Now, capitalizing on consumers' fear of germs, companies are adding tiny, powerful silver particles to cutting boards, underwear, yoga mats, running shirts, socks and an expanding array of other "antibacterial" goods.
Such products are made possible by recent advances in technology that allow manufacturers to create nano-sized silver and incorporate it into various materials. (A nanometer is one-billionth of a meter; a human hair is about 80,000 to 100,000 nanometers wide.)
But some scientists and environmental watchdog groups say putting nanosilver to widespread use may pose risks, as no one knows how chronic exposure to the particles may affect human health or the ecosystem in the long run.
They also worry that silver may lose its power to fight infections if bacteria become more resistant, a phenomenon already seen with other antibiotic drugs and with triclosan, an ingredient added to antibacterial soaps, cosmetics and other commercial goods.
"Some humans are quite frightened of bacteria, and the tendency to overuse silver is likely," said
"What concerns me is the explosion of products which we don't know are effective and that are not necessary," Luoma said. "It's important to prove it's worth taking the risk before we put it in the environment."
Consumers, meanwhile, often can't tell whether products contain nano-sized silver because nanomaterials aren't required to be labeled.
"The presence of nanotechnology in consumer products has gone underground," said
Silver kills germs when it oxidizes and releases silver ions, which are lethal to bacteria and yeast. Ancient civilizations used the metal to treat open wounds, and American pioneers tossed silver coins into water storage barrels to keep water fresh.
Nanoparticles of silver, which can occur naturally, are more powerful than bigger particles because their large surface area relative to their mass increases the number of ions released.
The silver industry says concerns about nanosilver are unfounded, pointing to silver's history as an effective antibiotic agent and noting that most applications use very small quantities.
It also says nano-sized silver isn't new and has been in continual use for more than a century. Nanoscale silver products have been safely used in swimming pools and drinking water purification since the 1970s, said
"It's the most sustainable and by now the most researched and safest biocide technology on the market," said
Some manufacturers also argue that treating textiles with silver nanoparticles benefits the environment by reducing the need for washing, allowing cleaning at lower temperatures and making the products more durable.
In general, experts agree that humans can safely tolerate fairly high doses of silver. What's new is that over the past decade nanoparticles have been manipulated and put in places where silver has never been before.
Some scientists and environmental health groups fear that the very properties that make these particles useful -- their vanishingly small size and high surface area -- may have unintended consequences once they get into the human body and the environment.
Both animal studies and computer models have found that nanoparticles, including nanosilver, can spread throughout the body into organs and tissues.
"Nanoparticles are little bundles of pure metal that can dissolve within the body if taken up as a particle," said Luoma.
"To me," he added, "the big risk is to ingest or breathe nanosilver when we don't know for sure if there's a unique risk in this form."
Like other germ-killers, including antibiotics, nanosilver also could upset the delicate balance of bacteria inside the digestive tract, said
Beyond the possible impact of nanosilver on human health, environmental concerns loom large.
"There is clear evidence that silver, and in particular nanosilver, is toxic to aquatic and terrestrial organisms" and to cells from mammals in laboratory research, the
Potential sources of nanosilver pollution include landfills, wastewater treatment plants and industrial facilities. Several lab studies also have found that nanosilver can leach out of products when washed.
"If we start using tons of silver nanoparticles and a lot more are going down the drain, then we might start to have an effect we haven't seen in the past," said
Some studies have found that nanosilver in wastewater tends to be transformed into silver sulfide, a much less toxic substance, said
Researchers there also found that the vast majority of nanosilver that enters a wastewater treatment center is removed from the water as it sticks to bacteria that feed off organic matter.
But these bacteria, Pruden said, eventually settle out to form biosolids, or "sludge," that is applied as fertilizer to agricultural land. Nanosilver in the sludge could affect the soil bacteria responsible for cycling nitrogen and carbon, she said.
Pesticides, which include chemicals that kill bacteria, must be registered with the
Environmental health organizations complain that the conditional approval process allows products to be marketed several years before all the safety studies have been submitted, potentially putting consumers at risk.
Last year, the
For consumers, the safety picture on nanosilver is murky to say the least. Products launched before the change in
"A specific human risk assessment for silver nanoparticles is not feasible as information on possible long-term effects are lacking," its report stated.
Until such data are collected, some argue that nanosilver shouldn't be used in consumer products, in part to preserve silver's ability to fight infections.
"Silver was and continues to be a valuable tool," said
But Maynard also said people may have taken the assumption of harm too far, in the absence of more concrete evidence. Using nanosilver in a product may make sense if it serves a medical purpose and proves effective, he said.
"Rather than a marketing ploy, it should be something that will benefit people," Maynard said. "We've survived for many years without having antimicrobial shirts and tools."
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