Microscopic bits of silver, known as nanoparticles, now appear as an anti-microbial ingredient in a wide variety of consumer products.

However, a growing body of evidence tarnishes silver nanoparticles' reputation. Studies published this year documented unhealthy reactions in human intestinal cells and aquatic algae after exposure to silver nanoparticles, reported Inside Science.

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Manufacturers now use silver nanoparticles in everything from skin creams to little black dresses to food containers. Commercial aliases for the nanoparticles include colloidal silver and nanosilver.

In 2013, the Environmental Protection Agency proposed registration of a pesticide containing silver nanoparticles. Consumers may benefit from the silver specks’ ability to inhibit the growth of bacteria, fungus and other microorganisms, including disease-causing Escherichia coli and Staphylococcus aureus, according to numerous studies.

However, silver nanoparticles might harm more than microorganisms. Inside Science pointed to two recent studies that suggested nanosilver can be harmful.

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In a study from January, algae (Chlamydomonas reinhardtii) reacted negatively to nanosilver. The algae’s rates of photosynthesis and levels of ATP, an energy storage and transport molecule, plummeted after exposure. The algae then mounted a defensive response to cleanse itself of the nanosilver and fight damage caused by the particles. The Proceedings of the National Academy of Sciences published the results.

Silver nanoparticles' tiny size allows them to enter parts of living things bodies that other molecules can’t reach. The other study mentioned by Inside Science and published in February by ACS Nano, found that human intestinal cells reacted negatively to silver nanoparticles of different sizes.

Smaller particles (20 nanometers) could enter cells and directly damaged the internal workings, while larger particles (100 nanometers) acted indirectly by influencing protein production and enzyme activity.

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The danger is that nanosilver may be entering the environment now, yet we don't know how it will affect living things in the long term. Scientists do know that various forms of silver, including nanoparticles, can be toxic to animals, including rainbow trout and rats, in laboratory experiments.

Photo: Nearly pure silver foil in a bottle. Credit: JanDerChemiker, Wikimedia Commons.