For all its toxicity to microbes, how much a threat silver nanoparticles pose to public health and the environment is not known, according to Luoma. Before the photographic industry became environmentally conscious during the 1970s, use of black-and-white film development procedures routinely released large quantities of silver particles into wastewater and then the open environment, Luoma points out. Thus, he and others began to study silver and its effects in the San Francisco Bay. Although these researchers tracked the collapse of the bivalve population, which formed much of the base of the fish and bird food webs, "we did not study the microbial community," he says.
Some observers worry that silver nanoparticles will poison wastewater treatment plants. Doing so "would take massive uses of silver in the commercial sector," Luoma says. However, if silver ion dishwashers, washing machines, and air conditioners come into wide use, leached levels of nanoparticles might bring this nowremote possibility closer to reality.
In terms of direct risks to humans, "all indications are you would have to have quite a lot of silver in your system to have any impact," he adds. Further, some people drink colloidal silver as a dietary supplement, and "seem to be healthy." Silver toxicity comes within its own early warning system. Thus, a blue tinge, called argeria, appears on skin before other ill effects are manifest. What about microorganisms developing resistance to silver? Products containing silver are used in dressings for trauma wounds, burns, and diabetic ulcers, according to Silver from UIC. While resistance genes are found in bacteria associated with such wounds, including methicillin-resistant Staphylococcus aureus, the silverimpregnated dressings remain effective, according to Christine Cochrane at Liverpool John Moores University in Liverpool, United Kingdom, and her collaborators.
In 2008, activists led by the Washington, D.C.-based Center for Food Safety and the International Center for Technology Assessment petitioned the U.S. Environmental Protection Agency (EPA), urging it to regulate silver nanoparticles as a pesticide. In November 2009, EPA convened one of its advisory panels, asking these outside experts to assess levels of exposure and hazard from current uses of silver nanoparticles, and to advise the agency on how or whether to regulate such products and protect public health and the environment. The agency is not yet saying what comes next. "There are currently no plans for rulemaking," says Dale Kemery, an EPA press officer, referring to silver nanoparticles. "We cannot comment on any aspect of the [scientific advisory panel] until their report is released."
"It's hard to work out how serious this is," Luoma says. "The biggest concern is the potential for encouraging resistance to develop in certain strains of microbes, and the possibility of impacting the ecosystem at a fairly low level if a lot of this is released." If nothing else, concerns over rampant use of silver nanoparticles echoes those arising from profligate antibiotic use. At the very least, improved public education is warranted in both cases.
David Holzman is the Microbe Journal Highlights Editor.