Toxic nanoparticles from air pollution have been found embedded in people's brain tissue for the first time, and research has tentatively linked these particles to a higher risk of Alzheimer's disease.
The particles were already known to be present in our brains, but researchers had assumed our bodies naturally produced them. Now a small study by UK researchers has found that they're the direct result of polluted air.
"This is a discovery finding, and now what should start is a whole new examination of this as a potentially very important environmental risk factor for Alzheimer's disease," one of the team, Barbara Maher from Lancaster University, told Damian Carrington at The Guardian.
"Now there is a reason to go on and do the epidemiology and the toxicity testing, because these particles are so prolific and people are exposed to them."
Maher and her team examined brain tissue from 37 people in Manchester, England, and Mexico City, aged between 3 years old and 92. Each of them contained particles of a type of iron oxide called magnetite, and not just traces of them - they were abundant.
"You are talking about millions of magnetite particles per gram of freeze-dried brain tissue - it is extraordinary," says Maher.
The next step was to figure out where these particles were coming from. When the team looked at the particles in the front regions of the brains of six of the volunteers, they found two types of magnetite in the tissue - round particles of magnetite and angular magnetite crystals, and the round ones outnumbered the crystals by about 100 to one.
"Crystal forms are more likely to have a natural source - such as iron that has come out of the body's cells," Clare Wilson explains for New Scientist. "But round particles normally come from melting iron at high temperatures, which happens when fuel is burned."
A couple of caveats here - the evidence is circumstantial, and the only way to really prove that these particles are sourced from air pollution is to actually trace them all the way from the atmosphere to the brain tissue.
But Maher says they also found particles of metals such as platinum that are very rarely found naturally in the body, but are found in many car engines.
The second big limitation here is that the sample size is tiny, and while the result of abundant round magnetite particles was found in 100 percent of the participants, it's far too early to extrapolate meaning from that for the wider population.
But if larger studies do end up finding similar results in a wider, more diverse group of participants, what are the implications?
"Magnetite in the brain is not something you want to have, because it is particularly toxic there," Maher told The Guardian, adding that they can produce reactive oxygen molecules called free radicals, which have been linked to ageing and neurological disease.
"Oxidative cell damage is one of the hallmark features of Alzheimer's disease, and this is why the presence of magnetite is so potentially significant, because it is so bioreactive," she says.
Previous research looking at cells grown in the lab has found that iron oxide could be present in the amyloid plaques that have been linked to the development of Alzheimer's disease, and a study from earlier in the year also linked the presence of magnetite to damage in the brains of Alzheimer's patients.
We're still very much in the early stages of figuring out what's going on here, but even if the Alzheimer's evidence is yet to be confirmed, what we do know is that air pollution is seriously bad for all of us.
As a study from 2015 found, air pollution is likely contributing to the premature deaths of some 3.3 million people around the world every year, and that figure could double by 2050, so whatever pollution is doing to us, one thing's for sure - it's nothing good.
Maher and her team's research has been published in Proceedings of the National Academy of Sciences (the paper has not been made public at the time of writing).