Cal Baier-Anderson, Ph.D., is a Health Scientist.
In 2004, Gunter Oberdorster and colleagues demonstrated that upon inhalation, ultrafine particles, the dimensions of which are measured in nanometers, can move from the nasal passages of rodents to the brain via a specialized nerve called the olfactory bulb. The evolutionary purpose of the olfactory bulb is to relay information about odors directly and rapidly from the nose to the brain.
The extent to which rapid transit via the olfactory bulb is a significant potential route of exposure to engineered nanomaterials is still an open question. But two new papers add support for the relevance of this intriguing exposure pathway, raising important questions regarding the safety of inhaled nanoparticles.
The first paper, by Jiangxue Wang and colleagues, followed the movement of nanoscale titanium dioxide (TiO2) particles placed directly in the nasal passages of mice to the brain via the olfactory bulb. When they looked to see where in the brain the TiO2 went, they found it went pretty much everywhere, although after 30 days the highest concentrations were found in the olfactory bulb and hippocampus. Moreover, the brain tissue of the exposed mice exhibited changes in structure and biochemistry consistent with damage from reactive oxygen compounds.
Nanoscale silver was the subject of the second paper by Jae Hyuck Sung and colleagues. Instead of a single exposure, rats were exposed to nanosilver in the air for 13 weeks. Like an earlier 28-day inhalation study, this one also found widespread distribution of nanosilver in the rats.
This study also sought to determine if there were any health effects associated with longer-term exposures. And they did find effects: inflammation in the lungs, and subtle cellular changes in the livers that are sometimes indicative of pre-cancerous conditions.
Nanosilver was detected in both the olfactory bulb and the brain but unfortunately the paper did not report on any effects that might have been associated with the presence of nanosilver. It is not clear from the description provided if the extent of examination of the brain would have been able to identify subtle effects if they were present.
So what should the next steps be? Elucidating the possible impacts of nanoscale materials on the brain is tricky, in part because the effects could be very diverse, requiring lots of different types of tests to capture them. While damage to brain cells may be relatively easy to discern, other effects, such as those altering brain development or biochemistry, may require more sophisticated testing.
While it is often assumed that inhalation exposure to nanomaterials will be limited primarily to workplaces, such materials are also being used in consumer products that can be widely dispersed. This is particularly true of nanosilver, which can be found in sprays that release nanosilver into the air. This is why we have repeatedly advocated that such dispersive uses be avoided until more is known about the potential adverse effects that could come with these kinds of exposures.