If ENMs get into the environment, new routes of exposure open up for humans (through drinking water, for example) and other organisms. Silver nanoparticles, which are used for their antimicrobial properties, have drawn research scrutiny for their environmental fate. Many researchers consider these particles quite likely to enter the aquatic environment because they can wash out of antimicrobial clothing and washing machines and into wastewater—although whether they do so in amounts large enough to matter has been subject to debate. They can also wind up in sewage sludge, which is often applied to farmland as fertilizer.
Silver nanoparticles have been shown to damage cells derived from human and mammalian skin, liver, lung, brain, vascular, and reproductive tissues when evaluated in vitro. At high doses, they have been shown to compromise the blood–brain barrier and cause neurotoxicity in rats and mice.,, A 2008 University of Florida study found that both silver and copper nanoparticles can be toxic to model aquatic organisms including zebrafish, two species of water flea, and the alga Pseudokirchneriella subcapiata.