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Silver Tongues

From The Economist print edition

ANCIENT Phoenicians stored their drinking water in silver vessels, but not for aesthetic reasons. They discovered that by doing so they remained healthier. The reason for that is now understood: silver has antimicrobial properties.

In the 21st century people have realised that if you fortify Phoenician wisdom with a dash of nanotechnology, silver can be made into a far more potent bactericide. Companies have quickly seized on this idea to produce a wide variety of products, from clothes to soap and even chopsticks, containing silver nanoparticles. The claim is that they destroy germs.

But silver can also accumulate in the environment and, at certain levels, prove toxic. Nor is the general safety of nanoparticles fully understood, not least because they can react in novel ways. Some scientists think more research is needed and perhaps more regulation too. A move in that direction now seems to be under way.

Silver's natural germ-killing ability stems from its extremely slow release of silver ions (electrically charged atoms, or groups of atoms). When made into particles only a few nanometres big—a nanometre is a billionth of a metre—they shed a lot more ions and so become more potent.

America's Environmental Protection Agency (EPA) is worried about a large number of products claiming antimicrobial abilities. One is “Silver Wash”, a washing machine made by Samsung, which claims to employ nanotechnology to release hundreds of billions of silver ions during a wash to sanitise fabrics.

The EPA has ruled that ion-generating devices that claim to kill germs must be registered as a pesticide and tested to show they pose no unreasonable risk. The EPA says its intention is to regulate ion-generating devices rather than nanotechnology itself. But it is hard to draw a distinction. Andrew Maynard, chief science adviser for the Project on Emerging Nanotechnologies at the Woodrow Wilson International Centre for Scholars in Washington, DC, says functionality is an important part of the definition. Turning silver into tiny particles that behave in new ways (for example, by shedding more ions) and putting those particles into new places (such as fabrics) qualifies—or so he thinks.

One consequence of dividing a substance into nanoparticles is that the surface area of the material greatly expands. “Nanosilver is so tiny it can go right to the surface of an organism and essentially shoot ions into the organism,” says Sam Luoma, a research scientist at the John Muir Institute of the Environment at the University of California, Davis. Although this makes silver nanoparticles an extremely effective antimicrobial agent, it also raises concerns about humans' ability to withstand relatively high exposures.

Despite the unknowns, Dr Luoma and others believe there is enormous potential for good from nanosilver. It can, for example, be used in small amounts to coat medical catheters to reduce the possibility of infection without causing environmental worries. “We need to separate out the truly beneficial uses,” he adds.

The EPA will not look at benefit or necessity, but is determined to make its registration stick. It has fined one company more than $200,000 for making unsubstantiated claims about unregistered nanosilver-coated computer mice and keyboards. Firms making claims about nanotechnology need to watch out.

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