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.