucial soil microbe, Bradyrhizobium canariense, to Ag NPs [52]. However, there is a large body of literature describing the penetration of chemical toxins into bacterial biofilms [24,53,54], with suggestions that such environmental pressures may encourage the development of ‘persister cells’ with high levels of antimicrobial resistance [55]. In fact, the increased resistance to antimicrobials [53] and community-based defense mechanisms [56] are hallmarks of microbial biofilms, a topic that has been studied for decades [57]. The relationship between Ag NPs and increased Ag+ resistance has not been studied, perhaps hindered in part by limitations in technologies to enable such investigations. Mechanisms of Ag NP toxicity Silver nanoparticles are frequently touted as being highly effective as antimicrobial agents while being nontoxic to mammals. However, numerous in vitro studies have demonstrated the toxic effects of Ag NPs on rat liver (BRL3A) and neuronal cells [58,59], human lung epithelial cells [60] and murine stem cells [61]. Ag NPs have also exhibited toxicity in aquatic organisms, including vertebrates [11,62–64]. There is also strong evidence that microorganisms and plants are capable of concentrating nanoparticulate materials, which increases the potential for Ag NPs to accumulate in the food chain [65]. It should be noted, however, that many of the studies to date have used concentrations of Ag NPs that are much higher (>1 ppm) than what could be considered environmentally relevant. So far, most of the information relevant to the mechanisms of Ag NP toxicity has been derived from in vitro studies. Only a handful of mechanistic studies have been conducted in vivo, reflecting a significant gap in knowledge on a topic of increasing concern to the environment as well as human health. Nanomedicine (Lond). Author manuscript; available in PMC 2012 May 24. NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptPDF Image | Toxicological studies on silver nanoparticles
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