Scientists found large quantities of magnetite, a poisonous compound produced when cars brake and burn fuel, that appear to have entered the brain through the nose.
Once it enters the brain, magnetite appears to act as a rallying point for the clusters of poisonous protein that are thought to cause Alzheimer’s. Globally, it is believed that 47 million people are living with the disease. Scientists believe this number will almost double every 20 years, reaching 74.7 million in 2030.
Researchers looked at brain slices taken from 29 dead people in Mexico City and eight in Manchester. The bodies were aged from three to 92.
It has been known for more than two decades that the brain naturally accumulates small cubic or octohedral crystals of magnetite but this study found a very different form.
The new particles were spherical and smooth, as though they had been shaped by high temperatures, and so fine - at about 20 millionths of a millimetre in diameter - that they could easily slip through the body’s defenses.
Barbara Maher, of the Centre for Environmental Magnetism and Palaeomagnetism at the University of Lancaster, said her team suspected that the magnetite found its way into the brain through the olfactory nerve, which carries information about smells.
Once there it speeds up the production of reactive oxygen, which can lead cells to die or kick off a cycle of inflammation, Professor Maher and her colleagues wrote in the journal PNAS.
“When you look at those samples, you find literally millions of these extra magnetites,” she said. “They look compellingly similar to those magnetites that we know are floating around in the atmosphere in air pollution.
“Excess metals in the brain are thought to be dangerous. Magnetite is a mixture of ferric iron and ferrous [less chemically stable] iron, and it’s the ferrous form that’s quite toxic.”
Professor Maher said that the ease with which the ore became magnetised meant that as people moved through electrical or magnetic fields the particles could shift around, possibly damaging surrounding cells.
But researchers have said they cannot say for sure whether there is a causal link between these nanoparticles and Alzheimer’s.
Dr David Reynolds, chief scientific officer at Alzheimer’s Research UK, said: ‘Little is known about the role of magnetite nanoparticles in the brain and whether their magnetic properties influence brain function.
‘It’s interesting to see further research investigating the presence of this mineral in the brain, but it’s too early to conclude that it may have a causal role in Alzheimer’s disease or any other brain disease.
‘We know that air pollution can have a negative impact on certain aspects of human health, but we can’t conclude from this study that magnetite nanoparticles carried in air pollution are harmful to brain health.’
He highlighted the fact that, while it is important to continue to study the impact of lifestyle and environment on brain health, age and genetic risk factors also play a role in influencing a person’s risk of Alzheimer’s.
Dr Clare Walton, Research Manager at Alzheimer’s Society, said: ‘Magnetite, a form of iron oxide, has previously been seen in amyloid plaques in the brains of people who have died with Alzheimer’s disease.
'This magnetite is generally thought to come from iron found naturally in the brain and there is no strong evidence to suggest that it causes Alzheimer’s disease or makes it worse.
‘This study offers convincing evidence that magnetite from air pollution can get into the brain, but it doesn’t tell us what effect this has on brain health or conditions such as Alzheimer’s disease.
‘The causes of dementia are complex and so far there hasn’t been enough research to say whether living in cities and polluted areas raises the risk of dementia.
'Further work in this area is important, but until we have more information people should not be unduly worried.
'There are more practical ways to lower your chances of developing dementia such as regular exercise, eating a healthy diet and avoiding smoking.’
The findings are published in the journal Proceedings of the National Academy of Sciences.