A paper published in PNAS titled “Magnetite pollution nanoparticles in the human brain,” makes the argument:
We identify the abundant presence in the human brain of magnetite nanoparticles that match precisely the high-temperature magnetite nanospheres, formed by combustion and/or friction-derived heating, which are prolific in urban, airborne particulate matter (PM). Because many of the airborne magnetite pollution particles are <200 nm in diameter, they can enter the brain directly through the olfactory nerve and by crossing the damaged olfactory unit. This discovery is important because nanoscale magnetite can respond to external magnetic fields, and is toxic to the brain, being implicated in production of damaging reactive oxygen species (ROS). Because enhanced ROS production is causally linked to neurodegenerative diseases such as Alzheimer’s disease, exposure to such airborne PM-derived magnetite nanoparticles might need to be examined as a possible hazard to human health.
It’s hard not the find the idea of pollution induced magnetite nanospheres in your brain disturbing. The paper reports:
The specific presence of magnetite in the brain is important because it has been causally linked with potential cellular responses to external magnetic fields
suggesting that the nanoparticles in conjunction with magnetic fields may be a factor. This may explain some of the inconsistent findings on the effects of magnetic fields on humans.