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They found that, for some forms of bacteria, nanosilver was not more toxic than either of the bulk silver species. But it was far more toxic to nitrifying bacteria, the bacteria type critical to removing ammonia from wastewater systems.

"So the potential consequence in wastewater facilities is enormous," Hu says. "If ammonia stays in the water, it causes significant problems like algae bloom."

Having established nanosilver's enhanced toxicity, Hu and Choi began investigating how nanosilver inhibits the bacteria's respiration. They discovered that bacteria exposed to nanosilver generate more of a compound called Reactive Oxygen Species, or ROS, which can damage cell DNA and RNA. The bacteria exposed to nanosilver generated more ROS both within their cells and without, but it was the extra ROS generation within the cells that was associated with slower respiration. "We think the smaller size of the nanoparticles allows them to directly enter the cell, while the large ones only stick to the cell membranes," Hu says, offering a hypothesis for why nanoparticles induce more of the harmful, intracellular ROS production, "But this is just our speculation."

Follow-up research will be needed to test that hypothesis, as well as explore other important questions, such as how Hu and Choi's nanosilver differs from nanosilver used in consumer products, at what concentration levels nanosilver becomes toxic and how nanosilver changes when exposed to other materials.

"It is too soon to conclude that nanosilver will cause problems with microbial processes in waste treatment plants," Luoma says. "It is increasingly evident that toxicity is possible, but toxicity at environmentally realistic concentrations is just beginning to be investigated, and such investigations are much more complicated than the initial screen studies for toxicity. Finally, as noted by Dr. Hu, we do not know why nanosilver is toxic, and this prevents generalizations about the potential risks." Hu has already been awarded $150,000 from the Water Environment Research Foundation to follow up on such questions.

Meanwhile, federal regulatory agencies are struggling to keep pace with the rapid pace of nanotechnology developments, both scientific and commercial. At present, for example, EPA regulations do not differentiate between nanomaterials and their bulk species forms. This must change, says Andrew Maynard, chief science advisor for the Project on Emerging Nanotechnologies in Washington, D.C. The not-for-profit group was established in April 2005 as a partnership between the Woodrow Wilson International Center for Scholars and the Pew Charitable Trusts.

"The first step to successful regulation is realizing that conventional thinking doesn't apply, that there has to be new thinking and that new questions have to be asked," he says. "Once you take that first step, then research like Dr. Hu's becomes very important because it points you in the right direction to ask the right questions for effective regulation.

What Dr. Hu's research shows is that the form of the material does matter. Hopefully, that feeds back to regulators who say: 'We've got to rethink this. We've got to add some questions to determine if something should be regulated or not.'"

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