Inhaled Silver Nanoparticles Easily Access the Brain

A study from UC Davis is the first to demonstrate airborne silver nanoparticles (AgNP) found in occupational environments can travel from the nose to the brain and remain for up to 2 months, possibly producing neurological effects. The finding raises the potential importance of workplace regulation of inhaled AgNP, suggests COEH faculty Kent Pinkerton and his research team.

Diagrams that illustrate the use of coated silver nanoparticles to understand the relationship of particle dissolution and bioavailability to cell and lung toxicological potential.
Diagrams that illustrate the use of coated silver nanoparticles to understand the relationship of particle dissolution and bioavailability to cell and lung toxicological potential.

Published in the May 2016 on-line issue of Environmental Health Perspectives, the study notes the significance of the long-term retention of measurable silver in the nose and forebrain after a single six hour inhalation dose, a period equivalent to a standard work shift.

“We know these particles deposit throughout the respiratory tract, but our study is the first to show that they easily gain access to the brain,” Kent Pinkerton said in a press release. Pinkerton is the director of the UC Davis Center for Health and the Environment.

Silver nanoparticles are commercially valued for their anti-microbial properties and used in antifungal, antibacterial, and anti-viral products, including homeopathic sprays for the treatment of respiratory infections. They are found in food storage containers, cosmetics, household items such as refrigerators and washing machines, electronics such as cell phones, laptops computers, and toys. Exposure can occur through dermal absorption, oral ingestion, inhalation, or eye contact.

For the study, researchers exposed adult male rats to a single dose of aerosolized AgNP particles measuring 20 or 110 nanometers, equivalent to one billionth of a meter. For comparison, a water molecule is less than one nanometer. “This dose was selected to approximate human exposure after one day of light work in a worst-case occupational scenario,” report the authors.

For both particle sizes, silver nanoparticles were deposited in the nose where they were retained and transported to the olfactory bulb in the forebrain. The total concentration of silver in the olfactory bulb remained significantly higher than in the control group through 8 weeks post-inhalation.

While the smaller particles 20 nanometers in size are small enough to quickly reach the olfactory bulb, the larger particles are not. Researchers noted the larger nanoparticles continuously shed silver ions, which may result in the gradual but progressive movement of AgNP to the olfactory bulb.

Almost immediately after exposure, researchers observed the activation of microglial cells in the brain. “Microglial cells are the phagocytic cells (macrophages) of the brain and can be associated with free radical generation, indicating the possibility of central nervous damage,” Pinkerton said in the press release, noting that, although the findings are not cause for concern, he and his co-investigators conclude “the potential to elicit neurological effects following inhalation of differently sized AgNP merits further attention.” The study received funding from the National Institute of Environmental Health Sciences.

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