New research has revealed tiny particles of pollution lodged inside samples of brain tissue.
The finding, described as "dreadfully shocking" by researchers, raises a host of new questions about the health risks of pollution which it is estimated kill 50,000 people in the UK every year with conditions linked to dirty air.
The research led by scientists at Lancaster University and is published in the Proceedings of the National Academy of Sciences (PNAS) shows the first evidence that minute particles of what is called magnetite, which can be derived from pollution, can find their way into the brain.
The team analysed samples of brain tissue from 37 people - 29 who had lived and died in Mexico City, a notorious pollution hotspot, and who were aged from 3 to 85.
The other eight came from Manchester, were aged 62-92 and some had died with varying severities of neurodegenerative disease.
Suspected of toxicity, the particles of iron oxide could conceivably contribute to diseases like Alzheimer's - though evidence for this is lacking.
Previous studies linked to air pollution have focused on the impact of dirty air on the lungs and heart.
Now this new research provides the first evidence that minute particles of what is called magnetite, which can be derived from pollution, can find their way into the brain.
The lead author of the research paper, Professor Barbara Maher, has previously identified magnetite particles in samples of air gathered beside a busy road in Lancaster and outside a power station.
She suspected that similar particles may be found in the brain samples, and that is what happened.
She told the BBC: "It's dreadfully shocking. When you study the tissue you see the particles distributed between the cells and when you do a magnetic extraction there are millions of particles, millions in a single gram of brain tissue - that's a million opportunities to do damage."
Further study revealed that the particles have a distinctive shape which provides a crucial clue to their origin.
Magnetite can occur naturally in the brain in tiny quantities but the particles formed that way are jagged. By contrast, the particles found in the study were not only far more numerous but also smooth and rounded - characteristics that can only be created in the high temperatures of a vehicle engine or braking systems.
Prof Maher said: "They are spherical shapes and they have little crystallites around their surfaces, and they occur with other metals like platinum which comes from catalytic converters.
"So for the first time we saw these pollution particles inside the human brain.
"It's a discovery finding. It's a whole new area to investigate to understand if these magnetite particles are causing or accelerating neurodegenerative disease."
Dubbed "nanospheres", the particles are less than 200 nanometres in diameter - by comparison, a human hair is at least 50,000 nanometres thick.
While large particles of pollution such as soot can be trapped inside the nose, smaller types can enter the lungs and even smaller ones can cross into the bloodstream.
But nanoscale particles of magnetite are believed to be small enough to pass from the nose into the olfactory bulb and then via the nervous system into the frontal cortex of the brain.
Professor David Allsop, a specialist in Alzheimer's and other neurodegenerative diseases, is a co-author of the study and also at Lancaster University.
He told the BBC pollution particles "could be an important risk factor" for these conditions.
"There is no absolutely proven link at the moment but there are lots of suggestive observations - other people have found these pollution particles in the middle of the plaques that accumulate in the brain in Alzheimer's disease so they could well be a contributor to plaque formation.
"These particles are made out of iron and iron is very reactive so it's almost certainly going to do some damage to the brain. It's involved in producing very reactive molecules called reactive oxygen species which produce oxidative damage and that's very well defined.
"We already know oxidative damage contributes to brain damage in Alzheimer's patients so if you've got iron in the brain it's very likely to do some damage. It can't be benign."
The best way to protect yourself from air pollution is to invest in an air purifier for your home, office or car.
The Healthy House recommends using an air purifier that is able to neutralise the tiny nanospheres as the particles are less than 200 nanometres in diameter as they are too tiny to be trapped in a normal filter.
Viruskiller VK Blue combines photo-catalytic oxidation with a True HEPA and carbon filtration to deliver unparalleled results against tiny particles.
To deliver superior results, the Viruskiller VK Blue employs a variety of filtration and sterilisation technologies. This combination enables the destruction or capture of even the tiniest particles and allergens.
So how does it achieve these kinds of results? First, a complex filter set comprising a washable pre-filter, a carbon filter, a medium filter and a thick HEPA filter tackles the larger particles. Then, the photo-catalytic oxidation (PCO) technology, originally created by NASAS, takes on bacteria, viruses and gases.
The layers of nano filter tubes in the filter chamber are coated with titanium dioxide (a photo-catalyst) which reacts with the UVC lights inside the chamber. This reaction creates hydroxyl radicals which destroy the cells of impurities breaking them down into harmless water and carbon dioxide.
The Radic8 Clean Air 200 Deodoriser and Steriliser also use PCO technology to neutralise tiny particles. In the process, titanium dioxide is exposed to UV light to kick-start a chemical reaction. Hydroxyl radicals are released into the air, which break down airborne pollutants.