Little Killers

As silver nanoparticles go mainstream, environmental concerns flow downstream. By Anita Neal Harrison

Nanoparticle silver has become a high-tech superstar, a marketing sensation that promises to free an ever-growing variety of household items from the scourges of odor and bacteria. But a growing number of scientists fear these tiny nanosilvers, while inarguably hygienic, could be less than completely benign.

MU's Zhiqiang Hu, an assistant professor of civil and environmental engineering, is among the most prominent. Hu says nanosilver first appeared on his investigative radar screen when he learned that public utilities in California were nervous about the tiny element's effect on wastewater. Hu specializes in wastewater treatment research, and he immediately recognized why California officials had reason to be worried: Nanosilvers can kill good bacteria along with the bad.

"If these nanoparticles go to our wastewater facilities, they may cause some problems because we rely on healthy bacteria [to remove contaminants from wastewater]," he says. Armed with a grant from the National Science Foundation, Hu set about to test his concerns. And sure enough, it wasn't long before he determined that nanosilver did, in fact, have a strong toxic effect, stronger even than that of other toxic silvers.

"The occurrence of toxicity is not a novel finding, necessarily, but Dr. Hu's observation that nanosilver is more toxic than the silver ion by two-fold is, indeed, a new finding," says Samuel Luoma, a researcher with the John Muir Institute of the Environment and editor-in-chief of San Francisco Estuary & Watershed Science. The concentration at which the

Bacteria toxicity occurs is also lower than conventional knowledge suggests, he says, and would be problematic for sewage treatment.

Sewage treatment plant operators worry about silver when concentrations are above one milligram per liter; Hu shows effects of nanosilver at 0.14 milligram per liter.

Nanosilver is a tiny form of silver, so small it almost defies comprehension. One nanometer is one billionth of a meter. To put that into perspective, consider this: You would have to magnify a one nanometer-sized object a million times before it reached the size of a red ant. Enlarge that ant to the same degree, and you'd have an ant that's 5 kilometers, or 3.1 miles, long.

Just being able to see materials at the nanoscale seems incredible. But the 1981 invention of the scanning tunneling microscope allowed scientists to go a step further and actually begin manipulating nanoscale materials. This advancement made possible what we today call nanotechnology, defined by the federal National Nanotechnology Initiative as "the understanding and control of matter at dimensions of roughly 1 to 100 nanometers, where unique phenomena enable novel applications." The tunneling microscope's inventors won the 1986 Nobel Prize in physics.

Novel applications are possible at the nanoscale because nanomaterials often have -- or can be engineered to have -- physical and chemical properties that differ from their "bulk" species counterparts. At the bulk scale, for example, gold is an excellent conductor of heat and electricity but not of light. Gold nanoparticles, on the other hand, can be structured to absorb light and turn that light into heat. This heat, according to the NNI website, can "act like miniature thermal scalpels that can kill unwanted cells in the body, such as cancer cells."

Other nanoscale materials differ from their bulk counterparts in how they conduct electricity, in their strength, in their magnetic properties and in their color. Nanoscale materials also have far more surface area than do bulk species. Depending on the nanoparticle size, one ounce of a nanomaterial could have the same surface area as one ton of the bulk material. This is an important distinction. Because interactions between materials take place at the surface, a larger surface area will create a greater opportunity for reactions.

Potential applications for nanomaterials stretch the imagination, ranging from stronger, lighter metals to revolutionary drug delivery systems. The Project on Emerging Nanotechnologies, established in April 2005 as a partnership between the Woodrow Wilson International Center for Scholars and the Pew Charitable Trusts, reports an estimated global research and development investment of nearly $9 billion per year in nanotechnology. The organization reports that between March 2006 and February 2008, the number of nanomaterial products on the market nearly tripled, from 212 to more than 600. These products are produced by some 300 companies located in 20 countries.

Nanosilver is by far the most commonly used nanomaterial. One estimate, published recently by the emerging nanotechnologies project, reports that nanosilvers can be found in about half of today's nanotech products. A sampling of these nanosilver goods would include household appliances, cleaners, bedding, clothing, food storage containers, soap, toothpaste, cosmetics, personal care products, drink supplements, pacifiers, stuffed animals and coated electronics.

For Hu, with his focus on wastewater, one of the most worrisome products are so-called "silver care" washing machines that produce either silver ions or nanosilver during each wash cycle. The idea is to use silver's antimicrobial power to disinfect clothes, thereby reducing the need for hot water and detergent -- perhaps even allowing the garments to be worn multiple times between washings.

These are all eco-friendly goals. But Hu points out that the nanosilver wash water might not be so eco-friendly when it goes down the drain.

Whether this is the case depends on a lot of factors, from the concentration of nanosilver in the water to how nanosilver, a highly reactive material, changes once it hits sewage, detergents and other materials. Another question is more basic: whether nanosilver, in fact, actually has the antibacterial property manufacturers claim. And if it does, which bacteria are most likely to be affected?

To address these questions Hu and his graduate student, Okkyoung Choi, exposed various kinds of bacteria to nanosilver they synthesized. This is a necessary step because commercial nanosilver tends to aggregate in water until the particles are no longer even on the nanoscale. Hu and Choi then monitored the respiration rates of the bacteria exposed to the nanosilver and compared them to those of control bacteria with slower rates, indicating a toxic effect. They did the same using silver ions and silver chloride colloids, two forms of bulk silver, in place of the nanosilver.

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.'"

Luoma, too, says the laws will have to be modified to fit nanomaterials. "It is likely that the approaches we have used traditionally to test the toxicity of materials do not work very well with nanomaterials," he says. "Chemical toxicity can be tested by dissolving the chemical in water and exposing a test species. Nanomaterials aggregate, change forms, stick to materials that are eaten, etc. They are much more complex to test, and again, the regulatory agencies do not yet have the methods to get around these difficulties."

Currently, most of the nanosilver products on the market are regulated by the EPA under the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA). That's because they tout silver's effectiveness at killing bacteria, a claim that categorizes them as a pesticide.

The act makes it illegal for anyone to sell, distribute or use a pesticide unless it is registered with the EPA, except for emergencies and certain minimum-risk pesticides. As silver meets neither of these exemptions, one might assume that the hundreds of nanosilver products on the market have passed the registration process.

But that's not the case. In fact, Dale Kemery, an EPA press officer, says the EPA "has neither received an application nor approved any pesticide product that contains a nanomaterial, including silver."

This seems to suggest nano-product manufacturers are out of compliance, but Kemery will not say so. Neither will he say whether the EPA has taken action against any manufacturers of nanosilver products.

"We don't discuss whether we have investigations ongoing or not," he explains. "We do this to protect the integrity of any possible ongoing investigations."

Kemery will say that in many cases, manufacturers of nanosilver products can avoid registration if they make no antimicrobial claims because, strangely enough, a product's claims usually have more bearing on its pesticide classification than do its components.

As Kemery explains, "Products not making a pesticidal claim would not be subject to FIFRA unless the active ingredients are inherently pesticidal in nature and have no other significant commercially valuable use as distributed or sold, other than use for pesticidal purpose."

Though this loophole may provide a pass to some nanosilver products, most are making antimicrobial claims of some sort. In May 2008, a coalition of consumer, health and environmental groups joined the International Center for Technology Assessment -- a self-described "nanotech watchdog" based in Washington D.C. -- to file a legal petition with the EPA. It "demanding the agency use its pesticide regulation authority to stop the sale of numerous consumer products now using nano-sized versions of silver."

While the petition itself cites hundreds of studies to build its case for potential harm, the press release regarding the petition references only two: a 2008 Arizona State University study documenting nanosilver "escapes" from laundered nanosilver-enhanced socks, and Hu's study.

"Hu's study and the Arizona State University study were the two newest studies, and they had the best evidence yet of the foreseeable environmental harms that the commercialization of these nanotechnologies may be having," says George Kimbrell, a lawyer with the International Center for Technology Assessment. Much of the science so far, Kimbrell adds, has focused on the potential good associated with nanosilvers' enhanced mobility and toxicity, barely mentioning that there may be unintended environmental impacts.

"Hu's was one of the first studies that said, 'Yes, in fact, that happens. Here is this beneficial bacteria that's being affected,'" Kimbrall says.

On the other side of the debate are groups such as the NanoBusiness Alliance. Sean Murdock, the group's executive director, has argued against a rush to regulate in a number of places, most notably the U.S. House Committee on Science and Technology.

At a hearing convened earlier this year to discuss the National Nanotechnology Initiative Amendments Act of 2008, Murdock told committee members that America faces stiff global competition in reaping nanotechnology's economic benefits. He urged Congress to develop "a broader understanding of nanotechnology before erecting an extensive new regulatory structure" and to "use the research authorized by this bill as a basis for the decision of what, if any, new regulation is needed."

At this point, neither nanosilver supporters or detractors can count Hu as an ally. "I don't think we have reached that level [of understanding]," he says, when asked for his thoughts on the right regulatory approach. "We have only studied it for about a year. Right now, we just want to raise the public awareness that if nanosilver is generated enormously and enters the wastewater system, it may cause problems. We need to be cautious."

Maynard, too, encourages a calm approach to the nano debate. "I think one of the important things to remember is that there isn't a lot of evidence that nanosilver is toxic," he says, "but there is this [Hu's] evidence of a toxic effect. So I think we've got to take responsible action in response to that. It's not scary information. It's not the end of the world, but it does mean we've got to sit up and take notice."