The idea of having magnetite particles in our brains, possibly making us sensitive to magnetic fields, sounds exciting. A career as a Marvel supervillain anyone? Sadly, the reality is much less attractive. We are absorbing tiny particles from air pollution, and those that reach the brain may be contributing to Alzheimer’s and other neurodegenerative diseases.
The presence of magnetic nanoparticles in the human brain was discovered in 1992, but little is known about where they come from or what effects they have. A new study indicates that most of the particles were formed in a high-temperature environment, suggesting they are not a product of some strange function within the human body. Their shape indicates they are probably a result of industrialization, rather than an external biological source.
Professor Barbara Maher of the University of Lancaster used a combination of magnetic analysis and electron microscopy to study the magnetite particles in brain samples taken from 37 people who once lived either in Mexico City or Manchester, England. She found what appear to be two distinct types of particles, which she attributes to different origins. One set is angular and appears to have grown inside the body.
However, in the Proceedings of the National Academy of Sciences Maher reports that the other type “displays [a] compelling similarity with the magnetite nanospheres formed by combustion, which are ubiquitous and prolific in urban, airborne particulate matter.” Specifically, some of the particles appeared to have been fused together in ways that are hard to explain without high temperatures.
Brain slices of people from Mexico city and Manchester, with magnetite nanoparticles circles. Maher et al/Proceedings of the National Academy of Sciences.
Magnetite (Fe3O4) is a naturally occurring substance. Pieces of it, known as lodestone, acquire magnetism through a (still debated) natural phenomenon, and served as humanity’s introduction to magnets, being made into the first compasses.
Without magnetite, humanity might never have discovered how to harness electricity. Magnetite particles may play a role in the capacity of many animals, most famously homing pigeons, to sense magnetic fields. Prior to Maher's research, it was unknown if the presence of these particles in our own brains was a vestigial remnant of our evolutionary past, rather than an outcome of living in polluted environments.
However, Maher concludes that industrially produced magnetite particles with diameters less than 20 nanometers get into the brain, apparently through the olfactory nerve at the top of our nose. Once inside the brain, the particles are thought to produce chemicals known as reactive oxygen species that have been linked to Alzheimer’s disease.
Although the median length of the longest side of the particles Maher found was just 18 nanometers some were as large as 150 nanometers, raising questions about how they reached the brain.
All the brains Maher studied were from city-dwellers, preventing comparisons that might prove the source of the particles. Nevertheless, the authors suggest the most likely explanation is that these particles are the product of urban air pollution, particularly from diesel engines and indoor open fires.