BEFORE there were antibiotics, doctors used silver in various forms to kill germs. Newborn babies had drops of silver nitrate popped in their eyes to prevent bacterial infections from turning them blind. Soldiers injured in battle had their wounds dressed with silver foil. People put silver coins in milk to prolong its freshness. Thanks to antibiotics, antiseptics and greater awareness, we don't do anything quite so silly these days. Or, rather, mostly we don't.

While silver in ionic form or as a compound with certain salts has some antimicrobial properties, doctors used it in the past only because they had nothing better. Sure, it turned the patient's skin a cadaverous grey or blue—a permanent affliction known as argyria. But they dismissed that as merely a cosmetic problem. Only later did they learn that silver accumulates in organs of the body just like mercury, lead, arsenic or any other toxic heavy metal. And while silver may not be quite as toxic as arsenic, many silver compounds are extremely poisonous. Quite a few are also carcinogens.

None of which has stopped snake-oil merchants from continuing to hawk silver elixirs as “super-antibiotics”. In fact, they have been positively encouraged by recent events.

Triclosan, a powerful antibacterial and antifungal disinfectant, is used nowadays in soap, toothpaste, mouthwash, deodorant and household cleaners. A theory that it mimics the behaviour of antibiotics has fed fears that it may trigger greater drug-resistance among superbugs. Although subsequently discounted, such concerns have prompted enough worried users to embrace quack medicines instead. According to Wired, silver solutions have become one of the most widely marketed health scams on the internet.

Ingot we trust

Now the clamour for consumer products containing an even more worrisome form of silver has eventually prompted officialdom to act. Last week America's Environmental Protection Agency reversed its policy on the use of extremely small particles of silver for killing germs. Henceforth manufacturers seeking to use “nanosilver” in consumer products will have to prove scientifically that it doesn't cause harm to public health.

This is the first time that American authorities have moved to regulate the country's burgeoning nanotechnology industry—the $3 billion-a-year business of developing, producing and finding uses for materials that have at least one dimension in the range 1-100 nanometres (billionths of a metre). Down at these near-atomic sizes, materials exhibit unique properties that dramatically alter their physical, chemical or biological behaviour.

Some of the earliest applications of nanomaterials have been in products such as stain-resistant fabrics, self-cleaning windows and harder-wearing tennis balls, but more and more are turning up in cosmetics and skin lotions such as sun-blockers. Over 350 products containing nanomaterials are now on the market, with silver as the most common ingredient by far. Nanosilver is found in bandages, food-storage containers, shoe-liners and sportswear. Samsung makes a washing machine that injects silver ions into the final rinse. Kohler has a line of toilets with silver embedded in the porcelain.

What worries health researchers is that when particles get down to nanoscale dimensions, they are virtually all surface—with, in effect, no “inside” inside them. Because reactions between solids take place on their surfaces rather than within their bulk, nanoparticles can be far more reactive and toxic than a similar mass made up of larger particles. Also, the atomic-like dimensions of nanoparticles make it easier for them to slip through the skin or, if inhaled, move from the lungs to other organs in the body, including the liver and brain.

The problem is that no-one knows what the long-term effects could be, especially when dealing with a toxic heavy metal such as silver. Some have wondered whether such nanoparticles will be the asbestos of the 21st century. That's probably scare-mongering a bit, given the advance in health and safety precautions over the past half-century. But in America at least, the EPA has shown it's not prepared to take that chance.