A possible causal link between air pollution and Alzheimer's disease has been suggested by scientists after they conducted detailed studies of brain tissue, in a joint British-Mexican project.

Study co-author Professor Barbara Maher, of Lancaster University, and her research team examined the brain tissue of 37 individuals who had lived in either Mexico City or Manchester, both air pollution hotspots.

Using microscopic and spectroscopic analysis, Maher's team found tiny magnetic particles from air pollution lodged in the brains, the first time such a discovery has been made.

She told Reuters: "The first thing we did was to make ultra-thin sections of tissue and we analyzed those using high resolution transmission electromicroscopy, a very highly resolved microscope, in Glasgow. We were able to examine those thin sections to identify if these particles were in the cells, their shape, size, and size distribution, and critically also conduct chemical analysis in the microscope to identify that these particles were magnetite."

Magnetite is a strongly magnetic, toxic, mineral, implicated in the production of reactive oxygen species (including free radicals) in the human brain. These have long been associated with neurodegenerative diseases, such as Alzheimer's.

Detailed analysis of the six most magnetic brain samples showed the majority of the magnetite particles to be spherical, distinguishing them from the angular magnetite particles which scientists believe form naturally within the brain. The range of particle sizes, from 5 nanometers (nm) diameter to 150 nm and fused surfaces both suggest high-temperature formation, which Maher's team posit came from industry, vehicle engines - mostly diesel - or open fires.

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Particles smaller than 200 nm can enter the brain directly through the olfactory nerve after breathing air pollution through the nose.

Maher says the particles found are "strikingly similar" to magnetite nanospheres abundant in airborne pollution found next to busy roads, formed by combustion or frictional heating from vehicle engines or brakes.

Although Maher had previously suspected a link between air pollution and magnetite particles in the brain, the number of particles found in brain samples surprised her.

"In a way, it was not surprising because I was familiar with how abundant the particles are in the atmosphere and the fact they have a ready entry route up the nose and through the olfactory nerves..... but on the human scale when you actually see the extracted particles and you see hundreds and thousands visible, then that's when it hits home that there's a prolific amount of magnetite in the brain that shouldn't be there."

Nanoparticles containing other metals, such as platinum, nickel, and cobalt, were also found in the brains.

"It's always been known that metals accumulate in the brain with Alzheimer's disease, sometimes with ageing, but it wasn't known if that was because they were coming in some other route and being solubilized and transported into the brain," said Maher. "Seeing these particles with their very distinctive morphologies and size distributions tells you that they haven't been dissolved. They look like they looked when they were in the atmosphere, and as well as being in the atmosphere they're now seen in the brain. So that's the novelty because magnetite is such a dangerous mineral for the brain, that's why it's so significant."

The researchers are not claiming to have found a definite link between the particles and the deadly degenerative brain disease, but they believe examining the possible link should be a priority for future research.

"If pollution derived magnetite particles are a substantial causal link involved in neurodegenerative disease, that drives a - individual level of trying to reduce their own exposure, but b - policy makers to try to do something to reduce the health burden. Alzheimer's disease may be a modern epidemic of our own making," said Maher.

The brain samples came from 37 individuals aged between three and 92-years-old. Maher worked with colleagues from the universities of Oxford, Glasgow, Manchester, Montana, and the Universidad Nacional Autonoma de Mexico. Their research was published by the Proceedings of the National Academy of Sciences journal.

According to Alzheimers.net, 44 million people globally suffer from some form of dementia, by far the most common of which is Alzheimer's.