Silver migrates from treated fabrics.
Geranio, L, M Heuberger, and B Nowack. 2009. The behavior of silver nanotextiles during washing. Environmental Science and Technology 43(21):8113–8118.
Silver nanoparticles used as antimicrobials in fabric can leach out of clothes as they are being washed. One brand lost over half of its silver content from the fabric with just two washings. The discovery raises questions about potential affects of human and environmental exposures.
The use of silver to kill microbes dates back to ancient times. However, in a modern twist, some manufacturers are adding tiny nanoparticles with silver to socks and other clothing to control odors caused by bacteria. The heavy use of silver nanoparticles has prompted questions about their human and environmental safety.
As a metal, silver is safer to people than lead, chromium and other metals. For aquatic organisms, though, the story is quite different. Silver is more toxic than most other metals to many fresh- and salt-water organisms, ranging from phytoplankton to marine invertebrates – such as oysters and snails – to different types of fish. Their immature stages are sometimes much more sensitive than adults.
Nanotechnology is a rapidly growing area of science. It involves manipulating or building new materials from atoms, which are very, very small. Carbon and metals are the most popular building blocks.
The materials – also called particles – are far smaller than a human hair and not visible to the naked eye. Most can only be seen with powerful microscopes. To put their size into perspective, the average atom is roughly one-third of a nanometer (a nanometer is a billionth of a meter) across. Nanoparticles are groups of atoms that are typically smaller than 100 nanometers in at least one dimension. A human hair is about 80,000 nanometers wide.
The tiny-sized materials often have unique chemical properties that differ from the properties of their larger scaled versions or the ions themselves. The particles can also group together, forming larger aggregates.
Nanomaterials are used in electronics, medicine, personal care products and many other applications. Their production and use are increasing as new uses are found and new types are developed.
The use of silver nanoparticles has expanded rapidly, and several hundred medical and consumer products that use the materials are currently on the market. Yet, it is not known how well the nanoparticles remain in products or whether exposure to silver nanoparticles that may leave the products is safe for the environment – particularly waterways, the organisms that live in the water and the people who drink and cook with the water.
A group of Swiss scientists tested how well silver nanoparticles stayed in treated fabrics under conditions similar to a washing machine. They considered mechanical stress and chemical factors such as bleaches, pH and surfactants.
First, they measured the silver content of several different brands and types of fabrics that had silver nanoparticles either incorporated into or bound to the cotton, nylon or polyester fibers. They also included a fabric lined with a layer of silver that was not in the nanoparticle form.
They then washed the fabrics either once or twice in detergent. They added steel balls to simulate mechanical stress that would be similar to normal washing conditions. Some of the fabrics were also treated with bleaching agents during washing. These chemicals contribute the oxidants – the molecules that donate oxygen and therefore "bleach" the fabric.
Finally, the amounts and sizes of the particles of silver that came out of the fabrics were measured.
The silver content of the fabrics varied enormously, and several products had relatively low silver content.
A big difference was seen between the silver lined and the nanoparticle treated fabrics. The fabric lined with silver contained more than 600 milligrams (mg) of silver per ounce of fabric, about 7,200 times more silver than the nanoparticle fabric with the smallest silver content.
In comparison, the nanoparticle-impregnated fabric with the highest silver content had about 75 mg per ounce of fabric. Among the different nanoparticle-treated fabrics, the silver content ranged up to 130 times higher than the fabric with the lowest silver content.
When the fabrics were washed in water with detergent only, the silver generally stayed in the fabrics. However, several fabrics released silver quite readily once the steel balls were added to mimic mechanical actions of the washing machine. Of the 7 nanoparticle fabrics subjected to mechanical stress, four lost roughly 20 percent to 35 percent of their silver with the first wash. The amount released was always less with a second wash. Yet, one brand lost over half of its silver content from the fabric with just two washings.
The silver-lined fabric did not lose such a large percent of its silver during the washing. The fabric, though, contained so much silver to begin with, that it released as much of the metal as some of the nanoparticle-impregnated fabrics.
Several of the fabrics treated with bleaching agents released generous levels of silver. Yet, when the bleaching agents were combined with washing, many did not release the expected amount of additional silver. The authors speculate that the effects are masked by the detergents, which neutralize the oxidants that are formed from the bleaching agents during washing.
The size of the silver particles released during washing was greater than 450 nanometers for most of the products. Only one nanoparticle-treated fabric released about half of its silver as free silver ion (Ag+). Also, fabrics where the nanoparticles were incorporated into the fibers lost less silver than those that were applied onto the fibers.
Silver by itself or as part of silver nanoparticles is escaping from treated clothing products during normal washings, most likely ending up in wastewater that is released into waterways. The amount of silver lost depended on a number of factors, including size (alone or in a nanomaterial), how it was applied to the fabric, if bleach was present and the acidity of the wash water.
This is one of the first times clothing has been identified as a source of the metal, and it is the first study to examine the loss of silver nanoparticles from fabrics under conditions simulating washing with detergent and bleaching. This study shows that nanoparticles do not always stay put on fabrics and thus could be a source of water pollution.
While not as toxic to people as other metals, silver can be deadly to many many fresh- and salt-water organisms – especially at their young stages of life. Many species of fish and shellfish, as well as their food, are susceptible to the metal. Widespread exposure to silver could impact some of these and disrupt ecosystem health.
Many waterways are just recovering from high levels of silver introduced by the photography industry during the 20th century. It is not clear if the spate of new silver nanoparticle products will result in high levels of silver being reintroduced into the rivers and lakes.
One of the key tenets of nanotechnology is that materials can have different properties when clustered into small groups. With this in mind, there is some concern about unanswered questions concerning the toxicity of silver nanoparticles that aggregate into larger groups. Most of what is known about silver’s effects on human and aquatic health is based on exposure to free silver ions, which are the smallest, elemental form of silver. Will aggregates of silver particles – as released from the fabrics in this study – have similar or different effects from silver ions?
The one thing that is clear is that silver does not necessarily remain in the products to which it is added. Silver's behavior and reactions "during wastewater treatment or in the environment needs to be studied," the authors' suggest.
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