Friday, November 20, 2009

Emerging Contaminant Alert: Nanosilver

There are known health risks involved with nanomolecules, most notably carbon nanotubes which can be highly toxic. Since the emergence of nanotechnology, metals such as gold and silver are being used in colloidal solutions for a variety of industrial and biomedical applications. Colloids are small particles….really small. They range from 10^-6 to 10^-9m (1um-1nm) in diameter. For reference, that’s about 100x smaller than a virus. Probably the most common colloid we are familiar with is milk, though I doubt anyone ever thinks of their morning cereal as a bowl of nanofat globules floating in suspension.

New research published in the journal Small, demonstrates that colloidal silver has teratogenic, or developmental, defects on zebrafish embryos. Silver, being a Class B metal, has a tendency to form ligands with sulfur complexes, such as those in amino acids (Cysteine for example). Once bound to protein structure-building acids, silver can accumulate within muscle tissues with a high residence time. While gold too is a Class B metal, it is biologically inert and nontoxic. In colloidal form, silver can be easily transported throughout the body (through the circulatory system, through the blood-brain barrier, etc) and its small size affords it’s a high surface-to-volume ratio, making it more chemically reactive, or in this case, biologically.

Zebrafish were used in this study because they are a powerful model for studying development of vertebrate species. So what happens when you mix zebrafish embryos and colloidal silver? Well, mainly death, at higher concentrations at least. It was not until embryos were dosed at less than 250uM were non-lethal effects seen. Zebrafish embryo malformations were scored 0-4 with 0 being normal(control) and 4 being death. “Some of the fish became extremely distorted, almost making a number nine or a comma instead of a linear fish,” said Dr. Darin Furgeson, author of the study in article written for Scientific American. Exposure to colloidal silver caused cranial and spinal deformities, pericardial edema (swelling around the heart) and in addition, toxicity was not only related to dosage, but also to the size of the colloids. By 24hrs of exposure, silver particles with a diameter of 3nm killed 80% of the embryos while particles of 100nm in size, only 3%. So the smaller the colloidal grain size; the more lethal it becomes.

But silver has never been a huge human health concern, with most commonly known affects being altered skin pigmentation. In the environment though, silver pollution from industry has been serious issue. Another characteristic of Class B metals is that they have a tendency to bind to particles and be transported via the atmosphere. So silver waste generated in coal production and burning in China, can adsorb onto particulate matter and be transported globally. That’s why atmospheric deposition is the major source of silver in the oceans. Colloidal silver is becoming more commonly used each year with its major use as antimicrobial to prevent bacteria growth on many consumer products. The fact that it has been shown that these colloidal silver solutions are not removed from standard waste treatment process should be of major concern. Anytime there is a potentially hazardous chemical mass produced and not removed before being discharged into the environment, red flags should be shooting up all over the place.

From Dr. Furgeson again, “I think we jumped the gun…[we] should take more time and really look at these new nano-systems before we start to throw them into personal products and shoot them into these ecosystems.”

So the question stands, just what lingering effects may we be seeing ten years from now? What concern is it to human health, environmental health? Like most emerging contaminants, we would expect to see the effects first in aquatic organisms because once something taints their environment, they are exposed via gills (respiration), diet, dermal contact…a combination that makes their health the first proxy we have for other organisms, and why fish embryo studies like this so imformative.

Bar-Ilan, O., R. M. Albrecht, et al. (2009). "Toxicity Assessments of Multisized Gold and Silver Nanoparticles in Zebrafish Embryos." Small 5(16): 1897-1910.

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