Scientists discovered, for the first time, the presence of “abundant” quantities of toxic nanoparticles from air pollution in the human brain. These tiny magnetic particles, they believe, could be the cause of Alzheimer’s disease.

The study published in the journal Proceedings of the National Academy of Sciences found that minute particles called magnetite formed by pollution can find their way to the brain. Previous research had shown that more people die of air pollution than of malaria and HIV/AIDS put together. The World Health Organization said in 2014 that nearly 7 million premature deaths can be attributed to air pollution every year.

“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,” Barbara Maher, lead author of the paper from the Lancaster University in Lancashire, U.K., said in a statement.

Researchers studied brain tissue of 37 people aged between three and 92 years living in Mexico City, Mexico, and Manchester, U.K. They found an abundant presence of magnetite nanoparticles, which are highly toxic and have been linked to the production of free radicals or reactive oxygen species in the brain. These free radicals are associated with neurodegenerative diseases like Alzheimer’s.

Angular magnetite particles form naturally in the brain but the ones observed by the researchers were spherical showing signs that they were present due to high-temperature formation. They were also accompanied by other nanoparticles, which contained metals like 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,” Maher added.

Particles that are smaller than 200 nanometer can enter the brain directly through the olfactory nerve as a result of breathing air pollution in through the nose. The spherical magnetite nanoparticles were about 150 nanometers in diameter.

“This finding opens up a whole new avenue for research into a possible environmental risk factor for a range of different brain diseases,” David Allsop, co-author of the study from the Lancaster University’s Faculty of Health and Medicine, said.

“There is no absolutely proven link at the moment but there are lots of suggestive observations - other people have found these pollution particles in the middle of the plaques that accumulate in the brain in Alzheimer's disease so they could well be a contributor to plaque formation,” he told BBC.