Researchers at Lancaster University in the UK found abundant magnetite nanoparticles in the brain tissue from 37 individuals aged three to 92-year-old who lived in Mexico City and Manchester.
This strongly magnetic mineral is toxic and has been implicated in the production of reactive oxygen species (free radicals) in the human brain, which are associated with neurodegenerative diseases including Alzheimer's disease.
Professor Barbara Maher, from Lancaster Environment Centre, used spectroscopic analysis to identify the particles as magnetite.
Unlike angular magnetite particles that are believed to form naturally within the brain, most of the observed particles were spherical, with diameters up to 150 nanometres (nm), some with fused surfaces, pointing to high-temperature formation - such as from vehicle engines or open fires.
The spherical particles are often accompanied by nanoparticles containing other metals, such as platinum, nickel, and cobalt.
Other sources of magnetite nanoparticles include open fires and poorly sealed stoves within homes.
Particles smaller than 200 nm are small enough to enter the brain directly through the olfactory nerve after breathing air pollution through the nose.
"Our results indicate that magnetite nanoparticles in the atmosphere can enter the human brain, where they might pose a risk to human health, including conditions such as Alzheimer's disease," said Maher.
"This finding opens up a whole new avenue for research into a possible environmental risk factor for a range of different brain diseases," said David Allsop, of Lancaster University's Faculty of Health and Medicine. The findings were published in the journal PNAS.