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Posted: June 25, 2008
Nanotechnology risks: the unclear fate of nanosilver
(Nanowerk News) Tiny particles of silver designed to kill germs are being put into socks to control odor. But as a recent story on ScienCentral explains, what happens to that nanosilver later is concerning some scientists.
Socks with a silver lining
Several manufacturers are incorporating nano-sized particles of silver into socks to kill bacteria that cause odor. But does the silver stay in the socks? And what happens to it if it washes out? Arizona State University's Troy Benn tested a variety of socks containing nanosilver. He wrote in the journal Environmental Science and Technology that some socks released nearly all of their nanosilver within the first four washings.
Surprisingly, says Benn, "Others that contained a lot of silver in the sock didn't release any silver that was detectable." He says there must be some differences in the manufacturing process. "We assume there is a way to contain the silver within sock because we did see a large difference between different manufacturers of the sock material."
For someone with diabetes or a soldier in the field, says Benn, a sock that kills microbes and prevents infection could be critical, but for other people, "The question is whether the benefits really outweigh the potential environmental cost"
Silver, the wonder drug
Silver is a naturally-occurring element, and is present in trace amounts in the environment. It is a natural antimicrobial. But as with many things, you can have too much of a good thing. In larger concentrations, silver is a pollutant. When it is used in industries like photographic film manufacturing, the US Environmental Protection Agency (EPA) places strict regulations on the amount of silver than can be released into the environment.
The Romans placed silver coins in wine to preserve freshness, and fixed them to wounds to help them heal with fewer infections. Silverware was also prized for preserving food and having some antimicrobial properties. Hospitals still use silver wound dressings for burn victims. And until recently, doctors routinely put silver nitrate drops in the eyes of newborns to prevent eye infections.
When things get small
Nanotechnology is the ability to manipulate matter on the nano scale, or billionths of a meter, the scale of individual molecules. Nanosilver is an unfathomably fine dust that thanks to its enormous surface area is a very potent microbicide. Silver has fairly low toxicity to humans, although drink enough of it and your skin will turn an alarming shade of gray or even blue, a permanent but otherwise apparently harmless condition called argyria.
Making silver into nanoparticles allows it to be mixed into soaps, plastic food containers, teddy bear stuffing, and toothpaste. And it is also used to coat everything from endotracheal tubes to elevator buttons to baby pacifiers.
In 2003, Samsung introduced a line of SilverCare washing machines that flush clothes with silver ions to disinfect them. (Silver ions are positively charged, reactive silver atoms.) The admirable goal was to reduce the amount of hot water (and energy) needed to disinfect clothes. Clothes doused in silver could be washed in cool water and still be fresh. A little silver might even stay in the clothes and keep them from stinking while you wear them.
But because silver kills "pests," the EPA considers it a pesticide. Products that are used to kill microbes are supposed to be registered with the EPA under FIFRA. That involves extensive safety and environmental impact testing related to the specific use of the product. Samsung is in the process of registering their SilverCare washing machines. They were pulled from U.S. markets in 2006, but they are on sale again during the application process, which can last a year or more.
Should the EPA regulate Nanosilver?
Unfortunately, silver has been shown to harm fish and other aquatic organisms. And since it is such a powerful microbicide, environmentalists worry that flushing silver-containing wash water down the drain and dumping nanosilver products into landfills could affect the base layer of the ecosystem, the tiny bacteria and other microorganisms that every other living thing depends on a scenario that is raising a lot of concern.
Zhiqiang Hu from the University of Missouri-Columbia has shown that silver ions and nanosilver can kill the kinds of microbes that are used in wastewater treatment plants to process waste. "We are really concerned if they are going to discharge into the wastewater system, they're probably going to cause some environmental impact."
Hu says introduction of large amounts of silver could interfere with the bacteria's ability to break down sewage. Also, solid biomass from treatment facilities is sometimes distributed to farms as fertilizer, and that could carry silver to the soil. "If the level of the metal made of those silver [particles] is high enough," says Hu, "that probably can cause environmental impact in the soil systems." Plants depend on nitrogen-fixing bacteria in soil. Kill that bacteria and crops and other plants won't grow.
There are several potential holes in the theory, though. Transport of nanosilver and silver ions through all these stages has yet to be documented. Hu and his colleagues are doing research to determine that now. Silver may combine with other elements along the way and become less harmful or precipitate out and get trapped by wastewater filters. Another consideration is that using a silver in washing machines may significantly reduce the amount of detergent and water needed to wash clothes, which would reduce pollution and conserve precious water.
But with nanosilver now in hundreds of products that are not required to be registered with the EPA, is this more useful but possibly more harmful form of silver becoming ubiquitous and unregulated?
The EPA tends to exclude two categories of microbicides from their regulations (except in cases where there is clear evidence of some toxic or environmental impact.) 1) Personal care products and cosmetics like hand-sanitizing gels and antibacterial toothpastes usually fall under FDA regulation. 2) Antimicrobials designed to protect the product itself, like those in mildew-resistant paint, tend to get a pass on EPA regulation. Only when the product disperses a pesticide or toxic chemical is the EPA really interested.
Many other products that contain nanosilver have since removed all mention of antimicrobial properties. This is a gray area, according to Dale Camry of the EPA. If the product still contains silver but makes no claim, it can fall through the regulatory cracks.
This has left citizens with less information about the product they choose.
One ounce = one ton
Another problem with regulating nanosilver is figuring out how to quantify its risks. Typically, pollution is regulated by the amount of contaminant in the environment by weight or volume. But these may not be useful measures of the effects either positive or negative of nano-sized particles. The reactive part of silver is on its surface, and nano-sized particles have thousands or even millions of times the surface area of bulk material. That means one ounce of nanosilver dust could have the same surface area as one ton of bulk silver, depending on the particle size. Eventually, regulations might have to be based on number of particles per volume or even a calculated surface area.
In 2008, the EPA launched the Nanoscale Materials Stewardship Program, which invites companies to send in any data they have of the characterization of nanomaterials and any safety and environmental research results that they have produced. The EPA also funds research of the impact of new materials.
But some people feel it's not enough. For one thing, the program is voluntary. Companies can exclude materials or results that they don't care to share with the public. And environmental groups are wary to let industry be responsible for its own safety testing. The U.S. has a long history of failing to respond to the danger of hazardous materials in a timely manner.
Sean Murdock of the Nanobusiness Alliance says many people don't realize how far we've come in predicting and responding to potential hazards. "In all of these things, manufacturers are doing testing to ensure that they're safe. And frankly, we've much more robust science to help understand both the short term safety as well as the long-term safety of the materials that we're working with."
Price of innovation
This year, the National Nanotechnology Initiative, the major government funding for nanotechnology research, is up for reauthorization. Andrew Maynard, chief science advisor to the policy group Project on Emerging Nanotechnologies, would like to see a larger portion of those funds put toward environmental, health, and safety research. "Nanotechnology industries are desperate for clarity and a reduction in uncertainty here. So they want the research that is going to show us which nanomaterials are harmful and how to use them safely, because without that clarity, they just cannot do business."