The samples, obtained after death, were taken from 29 people from Mexico City and eight people from Manchester.
Magnetite is formed naturally in small quantities in the body, but the shapes of the naturally formed particles are jagged and irregular, while the particles found in the brain samples were spherical with smooth, fused surfaces.
Magnetite may increase oxidative damage - damage caused at the molecular level - to brain cells, especially in the presence of amyloid beta protein, a key protein linked to Alzheimer's disease.
While it's worrying to think pollution particles can enter the brain, it's unclear what role, if any, these particles really have in the development of the disease.
The people studied did not have Alzheimer's disease, although some of the eight people from the UK had a neurodegenerative disease.
The researchers have called for more work to be done to establish whether or not magnetite particles from air pollution play a role in causing Alzheimer's disease.
Independent experts have reacted with caution, saying this is as yet unknown.
Air pollution levels have fallen significantly in the UK in the last 40 years, but there has not been a corresponding fall in Alzheimer's cases, possibly making the link between the two harder to determine.
The study was carried out by researchers from the University of Lancaster, the University of Oxford, the University of Glasgow, the University of Manchester, the University of Montana and Universidad Nacional Autonoma de Mexico.
It was funded by Alzheimer's Research UK, the Alzheimer's Society and the Medical Research Council.
The study was published in the peer-reviewed Proceedings of the National Academy of Sciences journal.
The UK media covered the study responsibly for the most part, making it clear that we don't know for sure whether these particles are a cause of Alzheimer's, and quoting experts unconnected with the study to balance the views of the researchers.
This experimental laboratory study analysed brain tissue samples using four types of particle analysis processes.
This type of study can show that these specific particles are present in the brains of the people studied, but nothing else.
It can't tell us whether these particles are found in everyone's brains or just in the brains of people who live in polluted areas, or whether they are more common in people with Alzheimer's disease.
Researchers took samples of brain tissue from 29 people from Mexico City aged 3 to 85 years, and eight people from Manchester in the UK aged 62 to 92 years.
They analysed the samples using four different scanning and analysis procedures to examine the minerals, shape and composition of nanoparticles found in the frontal cortex of the brains.
The researchers looked at the number and size of the particles. They also compared the qualities of the particles found with previously identified naturally occurring magnetite particles, and also with particles found in air samples taken at roadsides in Lancaster.
The researchers found all the brain samples contained "abundant" magnetite particles "that match precisely the high-temperature magnetite nanospheres formed by combustion and/or friction-derived heating, which are prolific in urban, airborne particulate matter".
The concentrations were mainly highest among older people, although some of the samples taken from much younger Mexico City residents were also very high. Mexico City is known to have high levels of air pollution.
The researchers say they found two types of particles: the jagged types thought to form naturally, and the spherical, smooth type consistent with particles produced by air pollution.
These rounded forms also varied in size much more than the smaller naturally occurring variety.
The researchers say their results may explain previous research, which found spherical particles of magnetite in the plaques and tangles of protein in brain tissue from people with Alzheimer's disease.
They also point to previous research from Taiwan, which found people living in areas with higher air pollution were more likely to get Alzheimer's disease.
They say theoretically these particles could get from air into the brain via the olfactory nerve, which carries information about smell from the nose to the brain.
"Because of their combination of ultrafine size, specific brain toxicity, and ubiquity within airborne particulate matter, pollution-derived magnetite nanoparticles might require consideration as a possible Alzheimer's disease risk factor," they conclude.
Air pollution is known to be hazardous for human health as a cause of heart and lung disease. This study suggests microscopic particles found in pollution may also enter the brain. While that's a worrying thought, we don't know yet what effect that could have.
The study is quite limited in what it tells us. We know the researchers found particles of magnetite in all the brain samples studied, but as there was no control group - for example, people without neurodegenerative disease in the UK, or people from a less polluted part of Mexico - we don't know the significance of the finding.
And we don't know whether brains of people with Alzheimer's disease are more or less likely to contain magnetite particles than any other brains.
It's important that scientists investigate these findings further to answer some of these questions. But it doesn't mean the rest of us need to panic.
Avoiding pollution is sensible for health reasons if you can manage it - for example, by walking away from the edge of a busy road, or cycling through back streets - but it's not always possible.
Although nothing guarantees that you won't develop Alzheimer's disease, there are plenty of things you can do to reduce your risk of the condition:
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