Living is a wasteful business. Just think of all the stuff we dispose of down are drains and toilets. Pharmaceuticals, oils, cosmetics, hair, condoms, glues, paints, nail polish removers, soap, urine, feces, food remnants, toilet paper, pesticides, dyes, cleaning agents, blood and even vomit. And there are plenty of bacteria and viruses that go down as well. The original answer to this onslaught to nature was that the “solution to pollution is dilution.” Basically that meant the mess would be diluted enough in natural water systems so that it would not come back to bite us in the rear when it came to recycling water. And of course water has to be recycled. They just don’t make the stuff any more. But as the complexity of society increased, and as more and more waste was generated, novel technologies had to be introduced to deal with waste water.
Numerous methods are used to deal with sewage but they all involve some sort of holding tank to allow solids to settle to the bottom and oily substances to float to the surface where they can be skimmed off. The water is then exposed to various microbes that are adept at decomposing organic waste. The sludge that remains is either buried, or in some cases, is used as fertilizer. Any remaining contaminants in the sludge, such as silver nanoparticles, can be a problem.
So, what are silver nanoparticles? Imagine taking your silver candlestick and putting it into some sort of grinder and grinding away until it has been reduced to tiny particles, so small that each one is less than one billionth of a millimeter in size. And on this scale, the “nano” scale, you’ll be looking at a different world. Of course, one single nanoparticle is way too small to be seen, but the collage of nanoparticles of silver will no longer appear as a bright, shiny metal because the tiny particles now absorb and reflect light in a different fashion, giving them a yellow colour. Long before anything was known about nanoparticles, artisans ground silver finely and added the particles to glass to give church windows a permanent yellow colour.
As particle size decreases, the ratio of the surface area to volume increases dramatically. Just a pinch of nanoparticles has a gigantic combined surface area. And this is what plays a pivotal role in the antibacterial effect attributed to nanosilver and explains why it is incorporated into textiles. The goal is to kill bacteria that are responsible for producing body odours which are the result of bacterial action mostly on the fats found in sweat. Enzymes called lipases produced by bacteria break down fats to yield small odiferous molecules such as butyric, propionic and isovaleric acids.
Insoles of athletic shoes are also sometimes treated with nanosilver particles, as are the inner surfaces of some appliances such as refrigerators, washing machines and air conditioners. In all these cases the intent is to make use of silver’s antibacterial and antifungal properties, which are now well understood. It turns out that metallic silver itself is inactive, but in the presence of moisture and oxygen, a process known as oxidative dissolution occurs. This leads to the formation of positively charged silver ions, a result of silver atoms losing an electron to oxygen. It is these ions that interfere with bacterial metabolism.
But there is a degree of concern with nanosilver particles being used in a wide array of consumer items because when textiles are washed, or when appliances are discarded, some of the silver, and consequently silver ions, end up in waste water. The resulting antibacterial action can interfere with the work of microbes in the treatment plants. And if sludge from these plants is used as fertilizer, which is common because of its high phosphorus content, soil microbes, including nitrogen fixing bacteria can suffer, causing damage to agricultural land. It has been estimated that if everyone buys just one silver-particle treated pair of socks a year, the silver concentration in waste water sludge can double. Of course this is not a realistic possibility.
There are other issues that arise with the commercialization of nanosilver, including some unsubstantiated hype. For example, some washing machines advertise that the billions of silver ions released in each wash cycle kill 99.99% of bacteria. But regular washing in hot water also kills over 99% of bacteria. Quibbling over a less than 1% difference in antibacterial effect is hardly good science. There are also questions about just how effective nanosilver in fabrics or shoes actually is in terms of preventing smells, and surveys have also shown that some products that say they contain silver don’t, and some that don’t make the claim, do. And in many cases the silver is actually not nano, which would reduce its disinfectant efficacy because, as already mentioned, it is the extremely high surface area to volume ratio that makes nanosilver an effective anti-bacterial agent. And if the silver isn’t effective enough, it can actually have an opposite effect to that desired. Exposure to sub-lethal doses of silver ions can improve bacterial survival rates and increase the chance of resistance.
While nanoparticles of silver are unlikely to pose any hazard to people, the same cannot be said for colloidal silver solutions which are promoted as a dietary supplement to stave of illness. Here we are talking about a much greater exposure to silver with the possibility of developing argyria, a permanent discoloration of the skin due to deposits of silver sulphide and silver selenide. When the colloidal silver particles hit the acidic environment in the stomach, they undergo oxidative dissolution and the resulting silver ions form complexes with glutathione, a naturally occurring antioxidant in the body. This complex is readily transported around the body by the bloodstream, and when exposed to ultraviolet light near the surface of the skin, undergoes a photo-reduction process whereby the silver ions are converted to metallic silver in the form of silver nanoparticles. These then react with sulphur and selenium compounds in the body to form the gray deposits that characterize argyria. And no silver polish will get rid of that.
