Iron nanoparticles were already known to be present in the brain -- but they were thought to come from the iron naturally found in our bodies, derived from food.
Now a closer look at their structure suggests the particles mostly come from air pollution sources, like traffic fumes and coal burning. The findings are a smoking gun, says Barbara Maher of Lancaster University in the UK.
Environmental pollution including carbon particles emitted by car exhaust, smoking and long term inhalation of dust of various origins have been recognised as risk factors causing chronic inflammation of the lungs. The link between smoking and autoimmune diseases such as rheumatoid arthritis has also been established. Diseases associated with inhaled nanoparticles include asthma, bronchitis, emphysema, lung cancer, and neurodegenerative diseases, such as Parkinson's and Alzheimer's diseases.
Increasing evidence indicates that damage specific parts of the brain is present in a wide range of neurodegenerative diseases including demyelinating and psychiatric disorders.
Iron is present harmlessly in our bodies in different forms, as it is part of many biological molecules. But the form known as magnetite, or iron oxide, which is highly reactive and magnetic, has been implicated in Alzheimer's disease.
Magnets on the Brain
Maher's team looked at the brains of 37 people who had lived either in Manchester in the UK or Mexico City. All contained millions of magnetite particles per gram of brain tissue, detected by measuring how magnetic the brain tissue was.
"We can confirm that larger cities expose their populations to a greater density of magnetite particles due to the nature of
environmental pollution in those areas," said Timothy Sweet, Professor of Environmental Science commenting on the study.
The surprise came when the team used electron microscopes to take a close look at particles in the front part of the brains of six people. Round particles of magnetite outnumbered angular magnetite crystals by about one hundred to one.
Crystal forms are more likely to have a natural source -- such as iron that has come out of the body's cells. But round particles normally come from melting iron at high temperatures, which happens when fuel is burned.
Maher says the shape of these particles is compelling evidence that they come from pollution. "There is iron as impurities in fuel, and there is iron in a car engine block," she says. "If you walk down the street you'll be breathing them in -- how could they not get into your system?"
These magnetite nanoparticles are less than 200 nanometres in diameter, so may be moving from the air into the nerve endings in our noses, and from there to the brain, says Maher's team.
The team also found that the brains contained nanoparticles of metals that are present in car engines but are rare in the body, such as platinum.
Jo Anne Shatkin, at US environmental health firm Vireo Advisors, says the findings are a cause for concern. "It's not that surprising because we have known for a long time we get exposed to these nanoparticles. We are just getting a better ability to look at them."
Previous work on cells grown in the lab has suggested that iron oxide is present in the protein plaques thought to play a role in Alzheimer's disease, and that it generates reactive compounds called free radicals, which can kill nerve cells.
Population studies have found that people who live nearer busy roads have a higher risk of mental impairment in old age. But these kinds of studies have also found that our risk of getting Alzheimer's by a particular age is falling over time, so if air pollution is contributing to the disease, it doesn't seem to be making it more common.
Even so, steps to reduce air pollution might cut our risk of Alzheimer's further, says Shatkin.