Anthropogenic (combustion-derived) nanoparticles of magnetite, a strongly magnetic mineral, have been discovered to be lodged in human brains — and scientists think these tiny particles could be a possible cause of Alzheimer’s disease.

Prof. Barbara Maher, lead author on the study and a researcher at the University of Lancaster, and her colleagues from Mexico and the United Kingdom found abundant magnetic particles in the brain tissue from 37 individuals aged three to 92-years-old who lived in Mexico City and Manchester.

“The brain magnetic analyses identify the presence in all of the samples of strongly magnetic, easily magnetized nanoparticles, with concentrations ranging from 0.2 to 12 µg/g dry tissue,” they said.

The researchers used spectroscopic analysis to identify the particles as magnetite.

This mineral can have potentially large impacts on the human brain. It has been implicated in the production of reactive oxygen species in the brain, which are associated with neurodegenerative diseases such as Alzheimer’s disease.

Unlike angular magnetite particles that are believed to form naturally within the brain, most of the particles observed by the team were spherical, with diameters up to 150 nm, some with fused surfaces, all characteristic of high-temperature formation – such as from vehicle (particularly diesel) engines or open fires.

The spherical particles are often accompanied by nanoparticles containing other metals, such as platinum, nickel, and cobalt.

“The particles we found are strikingly similar to the magnetite nanospheres that are abundant in the airborne pollution found in urban settings, especially next to busy roads, and which are formed by combustion or frictional heating from vehicle engines or brakes,” Prof. Maher said.

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,” Prof. Maher said.

The team’s results appear today in the Proceedings of the National Academy of Sciences.

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Barbara A. Maher et al. Magnetite pollution nanoparticles in the human brain. PNAS, published online September 6, 2016; doi: 10.1073/pnas.1605941113