Nanosilver Pesticides

Consumer products that contain nanoscale silver particles are increasingly showing up in the marketplace. Nanosilver, known for its antimicrobial properties, can be added to plastics—such as food containers, water bottles, countertops, shower curtains, and floor coverings—as well as to textiles and building materials—such as paints, caulks, and adhesives.

The job of figuring out how to best regulate this growing class of nanotech products falls on the Environmental Protection Agency. The agency has jurisdiction because antimicrobials are considered pesticides and therefore fall under the Federal Insecticide, Fungicide & Rodenticide Act (FIFRA).

The problem facing the agency is how to get a handle on the ever-changing science behind these emerging nanosilver products. This understanding is essential for EPA to determine what data it needs to assess potential environmental, health, and safety risks and the best way to get those data. Under FIFRA, the agency has broad authority to require manufacturers to provide whatever data it deems necessary.

On one side, industry asserts to EPA that little or no leaching of nanosilver will occur from nanosilver products, and therefore exposure to humans and the environment will be minimal. When leaching does occur, they say, it is in the form of silver ions, which have been used in pesticide products for decades with no unintended effects.

Countering this stance are numerous consumer, health, and environmental groups that emphasize that nanosilver products have novel properties that can pose different risks to humans and the environment than non-nanoscale forms of silver. Such groups are urging EPA to require the full gamut of toxicity data for all nanosilver products under its jurisdiction.

To help inform how to assess risks associated with nanosilver pesticide products, EPA held a four-day meeting of its FIFRA Scientific Advisory Panel earlier this month. The external group of scientists pondered over whether EPA should allow data for non-nanoscale forms of silver (particles that measure greater than approximately 100 nm in any dimension) to fill in for nanosilver data (particles that measure approximately 1–100 nm in any dimension), and whether data for one nanosilver product can substitute for data for another nanosilver product.

Several companies have approached EPA trying to register pesticide products that contain nanosilver, William L. Jordan, senior policy adviser to the director of EPA's Office of Pesticide Programs (OPP), told the advisory panel. And at least two nanosilver products have already been registered, he pointed out.

For those two products, EPA only has acute toxicity data to assess potential hazards. The data include acute oral toxicity, acute dermal toxicity, acute inhalation toxicity, eye irritation, dermal irritation, and skin sensitization, Jenny Tao, a toxicologist with OPP's antimicrobials division, told the panel.

EPA is also reviewing applications submitted by four different companies seeking to register nanosilver products, and it expects to receive many more, on the basis of numerous inquiries from manufacturers in the U.S. and Asia, Dennis Edwards, a branch chief in OPP's antimicrobials division, noted at the meeting.

According to Edwards, the four pending products are intended to be used as material preservatives. In such cases, nanosilver would be added to paint, plastic, or fabric to preserve the product by killing bacteria or other microbes that might spoil or contaminate it.

The use patterns for the four products are very similar to those of registered conventional silver products, Edwards noted. "The antimicrobial activity for all four products is attributed to the release of silver ions."

About 110 antimicrobials that contain non-nanoscale silver are currently registered by EPA, according to Edwards. Silver has been used as an antimicrobial agent for hundreds of years, and there are a lot of data in the literature on it, he stressed.

EPA agrees with the manufacturers that if there is exposure to silver ions from nanosilver products, then the hazards will be the same as those for pesticides that contain elemental silver, Jordan said. "But the question that keeps coming up for us is whether we have an adequate basis for assessing the potential exposure to the nanosilver particles themselves."

It is unclear whether nanosilver particles or silver ions are coming off the treated substrates in available leaching studies, Jordan noted. A recent study found that when silver is released from nanosilver textiles during washing, most of it is in the form of coarse (>450 nm in diameter) particles (C&EN, Oct. 5, page 12). The particles have yet to be fully characterized.

EPA is considering three broad approaches to regulate nanosilver pesticide products, Jordan told the advisory panel. The first is to evaluate industry's claim that there is limited exposure potential. In that case, EPA would only need to ask for data to confirm that there is minimal or no leaching of nanosilver. The agency would rely on the available toxicity data for elemental and ionic silver.

The second approach would focus on metabolism studies to determine whether nanoparticles and ionic silver behave the same way in various organisms. If they do, then EPA would assume that the data it has for elemental silver are sufficient to evaluate the risks associated with exposure to nanosilver products. If they behave differently, then the agency would shift to the third approach and ask for more data.

In the third approach, EPA would assume that nanosilver particles are significantly different from conventional silver. Nanosilver particles would be treated as new active ingredients and therefore would need to be supported by a full set of toxicity data, including subchronic and chronic mammalian toxicity studies, environmental fate and ecological effects studies, and a full complement of exposure studies.

No matter which approach or combination of approaches EPA decides to use, "we are going to need new guidelines for nanoparticle characterization," Jordan noted. Several EPA scientists echoed that statement during the meeting, emphasizing that without sufficient characterization of nanomaterials, it is difficult to reproduce results or compare results of one toxicity study with another.

During a presentation on hazard assessment of nanosilver, Jessica P. Ryman-Rasmussen, a toxicologist in OPP's health effects division, noted that the toxicology profiles for silver ions and nanosilver look very similar, "the similarity being that they are not really all that toxic." However, EPA doesn't have characterization data for the nanosilver-containing compounds, she said. "We don't really know what the animals got dosed with, how dispersed it was, or how agglomerated it was."

For incoming pesticide products, EPA traditionally asks for physicochemical properties such as physical state, chemical composition, solubility, density, and cation-anion exchange capacity, noted A. Najm Shamim, a chemist in OPP's antimicrobials division.

But for nanosilver and other nanometal products, the agency is considering asking for additional properties, including size and size distribution of nanoparticles, surface area, surface reactivity, zeta potential, surface charge, catalytic properties, and aggregation processes, he noted.

The problem is that EPA does not have test guidelines or standardized methods for most of those characteristics, Shamim acknowledged. In addition, there are no standards against which manufacturers can normalize or validate their tests. "Every manufacturer has a different product," he said.

Nanosilver products are made by many different processes, raising concerns for EPA about extrapolating toxicity data from one product to another. As an example, Shamim described two common manufacturing processes.

In one process, nanosized silver is incorporated into polymeric silica or attached to silica that is bonded with sulfur, forming a nanocomposite. The other process involves the addition of capping or dispersing agents—such as citric acid or its sodium salt—or polyvinyl pyrrolidone. These capping or dispersing agents keep the nanosilver dispersed in colloidal form and prevent aggregation, Shamim explained. "Nanoscale colloidal dispersions could be incorporated into polymeric materials like plastics or surface-coated on textile materials like socks."

In comments presented at the meeting, Jaydee Hanson, policy director of the nonprofit International Center for Technology Assessment (ICTA), urged EPA to clarify which nanosilver products are pesticide products that must be approved and registered by the agency. In May 2008, ICTA filed a petition with EPA on behalf of a coalition of consumer, health, and environmental advocacy groups, pointing out nearly 300 self-identified nanosilver products already on the market. The petition called on EPA to exercise its authority under FIFRA and require that such products be registered by the agency. It also recommended that EPA require all relevant data from nanosilver manufacturers to better assess the risks of nanosilver products.

Murray J. Height, chief technology officer of Switzerland-based HeiQ Materials, a manufacturer of nanosilver textiles, presented a much different perspective. He provided information to the panel on behalf of the Silver Nanotechnology Working Group, an industry effort to promote the benefits of nanosilver products.

Height argued that nanosilver is not a new material, but rather the same material as colloidal silver, which has been used for centuries. He asserted that EPA doesn't need more toxicity data on nanosilver products because those data already exist from products that have been on the market for decades. Height pointed to nanosilver biocides for killing algae in swimming pools and nanosilver-based carbon water filters.

Although most of the meeting was dedicated to nanosilver, EPA's Jordan did bring up concerns about other nanoscale metals, including copper, in consumer products. Jordan's remarks were spurred by comments submitted by the Natural Resources Defense Council. The environmental group recommended that the advisory panel consider nanoscale copper-based biocides for wood treatment. NRDC claimed that the product is in widespread use and that EPA has insufficient safety data on it.